. Preface . An Introduction to the Human Body 1. Overview 2. Structural Organization of the Human Body 3. Anatomical Terminology 4. Medical Imaging . Body Tissues 1. Types of Tissues 2. Epithelial Tissue 3. Connective Tissue Supports and Protects 4. Muscle Tissue and Motion 5. Nervous Tissue Mediates Perception and Response 6. Tissue Injury and Aging . Integumentary System 1. Layers of the Skin 2. Accessory Structures of the Skin 3. Diseases, Disorders, and Injuries of the Integumentary System . Bone 1. The Functions of the Skeletal System 2. Bone Classification 3. Bone Structure 4. Bone Formation and Development o. Fractures: Bone Repair . Axial Skeleton 1. Divisions of the Skeletal System 2. The Skull 3. The Vertebral Column 4. The Thoracic Cage . Appendicular Skeleton 1. The Pectoral Girdle 2. Bones of the Upper Limb 3. The Pelvic Girdle and Pelvis 4. Bones of the Lower Limb 8. Articulations 1. Classification of Joints 2. Fibrous Joints 3. Cartilaginous Joints 4. Synovial Joints 5. Types of Body Movements 6. Anatomy of Selected Synovial Joints 9. Skeletal Muscle 1. Overview of Muscle Tissues 2. Skeletal Muscle 3. Types of Muscle Fibers 4. Exercise and Muscle Performance 5. Cardiac Muscle Tissue 6. Smooth Muscle 7. Development and Regeneration of Muscle Tissue 8. Interactions of Skeletal Muscles, Their Fascicle Arrangement, and Their Lever Systems 9. Naming Skeletal Muscles 10. Axial Musculature 1. Muscles of the Head, Neck, and Back 2. Muscles of the Abdominal Wall and Thorax 11. Appendicular Musculature 1. Muscles of the Pectoral Girdle and Upper Limbs 2. Muscles of the Pelvic Girdle and Lower Limbs 12. Heart 1. Heart Anatomy 2. Cardiac Muscle and Electrical Activity 3. Cardiac Cycle 13. Blood Vessels 14. 15. 16. 17. 18. 19. 20. 21; 22. 1. Structure and Function of Blood Vessels 2. Circulatory Pathways Blood 1. An Overview of Blood 2. Production of the Formed Elements 3. Erythrocytes 4. Leukocytes and Platelets Nervous Tissue 1. Basic Structure and Function of the Nervous System 2. Nervous ‘Tissue Central Nervous System 1. Anatomy of the CNS 2. Circulation and the Central Nervous System Peripheral Nervous System 1. Nerves and ganglia Senses 1. Sensory Perception Autonomic Nervous System 1. Divisions of the ANS 2. Central Control Respiratory System 2. The Lungs Digestive System 1. Overview of the Digestive System 3. The Stomach 4. The Small and Large Intestines 5. Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder Lymphatic System 23. Urinary System 1. 2. 3: Gross Anatomy of Urine Transport Gross Anatomy of the Kidney Microscopic Anatomy of the Kidney 24. Endocrine System 1; . The Pituitary Gland and Hypothalamus . The Thyroid Gland . The Parathyroid Glands . The Adrenal Glands . The Pineal Gland . Gonadal and Placental Hormones . The Endocrine Pancreas 9. CON MU BW NHN An Overview of the Endocrine System Organs with Secondary Endocrine Functions 25. Reproductive System 1. 2. Male Anatomy Female Anatomy Preface Human Anatomy is designed for a semester-long course taken by life science and allied health students. The textbook is derived from OpenStax Human Anatomy and Physiology, and its coverage and organization were informed by hundreds of instructors who teach the course. Instructors can customize the book, adapting it to the approach that works best in their classroom. The artwork for this textbook is aimed focusing student learning through a powerful blend of traditional depictions and instructional innovations. Color is used sparingly, to emphasize the most important aspects of any given illustration. Significant use of micrographs from the University of Michigan complement the illustrations, and provide the students with a meaningful alternate depiction of each concept. Finally, enrichment elements provide relevance and deeper context for students, particularly in the areas of health, disease, and information relevant to their intended careers. Welcome to Human Anatomy, a resource designed for a semester-long course aimed at preparing undergraduate students for health-related programs. This book is derived from Human Anatomy and Physiology by OpenStax College. The source materials were created with several goals in mind: accessibility, customization, and student engagement—helping students reach high levels of academic scholarship. Instructors and students alike will find that this textbook offers a thorough introduction to the content in an accessible format. About OpenStax College OpenStax College is a nonprofit organization committed to improving student access to quality learning materials. Our free textbooks are developed and peer-reviewed by educators to ensure that they are readable, accurate, and organized in accordance with the scope and sequence requirements of today’s college courses. Unlike traditional textbooks, OpenStax College resources live online and are owned by the community of educators using them. Through partnerships with companies and foundations committed to reducing costs for students, we are working to improve access to higher education for all. OpenStax College is an initiative of Rice University and is made possible through the generous support of several philanthropic foundations. About OpenStax College’s Resources OpenStax College resources provide quality academic instruction. Three key features set our materials apart from others: 1) They can be easily customized by instructors for each class, 2) they are “living” resources that grow online through contributions from science educators, and 3) they are available for free or for a minimal cost. Customization OpenStax College learning resources are conceived and written with flexibility in mind so that they can be customized for each course. Our textbooks provide a solid foundation on which instructors can build their own texts. Instructors can select the sections that are most relevant to their curricula and create a textbook that speaks directly to the needs of their students. Instructors are encouraged to expand on existing examples in the text by adding unique context via geographically localized applications and topical connections. Human Anatomy and Physiology can be easily customized using our online platform (https://openstaxcollege.org/textbooks/anatomy-and- physiology/adapt). The text is arranged in a modular chapter format. Simply select the content most relevant to your syllabus and create a textbook that addresses the needs of your class. This customization feature will ensure that your textbook reflects the goals of your course. Curation To broaden access and encourage community curation, Human Anatomy and Physiology is “open source” under a Creative Commons Attribution (CC BY) license. Members of the scientific community are invited to submit examples, emerging research, and other feedback to enhance and strengthen the material, keeping it current and relevant for today’s students. Submit your suggestions to info@openstaxcollege.org, and check in on edition status, alternate versions, errata, and news on the StaxDash at http://openstaxcollege.org. Cost Our textbooks are available for free online, and in low-cost print and tablet editions. Contents 01. 02. 03. 04. 05. 06. 07. 08. 09. Preface An Introduction to the Human Body Body Tissues Integumentary System Bone Axial Skeleton Appendicular Skeleton Articulations Skeletal Muscle 10. Axial Musculature 11. Appendicular Musculature 12. Heart 13. Blood Vessels 14. Blood 15. Nervous Tissue 16. Central Nervous System 17. Peripheral Nervous System 18. Senses 19. Autonomic Nervous System 20. Respiratory System 21. Digestive System 22. Lymphatic System 23. Urinary System 24. Endocrine System 25. Reproductive System About Our Team Customization for One-Semester Human Anatomy Marcos Gridi-Papp University of the Pacific Senior Contributors J. Gordon Betts Tyler Junior College Peter Desaix University of North Carolina at Chapel Hill Eddie Johnson Central Oregon Community College Jody E. Johnson Arapahoe Community College Oksana Korol Aims Community College Dean Kruse Portland Community College Brandon Poe Springfield Technical Community College James A. Wise Hampton University Mark Womble Youngstown State University Kelly A. Young California State University, Long Beach Advisor Robin J. Heyden Other Contributors Kim Aquarius Institute; Triton College Aaronson Lopamudra Augusta Technical College Agarwal Gary Allen Dalhousie University Robert i ‘allison McLennan Community College Heather Southern Union State Community College Armbruster Timothy University of North Carolina Wilmington Ballard Matthew Eastern New Mexico University Barlow William Furman Universit Blaker Julie Bowers East Tennessee State University Emily Florida Southern College Bradshaw Nishi Bryska University of North Carolina, Charlotte Susan Caley Opsal Boyd Campbell Ann Caplea Marnie Chapman Barbara Christie-Pope Kenneth Crane Maurice Culver Heather Cushman Noelle Cutter Lynnette Danzl-Tauer Jane Davis AnnMarie DelliPizzi Susan Dentel Pamela Dobbins Illinois Valley Community College Southwest College of Naturopathic Medicine and Health Sciences Walsh University University of Alaska, Sitka Cornell College Texarkana College Florida State College at Jacksonville Tacoma Community College Molloy College Rock Valley College Aurora University Dominican College Washtenaw Community College Shelton State Community College Patty Dolan Sondra Dubowsky Peter Dukehart Ellen DuPré Elizabeth DuPriest Pam Elf Sharon Ellerton Carla Endres Myriam Feldman Greg Fitch Lynn Gargan Michael Giangrande Chaya Gopalan Victor Greco Susanna Pacific Lutheran University McLennan Community College Three Rivers Community College Central College Warner Pacific College University of Minnesota Queensborough Community College Utah State University - College of Eastern Utah: San Juan Campus Lake Washington Institute of Technology; Cascadia Community College Avila University Tarant County College Oakland Community College St. Louis College of Pharmacy Chattahoochee Technical College Skagit Valley College Heinze Ann Henninger Dale Horeth Michael Hortsch Rosemary Hubbard Mark Hubley Branko Jablanovic Norman Johnson Mark Jonasson Jeff Keyte William Kleinelp Leigh Kleinert Brenda Leady John Lepri Sarah Leupen Wartburg College Tidewater Community College University of Michigan Marymount University Prince George's Community College College of Lake County University of Massachusetts Amherst North Arkansas College College of Saint Mary Middlesex County College Grand Rapids Community College University of Toledo University of North Carolina, Greensboro University of Maryland, Baltimore County Lihua Liang Robert Mallet Bruce Maring Elisabeth Martin Natalie Maxwell Julie May Debra McLaughlin Nicholas Mitchell Shobhana Natarajan Phillip Nicotera Mary Jane Niles Ikemefuna Nwosu Betsy Ott Ivan Paul Aaron Payette Johns Hopkins University University of North Texas Health Science Center Daytona State College College of Lake County Carl Albert State College, Sallisaw William Carey University University of Maryland University College St. Bonaventure University Brookhaven College St. Petersburg College University of San Francisco Parkland College; Lake Land College Tyler Junior College John Wood Community College College of Southern Nevada Scott Payne Cameron Perkins David Pfeiffer Thomas Pilat Eileen Preston Mike Pyle Robert Rawding Jason Schreer Laird Sheldahl Brian Shmaefsky Douglas Sizemore Susan Spencer Cynthia Standley Robert Sullivan Kentucky Wesleyan College South Georgia College University of Alaska, Anchorage Illinois Central College Tarrant County College Olivet Nazarene University Gannon University State University of New York at Potsdam Mt. Hood Community College Lone Star College System Bevill State Community College Mount Hood Community College University of Arizona Marist College Eric Sun Tom Swenson Kathleen Tallman Rohinton Tarapore Elizabeth Tattersall Mark Thomas Janis Thompson Rita Thrasher David Van Wylen Lynn Wandrey Margaret Weck Kathleen Weiss Neil Westergaard David Wortham Middle Georgia State College Ithaca College Azusa Pacific University University of Pennsylvania Western Nevada College University of Northern Colorado Lorain County Community College Pensacola State College St. Olaf College Mott Community College St. Louis College of Pharmacy George Fox University Williston State College West Georgia Technical College Umesh Yadav Tony Yates Justin York Cheri Zao Elena Zoubina Shobhana Natarajan Special Thanks University of Texas Medical Branch Oklahoma Baptist University Glendale Community College North Idaho College Bridgewater State University; Massasoit Community College Alcon Laboratories, Inc. OpenStax College wishes to thank the Regents of University of Michigan Medical School for the use of their extensive micrograph collection. Many of the UM micrographs that appear in Human Anatomy and Physiology are interactive WebScopes, which students can explore by zooming in and out. We also wish to thank the Open Learning Initiative at Carnegie Mellon University, with whom we shared and exchanged resources during the development of Human Anatomy and Physiology. Overview By the end of this section, you will be able to: ¢ Compare and contrast anatomy and physiology, including their specializations and methods of study e Discuss the fundamental relationship between anatomy and physiology Human anatomy is the scientific study of the body’s structures. Some of these structures are very small and can only be observed and analyzed with the assistance of a microscope. Other larger structures can readily be seen, manipulated, measured, and weighed. The word “anatomy” comes from a Greek root that means “to cut apart.” Human anatomy was first studied by observing the exterior of the body and observing the wounds of soldiers and other injuries. Later, physicians were allowed to dissect bodies of the dead to augment their knowledge. When a body is dissected, its structures are cut apart in order to observe their physical attributes and their relationships to one another. Dissection is still used in medical schools, anatomy courses, and in pathology labs. In order to observe structures in living people, however, a number of imaging techniques have been developed. These techniques allow clinicians to visualize structures inside the living body such as a cancerous tumor or a fractured bone. Like most scientific disciplines, anatomy has areas of specialization. Gross anatomy is the study of the larger structures of the body, those visible without the aid of magnification ([link]a). Macro- means “large,” thus, gross anatomy is also referred to as macroscopic anatomy. In contrast, micro- means “small,” and microscopic anatomy is the study of structures that can be observed only with the use of a microscope or other magnification devices ({link]b). Microscopic anatomy includes cytology, the study of cells and histology, the study of tissues. As the technology of microscopes has advanced, anatomists have been able to observe smaller and smaller structures of the body, from slices of large structures like the heart, to the three-dimensional structures of large molecules in the body. Gross and Microscopic Anatomy (a) Gross anatomy considers large structures such as the brain. (b) Microscopic anatomy can deal with the same structures, though at a different scale. This is a micrograph of nerve cells from the brain. LM x 1600. (credit a: “WriterHound”/Wikimedia Commons; credit b: Micrograph provided by the Regents of University of Michigan Medical School © 2012) Anatomists take two general approaches to the study of the body’s structures: regional and systemic. Regional anatomy is the study of the interrelationships of all of the structures in a specific body region, such as the abdomen. Studying regional anatomy helps us appreciate the interrelationships of body structures, such as how muscles, nerves, blood vessels, and other structures work together to serve a particular body region. In contrast, systemic anatomy is the study of the structures that make up a discrete body system—that is, a group of structures that work together to perform a unique body function. For example, a systemic anatomical study of the muscular system would consider all of the skeletal muscles of the body. Whereas anatomy is about structure, physiology is about function. Human physiology is the scientific study of the chemistry and physics of the structures of the body and the ways in which they work together to support the functions of life. Much of the study of physiology centers on the body’s tendency toward homeostasis. Homeostasis is the state of steady internal conditions maintained by living things. The study of physiology certainly includes observation, both with the naked eye and with microscopes, as well as manipulations and measurements. However, current advances in physiology usually depend on carefully designed laboratory experiments that reveal the functions of the many structures and chemical compounds that make up the human body. Like anatomists, physiologists typically specialize in a particular branch of physiology. For example, neurophysiology is the study of the brain, spinal cord, and nerves and how these work together to perform functions as complex and diverse as vision, movement, and thinking. Physiologists may work from the organ level (exploring, for example, what different parts of the brain do) to the molecular level (such as exploring how an electrochemical signal travels along nerves). Form is closely related to function in all living things. For example, the thin flap of your eyelid can snap down to clear away dust particles and almost instantaneously slide back up to allow you to see again. At the microscopic level, the arrangement and function of the nerves and muscles that serve the eyelid allow for its quick action and retreat. At a smaller level of analysis, the function of these nerves and muscles likewise relies on the interactions of specific molecules and ions. Even the three-dimensional structure of certain molecules is essential to their function. Your study of anatomy and physiology will make more sense if you continually relate the form of the structures you are studying to their function. In fact, it can be somewhat frustrating to attempt to study anatomy without an understanding of the physiology that a body structure supports. Imagine, for example, trying to appreciate the unique arrangement of the bones of the human hand if you had no conception of the function of the hand. Fortunately, your understanding of how the human hand manipulates tools—from pens to cell phones—helps you appreciate the unique alignment of the thumb in opposition to the four fingers, making your hand a structure that allows you to pinch and grasp objects and type text messages. Chapter Review Human anatomy is the scientific study of the body’s structures. In the past, anatomy has primarily been studied via observing injuries, and later by the dissection of anatomical structures of cadavers, but in the past century, computer-assisted imaging techniques have allowed clinicians to look inside the living body. Human physiology is the scientific study of the chemistry and physics of the structures of the body. Physiology explains how the structures of the body work together to maintain life. It is difficult to study structure (anatomy) without knowledge of function (physiology). The two disciplines are typically studied together because form and function are closely related in all living things. Review Questions Exercise: Problem: Which of the following specialties might focus on studying all of the structures of the ankle and foot? a. microscopic anatomy b. muscle anatomy c. regional anatomy d. systemic anatomy Solution: C Exercise: Problem: A scientist wants to study how the body uses foods and fluids during a marathon run. This scientist is most likely a(n) a. exercise physiologist b. microscopic anatomist c. regional physiologist d. systemic anatomist Solution: A CRITICAL THINKING QUESTIONS Exercise: Problem: Name at least three reasons to study anatomy and physiology. Solution: An understanding of anatomy and physiology is essential for any career in the health professions. It can also help you make choices that promote your health, respond appropriately to signs of illness, make sense of health-related news, and help you in your roles as a parent, spouse, partner, friend, colleague, and caregiver. Exercise: Problem: For whom would an appreciation of the structural characteristics of the human heart come more easily: an alien who lands on Earth, abducts a human, and dissects his heart, or an anatomy and physiology student performing a dissection of the heart on her very first day of class? Why? Solution: A student would more readily appreciate the structures revealed in the dissection. Even though the student has not yet studied the workings of the heart and blood vessels in her class, she has experienced her heart beating every moment of her life, has probably felt her pulse, and likely has at least a basic understanding of the role of the heart in pumping blood throughout her body. This understanding of the heart’s function (physiology) would support her study of the heart’s form (anatomy). Glossary anatomy science that studies the form and composition of the body’s structures gross anatomy study of the larger structures of the body, typically with the unaided eye; also referred to macroscopic anatomy homeostasis steady state of body systems that living organisms maintain microscopic anatomy study of very small structures of the body using magnification physiology science that studies the chemistry, biochemistry, and physics of the body’s functions regional anatomy study of the structures that contribute to specific body regions systemic anatomy study of the structures that contribute to specific body systems Structural Organization of the Human Body By the end of this section, you will be able to: ¢ Describe the structure of the human body in terms of six levels of organization e List the eleven organ systems of the human body and identify at least one organ and one major function of each Before you begin to study the different structures and functions of the human body, it is helpful to consider its basic architecture; that is, how its smallest parts are assembled into larger structures. It is convenient to consider the structures of the body in terms of fundamental levels of organization that increase in complexity: subatomic particles, atoms, molecules, organelles, cells, tissues, organs, organ systems, organisms and biosphere ({link]). Levels of Structural Organization of the Human Body Oxygen atom Chemical level Atoms bond to form molecules with three- dimensional structures. Jd Water molecule Smooth muscle cell Cellular level A variety of molecules combine to form the fluid and organelles of a body cell. Organelle Cell fluid Organelle Smooth muscle tissue Tissue level A community of similar cells form a body tissue. Bladder Organ level Two or more different tissues combine to form an organ. Skeletal muscle Urinary tract system Kidney Organ system level Ureter Two or more organs work closely together to perform he functi \f . Bladder the functions of a body system. A, Ureth ra 1" Organismal level Many organ system work harmoniously together to perform the functions of an J independent organism. fo The organization of the body often is discussed in terms of six distinct levels of increasing complexity, from the smallest chemical building blocks to a unique human organism. The Levels of Organization To study the chemical level of organization, scientists consider the simplest building blocks of matter: subatomic particles, atoms and molecules. All matter in the universe is composed of one or more unique pure substances called elements, familiar examples of which are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of any of these pure substances (elements) is an atom. Atoms are made up of subatomic particles such as the proton, electron and neutron. Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things. Molecules are the chemical building blocks of all body structures. A cell is the smallest independently functioning unit of a living organism. Even bacteria, which are extremely small, independently-living organisms, have a cellular structure. Each bacterium is a single cell. All living structures of human anatomy contain cells, and almost all functions of human physiology are performed in cells or are initiated by cells. A human cell typically consists of flexible membranes that enclose cytoplasm, a water-based cellular fluid together with a variety of tiny functioning units called organelles. In humans, as in all organisms, cells perform all functions of life. A tissue is a group of many similar cells (though sometimes composed of a few related types) that work together to perform a specific function. An organ is an anatomically distinct structure of the body composed of two or more tissue types. Each organ performs one or more specific physiological functions. An organ system is a group of organs that work together to perform major functions or meet physiological needs of the body. This book covers eleven distinct organ systems in the human body ([Link] and [link]). Assigning organs to organ systems can be imprecise since organs that “belong” to one system can also have functions integral to another system. In fact, most organs contribute to more than one system. Organ Systems of the Human Body Integumentary System Skeletal System ¢ Encloses internal * Supports the body body structures ¢ Enables movement * Site of many (with muscular sensory receptors system) Muscular System Nervous System ¢ Enables movement * Detects and (with skeletal system) processes sensory ¢ Helps maintain body information temperature * Activates bodily responses Skeletal muscles Endocrine System Cardiovascular System Pituitary gland * Secretes hormones * Delivers oxygen . ¢ Regulates bodily and nutrients to Thyroid processes tissues gland * Equalizes temperature in the body Pancreas Adrenal glands Blood vessels Ovaries Organs that work together are grouped into organ systems. Organ Systems of the Human Body (continued) Lymphatic System ¢ Returns fluid to blood ¢ Defends against pathogens Digestive System ¢ Processes food for use by the body « Removes wastes from undigested food Stomach Liver Gall bladder Large intestine Small intestine Male Reproductive System ¢ Produces sex hormones and gametes * Delivers gametes to female Epididymis Respiratory System * Removes carbon dioxide from the body * Delivers oxygen to blood Nasal passage Trachea Lungs Urinary System * Controls water balance in the body « Removes wastes from blood and Kidneys excretes them Urinary bladder Female Reproductive System * Produces sex Mammary hormones glands and gametes * Supports embryo/ fetus until birth * Produces milk for : infant Ovaries Organs that work together are grouped into organ systems. The organism level is the highest level of organization. An organism is a living being that has a cellular structure and that can independently perform all physiologic functions necessary for life. In multicellular organisms, including humans, all cells, tissues, organs, and organ systems of the body work together to maintain the life and health of the organism. Chapter Review Life processes of the human body are maintained at several levels of structural organization. These include the chemical, cellular, tissue, organ, organ system, and the organism level. Higher levels of organization are built from lower levels. Therefore, molecules combine to form cells, cells combine to form tissues, tissues combine to form organs, organs combine to form organ systems, and organ systems combine to form organisms. Review Questions Exercise: Problem: The smallest independently functioning unit of an organism is a(n) a. cell b. molecule c. organ d. tissue Solution: A Exercise: Problem: A collection of similar tissues that performs a specific function is an a. organ b. organelle c. organism d. organ system Solution: A Exercise: Problem: The body system responsible for structural support and movement is the a. cardiovascular system b. endocrine system c. muscular system d. skeletal system Solution: D CRITICAL THINKING QUESTIONS Exercise: Problem:Name the six levels of organization of the human body. Solution: Chemical, cellular, tissue, organ, organ system, organism. Exercise: Problem: The female ovaries and the male testes are a part of which body system? Can these organs be members of more than one organ system? Why or why not? Solution: The female ovaries and the male testes are parts of the reproductive system. But they also secrete hormones, as does the endocrine system, therefore ovaries and testes function within both the endocrine and reproductive systems. Glossary cell smallest independently functioning unit of all organisms; in animals, a cell contains cytoplasm, composed of fluid and organelles organ functionally distinct structure composed of two or more types of tissues organ system group of organs that work together to carry out a particular function organism living being that has a cellular structure and that can independently perform all physiologic functions necessary for life tissue group of similar or closely related cells that act together to perform a specific function Anatomical Terminology By the end of this section, you will be able to: e Demonstrate the anatomical position e Describe the human body using directional and regional terms e Identify three planes most commonly used in the study of anatomy e Distinguish between the posterior (dorsal) and the anterior (ventral) body cavities, identifying their subdivisions and representative organs found in each e Describe serous membrane and explain its function Anatomists and health care providers use terminology that can be bewildering to the uninitiated. However, the purpose of this language is not to confuse, but rather to increase precision and reduce medical errors. For example, is a scar “above the wrist” located on the forearm two or three inches away from the hand? Or is it at the base of the hand? Is it on the palm-side or back-side? By using precise anatomical terminology, we eliminate ambiguity. Anatomical terms derive from ancient Greek and Latin words. Because these languages are no longer used in everyday conversation, the meaning of their words does not change. Anatomical terms are made up of roots, prefixes, and suffixes. The root of a term often refers to an organ, tissue, or condition, whereas the prefix or suffix often describes the root. For example, in the disorder hypertension, the prefix “hyper-” means “high” or “over,” and the root word “tension” refers to pressure, so the word “hypertension” refers to abnormally high blood pressure. Anatomical Position To further increase precision, anatomists standardize the way in which they view the body. Just as maps are normally oriented with north at the top, the standard body “map,” or anatomical position, is that of the body standing upright, with the feet at shoulder width and parallel, toes forward. The upper limbs are held out to each side, and the palms of the hands face forward as illustrated in [link]. Using this standard position reduces confusion. It does not matter how the body being described is oriented, the terms are used as if it is in anatomical position. For example, a scar in the “anterior (front) carpal (wrist) region” would be present on the palm side of the wrist. The term “anterior” would be used even if the hand were palm down on a table. Regions of the Human Body Frons or forehead (frontal). Oculus or eye (orbital or ocular) Cranium or skull (cranial) Bucca or cheek (buccal) Cephalon or head Shoulder (cephalic) (acromial) Facies or face (facial) Auris or ear (otic) Nasus or nose (nasal) Dorsum or back F Cervicis or neck Oris or mouth (oral) Cervicis or neck (cervical) Mentis or chin (dorsal) (mental) ; : Seaeun Axilla or armpit (thoracic) (brachial) (axillary) Mamma Olecranon . or breast or back bslhoral o (mammary) of elbow arm (brachial) ‘abdanner Trunk (olecranal) Antecubitis (abdominal) Lumbus or front of elbow or loin (antecubital) Umbilicus (lumbar) Upper or navel limb Antebrachium Sacrum (umbilical) or forearm (antebrachial) Carpus or wrist (carpal) Pollex Pelvis or thumb (pelvic) (manual) Palma or Inguen or groin Gluteus palm (palmar) ite inal) 9g sbatiock Digits (phalanges) (gluteal) ig or fingers (digital or phalangeal) aie or thigh Patella or (pubic) tone) ineecap Femur Poplit atellar) A opliteus or P ) . Naty back of knee Crus or (femoral) (popliteal) leg (crural) Sura or calf Tarsus (sural) or ankle (tarsal) Calcaneus or heel of foot Digits (phalanges) (calacaneal) or toes (digital or Pes or foot phalangeal) (pedal) Planta or sole ~ of foot (plantar) (a) Anterior view Antebrachium or forearm (antebrachial) (sacral) Manus or hand (b) Posterior view The human body is shown in anatomical position in an (a) anterior view and a (b) posterior view. The regions of the body are labeled in boldface. A body that is lying down is described as either prone or supine. Prone describes a face-down orientation, and supine describes a face up orientation. These terms are sometimes used in describing the position of the body during specific physical examinations or surgical procedures. Regional Terms The human body’s numerous regions have specific terms to help increase precision (see [link]). Notice that the term “brachium” or “arm” is reserved for the “upper arm” and “antebrachium” or “forearm” is used rather than “lower arm.” Similarly, “femur” or “thigh” is correct, and “leg” or “crus” is reserved for the portion of the lower limb between the knee and the ankle. You will be able to describe the body’s regions using the terms from the figure. Directional Terms Certain directional anatomical terms appear throughout this and any other anatomy textbook ({link]). These terms are essential for describing the relative locations of different body structures. For instance, an anatomist might describe one band of tissue as “inferior to” another or a physician might describe a tumor as “superficial to” a deeper body structure. Commit these terms to memory to avoid confusion when you are studying or describing the locations of particular body parts. e Anterior (or ventral) Describes the front or direction toward the front of the body. The toes are anterior to the foot. ¢ Posterior (or dorsal) Describes the back or direction toward the back of the body. The popliteus is posterior to the patella. e Superior (or cranial) describes a position above or higher than another part of the body proper. The orbits are superior to the oris. ¢ Inferior (or caudal) describes a position below or lower than another part of the body proper; near or toward the tail (in humans, the coccyx, or lowest part of the spinal column). The pelvis is inferior to the abdomen. e Lateral describes the side or direction toward the side of the body. The thumb (pollex) is lateral to the digits. e Medial describes the middle or direction toward the middle of the body. The hallux is the medial toe. ¢ Proximal describes a position in a limb that is nearer to the point of attachment or the trunk of the body. The brachium is proximal to the antebrachium. ¢ Distal describes a position in a limb that is farther from the point of attachment or the trunk of the body. The crus is distal to the femur. e Superficial describes a position closer to the surface of the body. The skin is superficial to the bones. ¢ Deep describes a position farther from the surface of the body. The brain is deep to the skull. Directional Terms Applied to the Human Body Superior Cranial -> Anterior or ventral Posterior +-7---- or dorsal Caudal Inferior Paired directional terms are shown as applied to the human body. Body Planes A section is a two-dimensional surface of a three-dimensional structure that has been cut. Modern medical imaging devices enable clinicians to obtain “virtual sections” of living bodies. We call these scans. Body sections and scans can be correctly interpreted, however, only if the viewer understands the plane along which the section was made. A plane is an imaginary two- dimensional surface that passes through the body. There are three planes commonly referred to in anatomy and medicine, as illustrated in [link]. e The sagittal plane is the plane that divides the body or an organ vertically into right and left sides. If this vertical plane runs directly down the middle of the body, it is called the midsagittal or median plane. If it divides the body into unequal right and left sides, it is called a parasagittal plane or less commonly a longitudinal section. e The frontal plane is the plane that divides the body or an organ into an anterior (front) portion and a posterior (rear) portion. The frontal plane is often referred to as a coronal plane. (“Corona” is Latin for “crown.”) e The transverse plane is the plane that divides the body or organ horizontally into upper and lower portions. Transverse planes produce images referred to as cross sections. Planes of the Body Frontal (coronal) plane Transverse The three planes most commonly used in anatomical and medical imaging are the sagittal, frontal (or coronal), and transverse plane. Body Cavities and Serous Membranes The body maintains its internal organization by means of membranes, sheaths, and other structures that separate compartments. The dorsal (posterior) cavity and the ventral (anterior) cavity are the largest body compartments ({link]). These cavities contain and protect delicate internal organs, and the ventral cavity allows for significant changes in the size and shape of the organs as they perform their functions. The lungs, heart, stomach, and intestines, for example, can expand and contract without distorting other tissues or disrupting the activity of nearby organs. Dorsal and Ventral Body Cavities Cranial cavity Vertebral cavity Thoracic cavity: Superior mediastinum Pleural cavity Pericardial cavity within the mediastinum Diaphragm Vertebral cavity Ventral body cavity (both thoracic and i i — 7 abdominopelvic Abdominal cavity Abdomino- | cavities) pelvic cavity Pelvic cavity Lateral view Anterior view The ventral cavity includes the thoracic and abdominopelvic cavities and their subdivisions. The dorsal cavity includes the cranial and spinal cavities. Subdivisions of the Posterior (Dorsal) and Anterior (Ventral) Cavities The posterior (dorsal) and anterior (ventral) cavities are each subdivided into smaller cavities. In the posterior (dorsal) cavity, the cranial cavity houses the brain, and the spinal cavity (or vertebral cavity) encloses the spinal cord. Just as the brain and spinal cord make up a continuous, uninterrupted structure, the cranial and spinal cavities that house them are also continuous. The brain and spinal cord are protected by the bones of the skull and vertebral column and by cerebrospinal fluid, a colorless fluid produced by the brain, which cushions the brain and spinal cord within the posterior (dorsal) cavity. The anterior (ventral) cavity has two main subdivisions: the thoracic cavity and the abdominopelvic cavity (see [link]). The thoracic cavity is the more superior subdivision of the anterior cavity, and it is enclosed by the rib cage. The thoracic cavity contains the lungs and the heart, which is located in the mediastinum. The diaphragm forms the floor of the thoracic cavity and separates it from the more inferior abdominopelvic cavity. The abdominopelvic cavity is the largest cavity in the body. Although no membrane physically divides the abdominopelvic cavity, it can be useful to distinguish between the abdominal cavity, the division that houses the digestive organs, and the pelvic cavity, the division that houses the organs of reproduction. Abdominal Regions and Quadrants To promote clear communication, for instance about the location of a patient’s abdominal pain or a suspicious mass, health care providers typically divide up the cavity into either nine regions or four quadrants ({link]). Regions and Quadrants of the Peritoneal Cavity (a) Abdominopelvic regions (b) Abdominopelvic quandrants There are (a) nine abdominal regions and (b) four abdominal quadrants in the peritoneal cavity. The more detailed regional approach subdivides the cavity with one horizontal line immediately inferior to the ribs and one immediately superior to the pelvis, and two vertical lines drawn as if dropped from the midpoint of each clavicle (collarbone). There are nine resulting regions. The simpler quadrants approach, which is more commonly used in medicine, subdivides the cavity with one horizontal and one vertical line that intersect at the patient’s umbilicus (navel). Membranes of the Anterior (Ventral) Body Cavity A serous membrane (also referred to a serosa) is one of the thin membranes that cover the walls and organs in the thoracic and abdominopelvic cavities. The parietal layers of the membranes line the walls of the body cavity (pariet- refers to a cavity wall). The visceral layer of the membrane covers the organs (the viscera). Between the parietal and visceral layers is a very thin, fluid-filled serous space, or cavity ([link]). Serous Membrane Visceral pericardium Pericardial cavity Parietal pericardium \/— Air space Balloon Serous membrane lines the pericardial cavity and reflects back to cover the heart—much the same way that an underinflated balloon would form two layers surrounding a fist. There are three serous cavities and their associated membranes. The pleura is the serous membrane that encloses the pleural cavity; the pleural cavity surrounds the lungs. The pericardium is the serous membrane that encloses the pericardial cavity; the pericardial cavity surrounds the heart. The peritoneum is the serous membrane that encloses the peritoneal cavity; the peritoneal cavity surrounds several organs in the abdominopelvic cavity. The serous membranes form fluid-filled sacs, or cavities, that are meant to cushion and reduce friction on internal organs when they move, such as when the lungs inflate or the heart beats. Both the parietal and visceral serosa secrete the thin, slippery serous fluid located within the serous cavities. The pleural cavity reduces friction between the lungs and the body wall. Likewise, the pericardial cavity reduces friction between the heart and the wall of the pericardium. The peritoneal cavity reduces friction between the abdominal and pelvic organs and the body wall. Therefore, serous membranes provide additional protection to the viscera they enclose by reducing friction that could lead to inflammation of the organs. Chapter Review Ancient Greek and Latin words are used to build anatomical terms. A standard reference position for mapping the body’s structures is the normal anatomical position. Regions of the body are identified using terms such as “occipital” that are more precise than common words and phrases such as “the back of the head.” Directional terms such as anterior and posterior are essential for accurately describing the relative locations of body structures. Images of the body’s interior commonly align along one of three planes: the sagittal, frontal, or transverse. The body’s organs are organized in one of two main cavities—dorsal (also referred to posterior) and ventral (also referred to anterior)—which are further sub-divided according to the structures present in each area. The serous membranes have two layers— parietal and visceral—surrounding a fluid filled space. Serous membranes cover the lungs (pleural serosa), heart (pericardial serosa), and some abdominopelvic organs (peritoneal serosa). Review Chapter Exercise: Problem: What is the position of the body when it is in the “normal anatomical position?” a. The person is prone with upper limbs, including palms, touching sides and lower limbs touching at sides. b. The person is standing facing the observer, with upper limbs extended out at a ninety-degree angle from the torso and lower limbs in a wide stance with feet pointing laterally c. The person is supine with upper limbs, including palms, touching sides and lower limbs touching at sides. d. None of the above Solution: D Exercise: Problem: To make a banana split, you halve a banana into two long, thin, right and left sides along the a. coronal plane b. longitudinal plane c. midsagittal plane d. transverse plane Solution: C Exercise: Problem: The lumbar region is a. inferior to the gluteal region b. inferior to the umbilical region c. superior to the cervical region d. superior to the popliteal region Solution: D Exercise: Problem: The heart is within the a. Cranial cavity b. mediastinum c. posterior (dorsal) cavity d. All of the above Solution: B Critical Thinking Question Exercise: Problem: In which direction would an MRI scanner move to produce sequential images of the body in the frontal plane, and in which direction would an MRI scanner move to produce sequential images of the body in the sagittal plane? Solution: If the body were supine or prone, the MRI scanner would move from top to bottom to produce frontal sections, which would divide the body into anterior and posterior portions, as in “cutting” a deck of cards. Again, if the body were supine or prone, to produce sagittal sections, the scanner would move from left to right or from right to left to divide the body lengthwise into left and right portions. Exercise: Problem: If a bullet were to penetrate a lung, which three anterior thoracic body cavities would it enter, and which layer of the serous membrane would it encounter first? Solution: The bullet would enter the ventral, thoracic, and pleural cavities, and it would encounter the parietal layer of serous membrane first. Glossary abdominopelvic cavity division of the anterior (ventral) cavity that houses the abdominal and pelvic viscera anatomical position standard reference position used for describing locations and directions on the human body anterior describes the front or direction toward the front of the body; also referred to as ventral anterior cavity larger body cavity located anterior to the posterior (dorsal) body cavity; includes the serous membrane-lined pleural cavities for the lungs, pericardial cavity for the heart, and peritoneal cavity for the abdominal and pelvic organs; also referred to as ventral cavity caudal describes a position below or lower than another part of the body proper; near or toward the tail (in humans, the coccyx, or lowest part of the spinal column); also referred to as inferior cranial describes a position above or higher than another part of the body proper; also referred to as superior cranial cavity division of the posterior (dorsal) cavity that houses the brain deep describes a position farther from the surface of the body distal describes a position farther from the point of attachment or the trunk of the body dorsal describes the back or direction toward the back of the body; also referred to as posterior dorsal cavity posterior body cavity that houses the brain and spinal cord; also referred to the posterior body cavity frontal plane two-dimensional, vertical plane that divides the body or organ into anterior and posterior portions inferior describes a position below or lower than another part of the body proper; near or toward the tail (in humans, the coccyx, or lowest part of the spinal column); also referred to as caudal lateral describes the side or direction toward the side of the body medial describes the middle or direction toward the middle of the body pericardium sac that encloses the heart peritoneum serous membrane that lines the abdominopelvic cavity and covers the organs found there plane imaginary two-dimensional surface that passes through the body pleura serous membrane that lines the pleural cavity and covers the lungs posterior describes the back or direction toward the back of the body; also referred to as dorsal posterior cavity posterior body cavity that houses the brain and spinal cord; also referred to as dorsal cavity prone face down proximal describes a position nearer to the point of attachment or the trunk of the body sagittal plane two-dimensional, vertical plane that divides the body or organ into right and left sides section in anatomy, a single flat surface of a three-dimensional structure that has been cut through serous Membrane membrane that covers organs and reduces friction; also referred to as serosa serosa membrane that covers organs and reduces friction; also referred to as serous Membrane spinal cavity division of the dorsal cavity that houses the spinal cord; also referred to as vertebral cavity superficial describes a position nearer to the surface of the body superior describes a position above or higher than another part of the body proper; also referred to as cranial supine face up thoracic cavity division of the anterior (ventral) cavity that houses the heart, lungs, esophagus, and trachea transverse plane two-dimensional, horizontal plane that divides the body or organ into superior and inferior portions ventral describes the front or direction toward the front of the body; also referred to as anterior ventral cavity larger body cavity located anterior to the posterior (dorsal) body cavity; includes the serous membrane-lined pleural cavities for the lungs, pericardial cavity for the heart, and peritoneal cavity for the abdominal and pelvic organs; also referred to as anterior body cavity Medical Imaging By the end of this section, you will be able to: e Discuss the uses and drawbacks of X-ray imaging e Identify four modern medical imaging techniques and how they are used For thousands of years, fear of the dead and legal sanctions limited the ability of anatomists and physicians to study the internal structures of the human body. An inability to control bleeding, infection, and pain made surgeries infrequent, and those that were performed—such as wound suturing, amputations, tooth and tumor removals, skull drilling, and cesarean births—did not greatly advance knowledge about internal anatomy. Theories about the function of the body and about disease were therefore largely based on external observations and imagination. During the fourteenth and fifteenth centuries, however, the detailed anatomical drawings of Italian artist and anatomist Leonardo da Vinci and Flemish anatomist Andreas Vesalius were published, and interest in human anatomy began to increase. Medical schools began to teach anatomy using human dissection; although some resorted to grave robbing to obtain corpses. Laws were eventually passed that enabled students to dissect the corpses of criminals and those who donated their bodies for research. Still, it was not until the late nineteenth century that medical researchers discovered non- surgical methods to look inside the living body. X-Rays German physicist Wilhelm R6éntgen (1845-1923) was experimenting with electrical current when he discovered that a mysterious and invisible “ray” would pass through his flesh but leave an outline of his bones on a screen coated with a metal compound. In 1895, R6ntgen made the first durable record of the internal parts of a living human: an “X-ray” image (as it came to be called) of his wife’s hand. Scientists around the world quickly began their own experiments with X-rays, and by 1900, X-rays were widely used to detect a variety of injuries and diseases. In 1901, R6ntgen was awarded the first Nobel Prize for physics for his work in this field. The X-ray is a form of high energy electromagnetic radiation with a short wavelength capable of penetrating solids and ionizing gases. As they are used in medicine, X-rays are emitted from an X-ray machine and directed toward a specially treated metallic plate placed behind the patient’s body. The beam of radiation results in darkening of the X-ray plate. X-rays are slightly impeded by soft tissues, which show up as gray on the X-ray plate, whereas hard tissues, such as bone, largely block the rays, producing a light-toned “shadow.” Thus, X-rays are best used to visualize hard body structures such as teeth and bones ({link]). Like many forms of high energy radiation, however, X-rays are capable of damaging cells and initiating changes that can lead to cancer. This danger of excessive exposure to X- rays was not fully appreciated for many years after their widespread use. X-Ray of a Hand High energy electromagnetic radiation allows the internal structures of the body, such as bones, to be seen in X- rays like these. (credit: Trace Meek/flickr) Refinements and enhancements of X-ray techniques have continued throughout the twentieth and twenty-first centuries. Although often supplanted by more sophisticated imaging techniques, the X-ray remains a “workhorse” in medical imaging, especially for viewing fractures and for dentistry. The disadvantage of irradiation to the patient and the operator is now attenuated by proper shielding and by limiting exposure. Modern Medical Imaging X-rays can depict a two-dimensional image of a body region, and only from a single angle. In contrast, more recent medical imaging technologies produce data that is integrated and analyzed by computers to produce three- dimensional images or images that reveal aspects of body functioning. Computed Tomography Tomography refers to imaging by sections. Computed tomography (CT) is a noninvasive imaging technique that uses computers to analyze several cross-sectional X-rays in order to reveal minute details about structures in the body ({link]a). The technique was invented in the 1970s and is based on the principle that, as X-rays pass through the body, they are absorbed or reflected at different levels. In the technique, a patient lies on a motorized platform while a computerized axial tomography (CAT) scanner rotates 360 degrees around the patient, taking X-ray images. A computer combines these images into a two-dimensional view of the scanned area, or “slice.” Medical Imaging Techniques (a) The results of a CT scan of the head are shown as successive transverse sections. (b) An MRI machine generates a magnetic field around a patient. (c) PET scans use radiopharmaceuticals to create images of active blood flow and physiologic activity of the organ or organs being targeted. (d) Ultrasound technology is used to monitor pregnancies because it is the least invasive of imaging techniques and uses no electromagnetic radiation. (credit a: Akira Ohgaki/flickr; credit b: “Digital Cate”/flickr; credit c: “Raziel”/Wikimedia Commons; credit d: “Tsis”/Wikimedia Commons) Since 1970, the development of more powerful computers and more sophisticated software has made CT scanning routine for many types of diagnostic evaluations. It is especially useful for soft tissue scanning, such as of the brain and the thoracic and abdominal viscera. Its level of detail is so precise that it can allow physicians to measure the size of a mass down to a millimeter. The main disadvantage of CT scanning is that it exposes patients to a dose of radiation many times higher than that of X-rays. In fact, children who undergo CT scans are at increased risk of developing cancer, as are adults who have multiple CT scans. Note: wae — : mess OPenstax COLLEGE A CT or CAT scan relies on a circling scanner that revolves around the patient’s body. Watch this video to learn more about CT and CAT scans. What type of radiation does a CT scanner use? Magnetic Resonance Imaging Magnetic resonance imaging (MRI) is a noninvasive medical imaging technique based on a phenomenon of nuclear physics discovered in the 1930s, in which matter exposed to magnetic fields and radio waves was found to emit radio signals. In 1970, a physician and researcher named Raymond Damadian noticed that malignant (cancerous) tissue gave off different signals than normal body tissue. He applied for a patent for the first MRI scanning device, which was in use clinically by the early 1980s. The early MRI scanners were crude, but advances in digital computing and electronics led to their advancement over any other technique for precise imaging, especially to discover tumors. MRI also has the major advantage of not exposing patients to radiation. Drawbacks of MRI scans include their much higher cost, and patient discomfort with the procedure. The MRI scanner subjects the patient to such powerful electromagnets that the scan room must be shielded. The patient must be enclosed in a metal tube-like device for the duration of the scan (see [link]b), sometimes as long as thirty minutes, which can be uncomfortable and impractical for ill patients. The device is also so noisy that, even with earplugs, patients can become anxious or even fearful. These problems have been overcome somewhat with the development of “open” MRI scanning, which does not require the patient to be entirely enclosed in the metal tube. Patients with iron-containing metallic implants (internal sutures, some prosthetic devices, and so on) cannot undergo MRI scanning because it can dislodge these implants. Functional MRIs ({MRIs), which detect the concentration of blood flow in certain parts of the body, are increasingly being used to study the activity in parts of the brain during various body activities. This has helped scientists learn more about the locations of different brain functions and more about brain abnormalities and diseases. Note: zis A patient undergoing an MRI is surrounded by a tube-shaped scanner. Watch this video to learn more about MRIs. What is the function of magnets in an MRI? Positron Emission Tomography Positron emission tomography (PET) is a medical imaging technique involving the use of so-called radiopharmaceuticals, substances that emit radiation that is short-lived and therefore relatively safe to administer to the body. Although the first PET scanner was introduced in 1961, it took 15 more years before radiopharmaceuticals were combined with the technique and revolutionized its potential. The main advantage is that PET (see [link]c) can illustrate physiologic activity—including nutrient metabolism and blood flow—of the organ or organs being targeted, whereas CT and MRI scans can only show static images. PET is widely used to diagnose a multitude of conditions, such as heart disease, the spread of cancer, certain forms of infection, brain abnormalities, bone disease, and thyroid disease. PET relies on radioactive substances administered several minutes before the scan. Watch this video to learn more about PET. How is PET used in chemotherapy? Ultrasonography Ultrasonography is an imaging technique that uses the transmission of high-frequency sound waves into the body to generate an echo signal that is converted by a computer into a real-time image of anatomy and physiology (see [link]d). Ultrasonography is the least invasive of all imaging techniques, and it is therefore used more freely in sensitive situations such as pregnancy. The technology was first developed in the 1940s and 1950s. Ultrasonography is used to study heart function, blood flow in the neck or extremities, certain conditions such as gallbladder disease, and fetal growth and development. The main disadvantages of ultrasonography are that the image quality is heavily operator-dependent and that it is unable to penetrate bone and gas. Chapter Review Detailed anatomical drawings of the human body first became available in the fifteenth and sixteenth centuries; however, it was not until the end of the nineteenth century, and the discovery of X-rays, that anatomists and physicians discovered non-surgical methods to look inside a living body. Since then, many other techniques, including CT scans, MRI scans, PET scans, and ultrasonography, have been developed, providing more accurate and detailed views of the form and function of the human body. Interactive Link Questions Exercise: Problem: A CT or CAT scan relies on a circling scanner that revolves around the patient’s body. Watch this video to learn more about CT and CAT scans. What type of radiation does a CT scanner use? Solution: X-rays. Exercise: Problem: A patient undergoing an MRI is surrounded by a tube-shaped scanner. Watch this video to learn more about MRIs. What is the function of magnets in an MRI? Solution: The magnets induce tissue to emit radio signals that can show differences between different types of tissue. Exercise: Problem: PET relies on radioactive substances administered several minutes before the scan. Watch this video to learn more about PET. How is PET used in chemotherapy? Solution: PET scans can indicate how patients are responding to chemotherapy. Review Questions Exercise: Problem: In 1901, Wilhelm Réntgen was the first person to win the Nobel Prize for physics. For what discovery did he win? a. nuclear physics b. radiopharmaceuticals c. the link between radiation and cancer d. X-rays Solution: D Exercise: Problem: Which of the following imaging techniques would be best to use to study the uptake of nutrients by rapidly multiplying cancer cells? a. CT b. MRI c. PET d. ultrasonography Solution: C Exercise: Problem: Which of the following imaging studies can be used most safely during pregnancy? a. CT scans b. PET scans c. ultrasounds d. X-rays Solution: C Exercise: Problem: What are two major disadvantages of MRI scans? a. release of radiation and poor quality images b. high cost and the need for shielding from the magnetic signals c. can only view metabolically active tissues and inadequate availability of equipment d. release of radiation and the need for a patient to be confined to metal tube for up to 30 minutes Solution: Critical Thinking Questions Exercise: Problem: Which medical imaging technique is most dangerous to use repeatedly, and why? Solution: CT scanning subjects patients to much higher levels of radiation than X-rays, and should not be performed repeatedly. Exercise: Problem: Explain why ultrasound imaging is the technique of choice for studying fetal growth and development. Solution: Ultrasonography does not expose a mother or fetus to radiation, to radiopharmaceuticals, or to magnetic fields. At this time, there are no known medical risks of ultrasonography. Glossary computed tomography (CT) medical imaging technique in which a computer-enhanced cross- sectional X-ray image is obtained magnetic resonance imaging (MRI) medical imaging technique in which a device generates a magnetic field to obtain detailed sectional images of the internal structures of the body positron emission tomography (PET) medical imaging technique in which radiopharmaceuticals are traced to reveal metabolic and physiological functions in tissues ultrasonography application of ultrasonic waves to visualize subcutaneous body structures such as tendons and organs X-ray form of high energy electromagnetic radiation with a short wavelength capable of penetrating solids and ionizing gases; used in medicine as a diagnostic aid to visualize body structures such as bones Types of Tissues By the end of this section, you will be able to: e Identify the four main tissue types e Discuss the functions of each tissue type e Relate the structure of each tissue type to their function e Discuss the embryonic origin of tissue e Identify the three major germ layers e Identify the main types of tissue membranes The term tissue is used to describe a group of cells found together in the body. The cells within a tissue share a common embryonic origin. Microscopic observation reveals that the cells in a tissue share morphological features and are arranged in an orderly pattern that achieves the tissue’s functions. From the evolutionary perspective, tissues appear in more complex organisms. For example, multicellular protists, ancient eukaryotes, do not have cells organized into tissues. Although there are many types of cells in the human body, they are organized into four broad categories of tissues: epithelial, connective, muscle, and nervous. Each of these categories is characterized by specific functions that contribute to the overall health and maintenance of the body. A disruption of the structure is a sign of injury or disease. Such changes can be detected through histology, the microscopic study of tissue appearance, organization, and function. The Four Types of Tissues Epithelial tissue, also referred to as epithelium, refers to the sheets of cells that cover exterior surfaces of the body, lines internal cavities and passageways, and forms certain glands. Connective tissue, as its name implies, binds the cells and organs of the body together and functions in the protection, support, and integration of all parts of the body. Muscle tissue is excitable, responding to stimulation and contracting to provide movement, and occurs as three major types: skeletal (voluntary) muscle, smooth muscle, and cardiac muscle in the heart. Nervous tissue is also excitable, allowing the propagation of electrochemical signals in the form of nerve impulses that communicate between different regions of the body ([link]). The next level of organization is the organ, where several types of tissues come together to form a working unit. Just as knowing the structure and function of cells helps you in your study of tissues, knowledge of tissues will help you understand how organs function. The epithelial and connective tissues are discussed in detail in this chapter. Muscle and nervous tissues will be discussed only briefly in this chapter. Four Types of Tissue: Body Nervous tissue Brain Spinal cord Nerves Muscle tissue Cardiac muscle Smooth muscle Skeletal muscle Epithelial tissue Lining of Gl tract organs and other hollow organs Skin surface (epidermis) Connective tissue Fat and other soft padding tissue Bone Tendon The four types of tissues are exemplified in nervous tissue, stratified squamous epithelial tissue, cardiac muscle tissue, and connective tissue in small intestine. Clockwise from nervous tissue, LM x 872, LM x 282, LM x 460, LM x 800. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) Embryonic Origin of Tissues The zygote, or fertilized egg, is a single cell formed by the fusion of an egg and sperm. After fertilization the zygote gives rise to rapid mitotic cycles, generating many cells to form the embryo. The first embryonic cells generated have the ability to differentiate into any type of cell in the body and, as such, are called totipotent, meaning each has the capacity to divide, differentiate, and develop into a new organism. As cell proliferation progresses, three major cell lineages are established within the embryo. As explained in a later chapter, each of these lineages of embryonic cells forms the distinct germ layers from which all the tissues and organs of the human body eventually form. Each germ layer is identified by its relative position: ectoderm (ecto- = “outer”), mesoderm (meso- = “middle”), and endoderm (endo- = “inner’’). [link] shows the types of tissues and organs associated with the each of the three germ layers. Note that epithelial tissue originates in all three layers, whereas nervous tissue derives primarily from the ectoderm and muscle tissue from mesoderm. Embryonic Origin of Tissues and Major Organs Ectoderm Epidermis, glands on skin, some cranial bones, pituitary and adrenal medulla, the nervous system, the mouth between cheek and gums, the anus = ue, ANZ 1Q( | a = is S P= y =. ] : y/ | . = Nos / Skin cells Neurons Pigment cell Mesoderm Connective tissues proper, bone, cartilage, blood, endothelium of blood vessels, muscle, synovial membranes, serous membranes lining body cavities, kidneys, lining of gonads _——— —— = — = — be _<<ff = = Cardiac Skeletal Tubule cell Red blood Smooth muscle muscle of kidney cells muscle Endoderm Lining of airways and digestive system except the mouth and distal part of digestive system (rectum and anal canal); glands (digestive glands, endocrine glands, adrenal cortex) Lung cell Thyroid cell Pancreatic cell = = : mmm §~OPENStax COLLEGE View this slideshow to learn more about stem cells. How do somatic stem cells differ from embryonic stem cells? Tissue Membranes A tissue membrane is a thin layer or sheet of cells that covers the outside of the body (for example, skin), the organs (for example, pericardium), internal passageways that lead to the exterior of the body (for example, abdominal mesenteries), and the lining of the moveable joint cavities. There are two basic types of tissue membranes: connective tissue and epithelial membranes ((link]). Tissue Membranes Mucous membranes line the digestive, respiratory, urinary, and reproductive tracts. They are coated with the secretions of mucous glands. Serous membranes line body cavities closed to the exterior of the body: the peritoneal, pleural, and pericardial cavities. Cutaneous membrane, or the skin, covers the body surface. Synovial membranes line joint cavities and produce the fluid within the joint. The two broad categories of tissue membranes in the body are (1) connective tissue membranes, which include synovial membranes, and (2) epithelial membranes, which include mucous membranes, serous membranes, and the cutaneous membrane, in other words, the skin. Connective Tissue Membranes The connective tissue membrane is formed solely from connective tissue. These membranes encapsulate organs, such as the kidneys, and line our movable joints. A synovial membrane is a type of connective tissue membrane that lines the cavity of a freely movable joint. For example, synovial membranes surround the joints of the shoulder, elbow, and knee. Fibroblasts in the inner layer of the synovial membrane release hyaluronan into the joint cavity. The hyaluronan effectively traps available water to form the synovial fluid, a natural lubricant that enables the bones of a joint to move freely against one another without much friction. This synovial fluid readily exchanges water and nutrients with blood, as do all body fluids. Epithelial Membranes The epithelial membrane is composed of epithelium attached to a layer of connective tissue, for example, your skin. The mucous membrane is also a composite of connective and epithelial tissues. Sometimes called mucosae, these epithelial membranes line the body cavities and hollow passageways that open to the external environment, and include the digestive, respiratory, excretory, and reproductive tracts. Mucous, produced by the epithelial exocrine glands, covers the epithelial layer. The underlying connective tissue, called the lamina propria (literally “own layer”), help support the fragile epithelial layer. A serous membrane is an epithelial membrane composed of mesodermally derived epithelium called the mesothelium that is supported by connective tissue. These membranes line the coelomic cavities of the body, that is, those cavities that do not open to the outside, and they cover the organs located within those cavities. They are essentially membranous bags, with mesothelium lining the inside and connective tissue on the outside. Serous fluid secreted by the cells of the thin squamous mesothelium lubricates the membrane and reduces abrasion and friction between organs. Serous membranes are identified according locations. Three serous membranes line the thoracic cavity; the two pleura that cover the lungs and the pericardium that covers the heart. A fourth, the peritoneum, is the serous membrane in the abdominal cavity that covers abdominal organs and forms double sheets of mesenteries that suspend many of the digestive organs. The skin is an epithelial membrane also called the cutaneous membrane. It is a stratified squamous epithelial membrane resting on top of connective tissue. The apical surface of this membrane is exposed to the external environment and is covered with dead, keratinized cells that help protect the body from desiccation and pathogens. Chapter Review The human body contains more than 200 types of cells that can all be classified into four types of tissues: epithelial, connective, muscle, and nervous. Epithelial tissues act as coverings controlling the movement of materials across the surface. Connective tissue integrates the various parts of the body and provides support and protection to organs. Muscle tissue allows the body to move. Nervous tissues propagate information. The study of the shape and arrangement of cells in tissue is called histology. All cells and tissues in the body derive from three germ layers in the embryo: the ectoderm, mesoderm, and endoderm. Different types of tissues form membranes that enclose organs, provide a friction-free interaction between organs, and keep organs together. Synovial membranes are connective tissue membranes that protect and line the joints. Epithelial membranes are formed from epithelial tissue attached to a layer of connective tissue. There are three types of epithelial membranes: mucous, which contain glands; serous, which secrete fluid; and cutaneous which makes up the skin. Interactive Link Questions Exercise: Problem: View this slideshow to learn more about stem cells. How do somatic stem cells differ from embryonic stem cells? Solution: Most somatic stem cells give rise to only a few cell types. Review Questions Exercise: Problem: Which of the following is not a type of tissue? a. muscle b. nervous c. embryonic d. epithelial Solution: c Exercise: Problem: The process by which a less specialized cell matures into a more specialized cell is called a. differentiation b. maturation c. modification d. specialization Solution: A Exercise: Problem: Differentiated cells in a developing embryo derive from a. endothelium, mesothelium, and epithelium b. ectoderm, mesoderm, and endoderm c. connective tissue, epithelial tissue, and muscle tissue d. epidermis, mesoderm, and endothelium Solution: B Exercise: Problem: Which of the following lines the body cavities exposed to the external environment? a. mesothelium b. lamina propria c. mesenteries d. mucosa Solution: D Critical Thinking Questions Exercise: Problem: Identify the four types of tissue in the body, and describe the major functions of each tissue. Solution: The four types of tissue in the body are epithelial, connective, muscle, and nervous. Epithelial tissue is made of layers of cells that cover the surfaces of the body that come into contact with the exterior world, line internal cavities, and form glands. Connective tissue binds the cells and organs of the body together and performs many functions, especially in the protection, support, and integration of the body. Muscle tissue, which responds to stimulation and contracts to provide movement, is divided into three major types: skeletal (voluntary) muscles, smooth muscles, and the cardiac muscle in the heart. Nervous tissue allows the body to receive signals and transmit information as electric impulses from one region of the body to another. Exercise: Problem: The zygote is described as totipotent because it ultimately gives rise to all the cells in your body including the highly specialized cells of your nervous system. Describe this transition, discussing the steps and processes that lead to these specialized cells. Solution: The zygote divides into many cells. As these cells become specialized, they lose their ability to differentiate into all tissues. At first they form the three primary germ layers. Following the cells of the ectodermal germ layer, they too become more restricted in what they can form. Ultimately, some of these ectodermal cells become further restricted and differentiate in to nerve cells. Exercise: Problem: What is the function of synovial membranes? Solution: Synovial membranes are a type of connective tissue membrane that supports mobility in joints. The membrane lines the joint cavity and contains fibroblasts that produce hyaluronan, which leads to the production of synovial fluid, a natural lubricant that enables the bones of a joint to move freely against one another. Glossary connective tissue type of tissue that serves to hold in place, connect, and integrate the body’s organs and systems connective tissue membrane connective tissue that encapsulates organs and lines movable joints cutaneous membrane skin; epithelial tissue made up of a stratified squamous epithelial cells that cover the outside of the body ectoderm outermost embryonic germ layer from which the epidermis and the nervous tissue derive endoderm innermost embryonic germ layer from which most of the digestive system and lower respiratory system derive epithelial membrane epithelium attached to a layer of connective tissue epithelial tissue type of tissue that serves primarily as a covering or lining of body parts, protecting the body; it also functions in absorption, transport, and secretion histology microscopic study of tissue architecture, organization, and function lamina propria areolar connective tissue underlying a mucous membrane mesoderm middle embryonic germ layer from which connective tissue, muscle tissue, and some epithelial tissue derive mucous membrane tissue membrane that is covered by protective mucous and lines tissue exposed to the outside environment muscle tissue type of tissue that is capable of contracting and generating tension in response to stimulation; produces movement. nervous tissue type of tissue that is capable of sending and receiving impulses through electrochemical signals. serous membrane type of tissue membrane that lines body cavities and lubricates them with serous fluid synovial membrane connective tissue membrane that lines the cavities of freely movable joints, producing synovial fluid for lubrication tissue group of cells that are similar in form and perform related functions tissue membrane thin layer or sheet of cells that covers the outside of the body, organs, and internal cavities totipotent embryonic cells that have the ability to differentiate into any type of cell and organ in the body Epithelial Tissue By the end of this section, you will be able to: e Explain the structure and function of epithelial tissue e Distinguish between tight junctions, anchoring junctions, and gap junctions e Distinguish between simple epithelia and stratified epithelia, as well as between squamous, cuboidal, and columnar epithelia e Describe the structure and function of endocrine and exocrine glands and their respective secretions Most epithelial tissues are essentially large sheets of cells covering all the surfaces of the body exposed to the outside world and lining the outside of organs. Epithelium also forms much of the glandular tissue of the body. Skin is not the only area of the body exposed to the outside. Other areas include the airways, the digestive tract, as well as the urinary and reproductive systems, all of which are lined by an epithelium. Hollow organs and body cavities that do not connect to the exterior of the body, which includes, blood vessels and serous membranes, are lined by endothelium (plural = endothelia), which is a type of epithelium. Epithelial cells derive from all three major embryonic layers. The epithelia lining the skin, parts of the mouth and nose, and the anus develop from the ectoderm. Cells lining the airways and most of the digestive system originate in the endoderm. The epithelium that lines vessels in the lymphatic and cardiovascular system derives from the mesoderm and is called an endothelium. All epithelia share some important structural and functional features. This tissue is highly cellular, with little or no extracellular material present between cells. Adjoining cells form a specialized intercellular connection between their cell membranes called a cell junction. The epithelial cells exhibit polarity with differences in structure and function between the exposed or apical facing surface of the cell and the basal surface close to the underlying body structures. The basal lamina, a mixture of glycoproteins and collagen, provides an attachment site for the epithelium, separating it from underlying connective tissue. The basal lamina attaches to a reticular lamina, which is secreted by the underlying connective tissue, forming a basement membrane that helps hold it all together. Epithelial tissues are nearly completely avascular. For instance, no blood vessels cross the basement membrane to enter the tissue, and nutrients must come by diffusion or absorption from underlying tissues or the surface. Many epithelial tissues are capable of rapidly replacing damaged and dead cells. Sloughing off of damaged or dead cells is a characteristic of surface epithelium and allows our airways and digestive tracts to rapidly replace damaged cells with new cells. Generalized Functions of Epithelial Tissue Epithelial tissues provide the body’s first line of protection from physical, chemical, and biological wear and tear. The cells of an epithelium act as gatekeepers of the body controlling permeability and allowing selective transfer of materials across a physical barrier. All substances that enter the body must cross an epithelium. Some epithelia often include structural features that allow the selective transport of molecules and ions across their cell membranes. Many epithelial cells are capable of secretion and release mucous and specific chemical compounds onto their apical surfaces. The epithelium of the small intestine releases digestive enzymes, for example. Cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles. A glandular epithelium contains many secretory cells. The Epithelial Cell Epithelial cells are typically characterized by the polarized distribution of organelles and membrane-bound proteins between their basal and apical surfaces. Particular structures found in some epithelial cells are an adaptation to specific functions. Certain organelles are segregated to the basal sides, whereas other organelles and extensions, such as cilia, when present, are on the apical surface. Cilia are microscopic extensions of the apical cell membrane that are supported by microtubules. They beat in unison and move fluids as well as trapped particles. Ciliated epithelium lines the ventricles of the brain where it helps circulate the cerebrospinal fluid. The ciliated epithelium of your airway forms a mucociliary escalator that sweeps particles of dust and pathogens trapped in the secreted mucous toward the throat. It is called an escalator because it continuously pushes mucous with trapped particles upward. In contrast, nasal cilia sweep the mucous blanket down towards your throat. In both cases, the transported materials are usually swallowed, and end up in the acidic environment of your stomach. Cell to Cell Junctions Cells of epithelia are closely connected and are not separated by intracellular material. Three basic types of connections allow varying degrees of interaction between the cells: tight junctions, anchoring junctions, and gap junctions ([link]). Types of Cell Junctions Tight junction plasma membranes Strands of <@ transmembrane proteins Intercellullar Anchoring junctions Gap junction Adjacent plasma membranes Gap between cells Connexons (composed of connexins) Adjacent plasma membranes Plaque Transmembrane Integrins Basal lamina ) ( Hemidesmosome glycoprotein glycoprotein (cadherin) (cadherin) Intermediate Actin filament filament (keratin) Intercellullar ——_-i Intercellullar space space _ it ee il Desmosome ( Adherens ) C) —— Adjacent plasma membranes Plaque Transmembrane The three basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions. At one end of the spectrum is the tight junction, which separates the cells into apical and basal compartments. When two adjacent epithelial cells form a tight junction, there is no extracellular space between them and the movement of substances through the extracellular space between the cells is blocked. This enables the epithelia to act as selective barriers. An anchoring junction includes several types of cell junctions that help stabilize epithelial tissues. Anchoring junctions are common on the lateral and basal surfaces of cells where they provide strong and flexible connections. There are three types of anchoring junctions: desmosomes, hemidesmosomes, and adherens. Desmosomes occur in patches on the membranes of cells. The patches are structural proteins on the inner surface of the cell’s membrane. The adhesion molecule, cadherin, is embedded in these patches and projects through the cell membrane to link with the cadherin molecules of adjacent cells. These connections are especially important in holding cells together. Hemidesmosomes, which look like half a desmosome, link cells to the extracellular matrix, for example, the basal lamina. While similar in appearance to desmosomes, they include the adhesion proteins called integrins rather than cadherins. Adherens junctions use either cadherins or integrins depending on whether they are linking to other cells or matrix. The junctions are characterized by the presence of the contractile protein actin located on the cytoplasmic surface of the cell membrane. The actin can connect isolated patches or form a belt-like structure inside the cell. These junctions influence the shape and folding of the epithelial tissue. In contrast with the tight and anchoring junctions, a gap junction forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells. These junctions allow electrical and metabolic coupling of adjacent cells, which coordinates function in large groups of cells. Classification of Epithelial Tissues Epithelial tissues are classified according to the shape of the cells and number of the cell layers formed ((link]). Cell shapes can be squamous (flattened and thin), cuboidal (boxy, as wide as it is tall), or columnar (rectangular, taller than it is wide). Similarly, the number of cell layers in the tissue can be one—where every cell rests on the basal lamina—which is a simple epithelium, or more than one, which is a stratified epithelium and only the basal layer of cells rests on the basal lamina. Pseudostratified (pseudo- = “false”) describes tissue with a single layer of irregularly shaped cells that give the appearance of more than one layer. Transitional describes a form of specialized stratified epithelium in which the shape of the cells can vary. Cells of Epithelial Tissue [Simp Stratified Squamous —S—S==—= Simple squamous epithelium Cuboidal elelelelele Simple cuboidal epithelium Stratified cuboidal epithelium Pseudostratified Simple columnar epithelium Stratified columnar epithelium Pseudostratified columnar epithelium Simple epithelial tissue is organized as a single layer of cells and stratified epithelial tissue is formed by several layers of cells. Simple Epithelium The shape of the cells in the single cell layer of simple epithelium reflects the functioning of those cells. The cells in simple squamous epithelium have the appearance of thin scales. Squamous cell nuclei tend to be flat, horizontal, and elliptical, mirroring the form of the cell. The endothelium is the epithelial tissue that lines vessels of the lymphatic and cardiovascular system, and it is made up of a single layer of squamous cells. Simple Squamous epithelium, because of the thinness of the cell, is present where rapid passage of chemical compounds is observed. The alveoli of lungs where gases diffuse, segments of kidney tubules, and the lining of capillaries are also made of simple squamous epithelial tissue. The mesothelium is a simple squamous epithelium that forms the surface layer of the serous membrane that lines body cavities and internal organs. Its primary function is to provide a smooth and protective surface. Mesothelial cells are squamous epithelial cells that secrete a fluid that lubricates the mesothelium. In simple cuboidal epithelium, the nucleus of the box-like cells appears round and is generally located near the center of the cell. These epithelia are active in the secretion and absorptions of molecules. Simple cuboidal epithelia are observed in the lining of the kidney tubules and in the ducts of glands. In simple columnar epithelium, the nucleus of the tall column-like cells tends to be elongated and located in the basal end of the cells. Like the cuboidal epithelia, this epithelium is active in the absorption and secretion of molecules. Simple columnar epithelium forms the lining of some sections of the digestive system and parts of the female reproductive tract. Ciliated columnar epithelium is composed of simple columnar epithelial cells with cilia on their apical surfaces. These epithelial cells are found in the lining of the fallopian tubes and parts of the respiratory system, where the beating of the cilia helps remove particulate matter. Pseudostratified columnar epithelium is a type of epithelium that appears to be stratified but instead consists of a single layer of irregularly shaped and differently sized columnar cells. In pseudostratified epithelium, nuclei of neighboring cells appear at different levels rather than clustered in the basal end. The arrangement gives the appearance of stratification; but in fact all the cells are in contact with the basal lamina, although some do not reach the apical surface. Pseudostratified columnar epithelium is found in the respiratory tract, where some of these cells have cilia. Both simple and pseudostratified columnar epithelia are heterogeneous epithelia because they include additional types of cells interspersed among the epithelial cells. For example, a goblet cell is a mucous-secreting unicellular “gland” interspersed between the columnar epithelial cells of mucous membranes ([link]). Goblet Cell Microvilli Secretory vesicles containing mucin Nucleus (a) In the lining of the small intestine, columnar epithelium cells are interspersed with goblet cells. (b) The arrows in this micrograph point to the mucous- secreting goblet cells. LM x 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: CEs.) a — mss Openstax COLLEGE View the University of Michigan WebScope to explore the tissue sample in greater detail. Stratified Epithelium A stratified epithelium consists of several stacked layers of cells. This epithelium protects against physical and chemical wear and tear. The stratified epithelium is named by the shape of the most apical layer of cells, closest to the free space. Stratified squamous epithelium is the most common type of stratified epithelium in the human body. The apical cells are squamous, whereas the basal layer contains either columnar or cuboidal cells. The top layer may be covered with dead cells filled with keratin. Mammalian skin is an example of this dry, keratinized, stratified squamous epithelium. The lining of the mouth cavity is an example of an unkeratinized, stratified squamous epithelium. Stratified cuboidal epithelium and stratified columnar epithelium can also be found in certain glands and ducts, but are uncommon in the human body. Another kind of stratified epithelium is transitional epithelium, so-called because of the gradual changes in the shapes of the apical cells as the bladder fills with urine. It is found only in the urinary system, specifically the ureters and urinary bladder. When the bladder is empty, this epithelium is convoluted and has cuboidal apical cells with convex, umbrella shaped, apical surfaces. As the bladder fills with urine, this epithelium loses its convolutions and the apical cells transition from cuboidal to squamous. It appears thicker and more multi-layered when the bladder is empty, and more stretched out and less stratified when the bladder is full and distended. [link] summarizes the different categories of epithelial cell tissue cells. Summary of Epithelial Tissue Cells Simple cuboidal epithelium Stratified cuboidal epithelium Air sacs of lungs and the lining of the heart, blood vessels, and lymphatic vessels In ducts and secretory portions of small glands and in kidney tubules Ciliated tissues are in bronchi, uterine tubes, and uterus; smooth (nonciliated tissues) are in the digestive tract, bladder Ciliated tissue lines the trachea and much of the upper respiratory tract Lines the esophagus, mouth, and vagina Sweat glands, salivary glands, and the mammary glands The male urethra and the ducts of some glands Lines the bladder, uretha, and the ureters SS Simple squamous epithelium Allows materials to pass through by diffusion and filtration, and secretes lubricating substance Secretes and absorbs Absorbs; it also secretes mucous and enzymes Secretes mucus; ciliated tissue moves mucus Protects against abrasion Protective tissue Secretes and protects Allows the urinary organs to expand and stretch Watch this video to find out more about the anatomy of epithelial tissues. Where in the body would one find non-keratinizing stratified squamous epithelium? Glandular Epithelium A gland is a structure made up of one or more cells modified to synthesize and secrete chemical substances. Most glands consist of groups of epithelial cells. A gland can be classified as an endocrine gland, a ductless gland that releases secretions directly into surrounding tissues and fluids (endo- = “inside”), or an exocrine gland whose secretions leave through a duct that opens directly, or indirectly, to the external environment (exo- = “outside’”’). Endocrine Glands The secretions of endocrine glands are called hormones. Hormones are released into the interstitial fluid, diffused into the bloodstream, and delivered to targets, in other words, cells that have receptors to bind the hormones. The endocrine system is part of a major regulatory system coordinating the regulation and integration of body responses. A few examples of endocrine glands include the anterior pituitary, thymus, adrenal cortex, and gonads. Exocrine Glands Exocrine glands release their contents through a duct that leads to the epithelial surface. Mucous, sweat, saliva, and breast milk are all examples of secretions from exocrine glands. They are all discharged through tubular ducts. Secretions into the lumen of the gastrointestinal tract, technically outside of the body, are of the exocrine category. Glandular Structure Exocrine glands are classified as either unicellular or multicellular. The unicellular glands are scattered single cells, such as goblet cells, found in the mucous membranes of the small and large intestine. The multicellular exocrine glands known as serous glands develop from simple epithelium to form a secretory surface that secretes directly into an inner cavity. These glands line the internal cavities of the abdomen and chest and release their secretions directly into the cavities. Other multicellular exocrine glands release their contents through a tubular duct. The duct is single in a simple gland but in compound glands is divided into one or more branches ([link]). In tubular glands, the ducts can be straight or coiled, whereas tubes that form pockets are alveolar (acinar), such as the exocrine portion of the pancreas. Combinations of tubes and pockets are known as tubuloalveolar (tubuloacinar) compound glands. In a branched gland, a duct is connected to more than one secretory group of cells. Types of Exocrine Glands Simple ducts a Alveolar (acinar) Not found in adult; a stage in development of simple branched glands Simp! Sebaceous (oil) glands Compound alveolar (acinar) RRRROESSAS Compound tubuloalveolar > o ix y ing a - Salivary glands; glands of respiratory passages; and pancreas Mammary glands Simple tubular — Simple branched tubular aTIr- - AP Intestinal glands Merocrine sweat glands Gastric glands; and mucous glands of esophagus, tongue, duodenum Compound tubular DOOUDWODLE ot ee oes Mucous glands (in mouth); bulbourethral glands (in male reproductive system); and testes (seminiferous tubules) ) Compound ducts 4 Exocrine glands are classified by their structure. Methods and Types of Secretion Exocrine glands can be classified by their mode of secretion and the nature Tubular of the substances released, as well as by the structure of the glands and shape of ducts ([link]). Merocrine secretion is the most common type of exocrine secretion. The secretions are enclosed in vesicles that move to the apical surface of the cell where the contents are released by exocytosis. For example, watery mucous containing the glycoprotein mucin, a lubricant that offers some pathogen protection is a merocrine secretion. The eccrine glands that produce and secrete sweat are another example. Modes of Glandular Secretion Secretion Secretory vesicle (a) Merocrine Golgi complex secretion Nucleus Pinched off portion of cell is the secretion (b) Apocrine secretion Mature cell dies and becomes secretory product (c) Holocrine secretion (a) In merocrine secretion, the cell remains intact. (b) In apocrine secretion, the apical portion of the cell is released, as well. (c) In holocrine secretion, the cell is destroyed as it releases its product and the cell itself becomes part of the secretion. Apocrine secretion accumulates near the apical portion of the cell. That portion of the cell and its secretory contents pinch off from the cell and are released. Apocrine sweat glands in the axillary and genital areas release fatty secretions that local bacteria break down; this causes body odor. Both merocrine and apocrine glands continue to produce and secrete their contents with little damage caused to the cell because the nucleus and golgi regions remain intact after secretion. In contrast, the process of holocrine secretion involves the rupture and destruction of the entire gland cell. The cell accumulates its secretory products and releases them only when it bursts. New gland cells differentiate from cells in the surrounding tissue to replace those lost by secretion. The sebaceous glands that produce the oils on the skin and hair are holocrine glands/cells ({link]). Sebaceous Glands Hair Sebaceous gland These glands secrete oils that lubricate and protect the skin. They are holocrine glands and they are destroyed after releasing their contents. New glandular cells form to replace the cells that are lost. LM x 400. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Glands are also named after the products they produce. The serous gland produces watery, blood-plasma-like secretions rich in enzymes such as alpha amylase, whereas the mucous gland releases watery to viscous products rich in the glycoprotein mucin. Both serous and mucous glands are common in the salivary glands of the mouth. Mixed exocrine glands contain both serous and mucous glands and release both types of secretions. Chapter Review In epithelial tissue, cells are closely packed with little or no extracellular matrix except for the basal lamina that separates the epithelium from underlying tissue. The main functions of epithelia are protection from the environment, coverage, secretion and excretion, absorption, and filtration. Cells are bound together by tight junctions that form an impermeable barrier. They can also be connected by gap junctions, which allow free exchange of soluble molecules between cells, and anchoring junctions, which attach cell to cell or cell to matrix. The different types of epithelial tissues are characterized by their cellular shapes and arrangements: squamous, cuboidal, or columnar epithelia. Single cell layers form simple epithelia, whereas stacked cells form stratified epithelia. Very few capillaries penetrate these tissues. Glands are secretory tissues and organs that are derived from epithelial tissues. Exocrine glands release their products through ducts. Endocrine glands secrete hormones directly into the interstitial fluid and blood stream. Glands are classified both according to the type of secretion and by their structure. Merocrine glands secrete products as they are synthesized. Apocrine glands release secretions by pinching off the apical portion of the cell, whereas holocrine gland cells store their secretions until they rupture and release their contents. In this case, the cell becomes part of the secretion. Interactive Link Questions Exercise: Problem: Watch this video to find out more about the anatomy of epithelial tissues. Where in the body would one find non-keratinizing stratified squamous epithelium? Solution: The inside of the mouth, esophagus, vaginal canal, and anus. Review Questions Exercise: Problem: In observing epithelial cells under a microscope, the cells are arranged in a single layer and look tall and narrow, and the nucleus is located close to the basal side of the cell. The specimen is what type of epithelial tissue? a. columnar b. stratified c. squamous d. transitional Solution: A Exercise: Problem: Which of the following is the epithelial tissue that lines the interior of blood vessels? a. columnar b. pseudostratified c. simple squamous d. transitional Solution: C Exercise: Problem: Which type of epithelial tissue specializes in moving particles across its surface and is found in airways and lining of the oviduct? a. transitional b. stratified columnar c. pseudostratified ciliated columnar d. stratified squamous Solution: B Exercise: Problem: The exocrine gland stores its secretion until the glandular cell ruptures, whereas the gland releases its apical region and reforms. a. holocrine; apocrine b. eccrine; endocrine c. apocrine; holocrine d. eccrine; apocrine Solution: A Critical Thinking Questions Exercise: Problem: The structure of a tissue usually is optimized for its function. Describe how the structure of the mucosa and its cells match its function of nutrient absorption. Solution: The mucosa of the intestine is highly folded, increasing the surface area for nutrient absorption. A greater surface area for absorption allows more nutrients to be absorbed per unit time. In addition, the nutrient-absorbing cells of the mucosa have finger-like projections called microvilli that further increase the surface area for nutrient absorption. Glossary anchoring junction mechanically attaches adjacent cells to each other or to the basement membrane apical that part of a cell or tissue which, in general, faces an open space apocrine secretion release of a substance along with the apical portion of the cell basal lamina thin extracellular layer that lies underneath epithelial cells and separates them from other tissues basement membrane in epithelial tissue, a thin layer of fibrous material that anchors the epithelial tissue to the underlying connective tissue; made up of the basal lamina and reticular lamina cell junction point of cell-to-cell contact that connects one cell to another in a tissue endocrine gland groups of cells that release chemical signals into the intercellular fluid to be picked up and transported to their target organs by blood endothelium tissue that lines vessels of the lymphatic and cardiovascular system, made up of a simple squamous epithelium exocrine gland group of epithelial cells that secrete substances through ducts that open to the skin or to internal body surfaces that lead to the exterior of the body gap junction allows cytoplasmic communications to occur between cells goblet cell unicellular gland found in columnar epithelium that secretes mucous holocrine secretion release of a substance caused by the rupture of a gland cell, which becomes part of the secretion merocrine secretion release of a substance from a gland via exocytosis mesothelium simple squamous epithelial tissue which covers the major body cavities and is the epithelial portion of serous membranes mucous gland group of cells that secrete mucous, a thick, slippery substance that keeps tissues moist and acts as a lubricant pseudostratified columnar epithelium tissue that consists of a single layer of irregularly shaped and sized cells that give the appearance of multiple layers; found in ducts of certain glands and the upper respiratory tract reticular lamina matrix containing collagen and elastin secreted by connective tissue; a component of the basement membrane serous gland group of cells within the serous membrane that secrete a lubricating substance onto the surface simple columnar epithelium tissue that consists of a single layer of column-like cells; promotes secretion and absorption in tissues and organs simple cuboidal epithelium tissue that consists of a single layer of cube-shaped cells; promotes secretion and absorption in ducts and tubules simple squamous epithelium tissue that consists of a single layer of flat scale-like cells; promotes diffusion and filtration across surface stratified columnar epithelium tissue that consists of two or more layers of column-like cells, contains glands and is found in some ducts stratified cuboidal epithelium tissue that consists of two or more layers of cube-shaped cells, found in some ducts stratified squamous epithelium tissue that consists of multiple layers of cells with the most apical being flat scale-like cells; protects surfaces from abrasion tight junction forms an impermeable barrier between cells transitional epithelium form of stratified epithelium found in the urinary tract, characterized by an apical layer of cells that change shape in response to the presence of urine Connective Tissue Supports and Protects By the end of this section, you will be able to: e Identify and distinguish between the types of connective tissue: proper, supportive, and fluid e Explain the functions of connective tissues As may be obvious from its name, one of the major functions of connective tissue is to connect tissues and organs. Unlike epithelial tissue, which is composed of cells closely packed with little or no extracellular space in between, connective tissue cells are dispersed in a matrix. The matrix usually includes a large amount of extracellular material produced by the connective tissue cells that are embedded within it. The matrix plays a major role in the functioning of this tissue. The major component of the matrix is a ground substance often crisscrossed by protein fibers. This ground substance is usually a fluid, but it can also be mineralized and solid, as in bones. Connective tissues come in a vast variety of forms, yet they typically have in common three characteristic components: cells, large amounts of amorphous ground substance, and protein fibers. The amount and structure of each component correlates with the function of the tissue, from the rigid ground substance in bones supporting the body to the inclusion of specialized cells; for example, a phagocytic cell that engulfs pathogens and also rids tissue of cellular debris. Functions of Connective Tissues Connective tissues perform many functions in the body, but most importantly, they support and connect other tissues; from the connective tissue sheath that surrounds muscle cells, to the tendons that attach muscles to bones, and to the skeleton that supports the positions of the body. Protection is another major function of connective tissue, in the form of fibrous capsules and bones that protect delicate organs and, of course, the skeletal system. Specialized cells in connective tissue defend the body from microorganisms that enter the body. Transport of fluid, nutrients, waste, and chemical messengers is ensured by specialized fluid connective tissues, such as blood and lymph. Adipose cells store surplus energy in the form of fat and contribute to the thermal insulation of the body. Embryonic Connective Tissue All connective tissues derive from the mesodermal layer of the embryo (see [link]). The first connective tissue to develop in the embryo is mesenchyme, the stem cell line from which all connective tissues are later derived. Clusters of mesenchymal cells are scattered throughout adult tissue and supply the cells needed for replacement and repair after a connective tissue injury. A second type of embryonic connective tissue forms in the umbilical cord, called mucous connective tissue or Wharton’s jelly. This tissue is no longer present after birth, leaving only scattered mesenchymal cells throughout the body. Classification of Connective Tissues The three broad categories of connective tissue are classified according to the characteristics of their ground substance and the types of fibers found within the matrix ({link]). Connective tissue proper includes loose connective tissue and dense connective tissue. Both tissues have a variety of cell types and protein fibers suspended in a viscous ground substance. Dense connective tissue is reinforced by bundles of fibers that provide tensile strength, elasticity, and protection. In loose connective tissue, the fibers are loosely organized, leaving large spaces in between. Supportive connective tissue—bone and cartilage—provide structure and strength to the body and protect soft tissues. A few distinct cell types and densely packed fibers in a matrix characterize these tissues. In bone, the matrix is rigid and described as calcified because of the deposited calcium salts. In fluid connective tissue, in other words, lymph and blood, various specialized cells circulate in a watery fluid containing salts, nutrients, and dissolved proteins. Connective Tissue Examples Connective tisssiee Exaifipjasortive Fluid connective proper connective tissue tissue Connective tissue Supportive Fluid connective proper connective tissue tissue Loose connective : an Cartilage e Hyaline e Areolar y Blood a aadinose e Fibrocartilage e Reticular Pane Dense connective tissue Bones Regular ; ere e Compact bone Lymph eicsalat e Cancellous bone elastic Connective Tissue Proper Fibroblasts are present in all connective tissue proper ([link]). Fibrocytes, adipocytes, and mesenchymal cells are fixed cells, which means they remain within the connective tissue. Other cells move in and out of the connective tissue in response to chemical signals. Macrophages, mast cells, lymphocytes, plasma cells, and phagocytic cells are found in connective tissue proper but are actually part of the immune system protecting the body. Connective Tissue Proper Reticular fibers Adipocytes Mesenchymal cell Elastic fibers Collagen fibers Fibroblast Macrophage Fibroblasts produce this fibrous tissue. Connective tissue proper includes the fixed cells fibrocytes, adipocytes, and mesenchymal cells. LM x 400. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Cell Types The most abundant cell in connective tissue proper is the fibroblast. Polysaccharides and proteins secreted by fibroblasts combine with extra- cellular fluids to produce a viscous ground substance that, with embedded fibrous proteins, forms the extra-cellular matrix. As you might expect, a fibrocyte, a less active form of fibroblast, is the second most common cell type in connective tissue proper. Adipocytes are cells that store lipids as droplets that fill most of the cytoplasm. There are two basic types of adipocytes: white and brown. The brown adipocytes store lipids as many droplets, and have high metabolic activity. In contrast, white fat adipocytes store lipids as a single large drop and are metabolically less active. Their effectiveness at storing large amounts of fat is witnessed in obese individuals. The number and type of adipocytes depends on the tissue and location, and vary among individuals in the population. The mesenchymal cell is a multipotent adult stem cell. These cells can differentiate into any type of connective tissue cells needed for repair and healing of damaged tissue. The macrophage cell is a large cell derived from a monocyte, a type of blood cell, which enters the connective tissue matrix from the blood vessels. The macrophage cells are an essential component of the immune system, which is the body’s defense against potential pathogens and degraded host cells. When stimulated, macrophages release cytokines, small proteins that act as chemical messengers. Cytokines recruit other cells of the immune system to infected sites and stimulate their activities. Roaming, or free, macrophages move rapidly by amoeboid movement, engulfing infectious agents and cellular debris. In contrast, fixed macrophages are permanent residents of their tissues. The mast cell, found in connective tissue proper, has many cytoplasmic granules. These granules contain the chemical signals histamine and heparin. When irritated or damaged, mast cells release histamine, an inflammatory mediator, which causes vasodilation and increased blood flow at a site of injury or infection, along with itching, swelling, and redness you recognize as an allergic response. Like blood cells, mast cells are derived from hematopoietic stem cells and are part of the immune system. Connective Tissue Fibers and Ground Substance Three main types of fibers are secreted by fibroblasts: collagen fibers, elastic fibers, and reticular fibers. Collagen fiber is made from fibrous protein subunits linked together to form a long and straight fiber. Collagen fibers, while flexible, have great tensile strength, resist stretching, and give ligaments and tendons their characteristic resilience and strength. These fibers hold connective tissues together, even during the movement of the body. Elastic fiber contains the protein elastin along with lesser amounts of other proteins and glycoproteins. The main property of elastin is that after being stretched or compressed, it will return to its original shape. Elastic fibers are prominent in elastic tissues found in skin and the elastic ligaments of the vertebral column. Reticular fiber is also formed from the same protein subunits as collagen fibers; however, these fibers remain narrow and are arrayed in a branching network. They are found throughout the body, but are most abundant in the reticular tissue of soft organs, such as liver and spleen, where they anchor and provide structural support to the parenchyma (the functional cells, blood vessels, and nerves of the organ). All of these fiber types are embedded in ground substance. Secreted by fibroblasts, ground substance is made of polysaccharides, specifically hyaluronic acid, and proteins. These combine to form a proteoglycan with a protein core and polysaccharide branches. The proteoglycan attracts and traps available moisture forming the clear, viscous, colorless matrix you now know as ground substance. Loose Connective Tissue Loose connective tissue is found between many organs where it acts both to absorb shock and bind tissues together. It allows water, salts, and various nutrients to diffuse through to adjacent or imbedded cells and tissues. Adipose tissue consists mostly of fat storage cells, with little extracellular matrix ({link]). A large number of capillaries allow rapid storage and mobilization of lipid molecules. White adipose tissue is most abundant. It can appear yellow and owes its color to carotene and related pigments from plant food. White fat contributes mostly to lipid storage and can serve as insulation from cold temperatures and mechanical injuries. White adipose tissue can be found protecting the kidneys and cushioning the back of the eye. Brown adipose tissue is more common in infants, hence the term “baby fat.” In adults, there is a reduced amount of brown fat and it is found mainly in the neck and clavicular regions of the body. The many mitochondria in the cytoplasm of brown adipose tissue help explain its efficiency at metabolizing stored fat. Brown adipose tissue is thermogenic, meaning that as it breaks down fats, it releases metabolic heat, rather than producing adenosine triphosphate (ATP), a key molecule used in metabolism. Adipose Tissue | es ————— This is a loose connective tissue that consists of fat cells with little extracellular matrix. It stores fat for energy and provides insulation. LM x 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Areolar tissue shows little specialization. It contains all the cell types and fibers previously described and is distributed in a random, web-like fashion. It fills the spaces between muscle fibers, surrounds blood and lymph vessels, and supports organs in the abdominal cavity. Areolar tissue underlies most epithelia and represents the connective tissue component of epithelial membranes, which are described further in a later section. Reticular tissue is a mesh-like, supportive framework for soft organs such as lymphatic tissue, the spleen, and the liver ([link]). Reticular cells produce the reticular fibers that form the network onto which other cells attach. It derives its name from the Latin reticulus, which means “little net.” Reticular Tissue This is a loose connective tissue made up of a network of reticular fibers that provides a supportive framework for soft organs. LM x 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Dense Connective Tissue Dense connective tissue contains more collagen fibers than does loose connective tissue. As a consequence, it displays greater resistance to stretching. There are two major categories of dense connective tissue: regular and irregular. Dense regular connective tissue fibers are parallel to each other, enhancing tensile strength and resistance to stretching in the direction of the fiber orientations. Ligaments and tendons are made of dense regular connective tissue, but in ligaments not all fibers are parallel. Dense regular elastic tissue contains elastin fibers in addition to collagen fibers, which allows the ligament to return to its original length after stretching. The ligaments in the vocal folds and between the vertebrae in the vertebral column are elastic. In dense irregular connective tissue, the direction of fibers is random. This arrangement gives the tissue greater strength in all directions and less strength in one particular direction. In some tissues, fibers crisscross and form a mesh. In other tissues, stretching in several directions is achieved by alternating layers where fibers run in the same orientation in each layer, and it is the layers themselves that are stacked at an angle. The dermis of the skin is an example of dense irregular connective tissue rich in collagen fibers. Dense irregular elastic tissues give arterial walls the strength and the ability to regain original shape after stretching ([link]). Dense Connective Tissue Fibroblast nuclei Fibroblast nuclei Collagen fiber bundles (b) Irregular dense (a) Dense regular connective tissue consists of collagenous fibers packed into parallel bundles. (b) Dense irregular connective tissue consists of collagenous fibers interwoven into a mesh-like network. From top, LM x 1000, LM x 200. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) Note: Disorders of the... Connective Tissue: Tendinitis Your opponent stands ready as you prepare to hit the serve, but you are confident that you will smash the ball past your opponent. As you toss the ball high in the air, a bumming pain shoots across your wrist and you drop the tennis racket. That dull ache in the wrist that you ignored through the summer is now an unbearable pain. The game is over for now. After examining your swollen wrist, the doctor in the emergency room announces that you have developed wrist tendinitis. She recommends icing the tender area, taking non-steroidal anti-inflammatory medication to ease the pain and to reduce swelling, and complete rest for a few weeks. She interrupts your protests that you cannot stop playing. She issues a stern warning about the risk of aggravating the condition and the possibility of surgery. She consoles you by mentioning that well known tennis players such as Venus and Serena Williams and Rafael Nadal have also suffered from tendinitis related injuries. What is tendinitis and how did it happen? Tendinitis is the inflammation of a tendon, the thick band of fibrous connective tissue that attaches a muscle to a bone. The condition causes pain and tenderness in the area around a joint. On rare occasions, a sudden serious injury will cause tendinitis. Most often, the condition results from repetitive motions over time that strain the tendons needed to perform the tasks. Persons whose jobs and hobbies involve performing the same movements over and over again are often at the greatest risk of tendinitis. You hear of tennis and golfer’s elbow, jumper's knee, and swimmer’s shoulder. In all cases, overuse of the joint causes a microtrauma that initiates the inflammatory response. Tendinitis is routinely diagnosed through a clinical examination. In case of severe pain, X-rays can be examined to rule out the possibility of a bone injury. Severe cases of tendinitis can even tear loose a tendon. Surgical repair of a tendon is painful. Connective tissue in the tendon does not have abundant blood supply and heals slowly. While older adults are at risk for tendinitis because the elasticity of tendon tissue decreases with age, active people of all ages can develop tendinitis. Young athletes, dancers, and computer operators; anyone who performs the same movements constantly is at risk for tendinitis. Although repetitive motions are unavoidable in many activities and may lead to tendinitis, precautions can be taken that can lessen the probability of developing tendinitis. For active individuals, stretches before exercising and cross training or changing exercises are recommended. For the passionate athlete, it may be time to take some lessons to improve technique. All of the preventive measures aim to increase the strength of the tendon and decrease the stress put on it. With proper rest and managed care, you will be back on the court to hit that slice-spin serve over the net. Note: Pelee Le Watch this animation to learn more about tendonitis, a painful condition caused by swollen or injured tendons. Supportive Connective Tissues Two major forms of supportive connective tissue, cartilage and bone, allow the body to maintain its posture and protect internal organs. Cartilage The distinctive appearance of cartilage is due to polysaccharides called chondroitin sulfates, which bind with ground substance proteins to form proteoglycans. Embedded within the cartilage matrix are chondrocytes, or cartilage cells, and the space they occupy are called lacunae (singular = lacuna). A layer of dense irregular connective tissue, the perichondrium, encapsulates the cartilage. Cartilaginous tissue is avascular, thus all nutrients need to diffuse through the matrix to reach the chondrocytes. This is a factor contributing to the very slow healing of cartilaginous tissues. The three main types of cartilage tissue are hyaline cartilage, fibrocartilage, and elastic cartilage ([{link]). Hyaline cartilage, the most common type of cartilage in the body, consists of short and dispersed collagen fibers and contains large amounts of proteoglycans. Under the microscope, tissue samples appear clear. The surface of hyaline cartilage is smooth. Both strong and flexible, it is found in the rib cage and nose and covers bones where they meet to form moveable joints. It makes up a template of the embryonic skeleton before bone formation. A plate of hyaline cartilage at the ends of bone allows continued growth until adulthood. Fibrocartilage is tough because it has thick bundles of collagen fibers dispersed through its matrix. Menisci in the knee joint and the intervertebral discs are examples of fibrocartilage. Elastic cartilage contains elastic fibers as well as collagen and proteoglycans. This tissue gives rigid support as well as elasticity. Tug gently at your ear lobes, and notice that the lobes return to their initial shape. The external ear contains elastic cartilage. Types of Cartilage (a) Hyaline cartilage Chondrocytes in lacunae Matrix (b) Fibrocartilage Chondrocyte in lacuna Collagen fiber in matrix (c) Elastic cartilage Chondrocyte in lacuna Elastic fibers in matrix Cartilage is a connective tissue consisting of collagenous fibers embedded in a firm matrix of chondroitin sulfates. (a) Hyaline cartilage provides support with some flexibility. The example is from dog tissue. (b) Fibrocartilage provides some compressibility and can absorb pressure. (c) Elastic cartilage provides firm but elastic support. From top, LM x 300, LM x 1200, LM x 1016. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) Bone Bone is the hardest connective tissue. It provides protection to internal organs and supports the body. Bone’s rigid extracellular matrix contains mostly collagen fibers embedded in a mineralized ground substance containing hydroxyapatite, a form of calcium phosphate. Both components of the matrix, organic and inorganic, contribute to the unusual properties of bone. Without collagen, bones would be brittle and shatter easily. Without mineral crystals, bones would flex and provide little support. Osteocytes, bone cells like chondrocytes, are located within lacunae. The histology of transverse tissue from long bone shows a typical arrangement of osteocytes in concentric circles around a central canal. Bone is a highly vascularized tissue. Unlike cartilage, bone tissue can recover from injuries in a relatively short time. Cancellous bone looks like a sponge under the microscope and contains empty spaces between trabeculae, or arches of bone proper. It is lighter than compact bone and found in the interior of some bones and at the end of long bones. Compact bone is solid and has greater structural strength. Fluid Connective Tissue Blood and lymph are fluid connective tissues. Cells circulate in a liquid extracellular matrix. The formed elements circulating in blood are all derived from hematopoietic stem cells located in bone marrow ([link]). Erythrocytes, red blood cells, transport oxygen and some carbon dioxide. Leukocytes, white blood cells, are responsible for defending against potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some white blood cells have the ability to cross the endothelial layer that lines blood vessels and enter adjacent tissues. Nutrients, salts, and wastes are dissolved in the liquid matrix and transported through the body. Lymph contains a liquid matrix and white blood cells. Lymphatic capillaries are extremely permeable, allowing larger molecules and excess fluid from interstitial spaces to enter the lymphatic vessels. Lymph drains into blood vessels, delivering molecules to the blood that could not otherwise directly enter the bloodstream. In this way, specialized lymphatic capillaries transport absorbed fats away from the intestine and deliver these molecules to the blood. Blood: A Fluid Connective Tissue —, o AY q fe) € Ne) G e oe ° ie 3 =e) ot 38 oe 8% 0 266.0 0 206 Blood is a fluid connective tissue containing erythrocytes and various types of leukocytes that circulate in a liquid extracellular matrix. LM x 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: ac| openstax COLLEGE” Wii ae View the University of Michigan Webscope to explore the tissue sample in greater detail. Note: =" meee OFENStAX COLLEGE” Visit this link to test your connective tissue knowledge with this 10- question quiz. Can you name the 10 tissue types shown in the histology slides? Chapter Review Connective tissue is a heterogeneous tissue with many cell shapes and tissue architecture. Structurally, all connective tissues contain cells that are embedded in an extracellular matrix stabilized by proteins. The chemical nature and physical layout of the extracellular matrix and proteins vary enormously among tissues, reflecting the variety of functions that connective tissue fulfills in the body. Connective tissues separate and cushion organs, protecting them from shifting or traumatic injury. Connect tissues provide support and assist movement, store and transport energy molecules, protect against infections, and contribute to temperature homeostasis. Many different cells contribute to the formation of connective tissues. They originate in the mesodermal germ layer and differentiate from mesenchyme and hematopoietic tissue in the bone marrow. Fibroblasts are the most abundant and secrete many protein fibers, adipocytes specialize in fat storage, hematopoietic cells from the bone marrow give rise to all the blood cells, chondrocytes form cartilage, and osteocytes form bone. The extracellular matrix contains fluid, proteins, polysaccharide derivatives, and, in the case of bone, mineral crystals. Protein fibers fall into three major groups: collagen fibers that are thick, strong, flexible, and resist stretch; reticular fibers that are thin and form a supportive mesh; and elastin fibers that are thin and elastic. The major types of connective tissue are connective tissue proper, supportive tissue, and fluid tissue. Loose connective tissue proper includes adipose tissue, areolar tissue, and reticular tissue. These serve to hold organs and other tissues in place and, in the case of adipose tissue, isolate and store energy reserves. The matrix is the most abundant feature for loose tissue although adipose tissue does not have much extracellular matrix. Dense connective tissue proper is richer in fibers and may be regular, with fibers oriented in parallel as in ligaments and tendons, or irregular, with fibers oriented in several directions. Organ capsules (collagenous type) and walls of arteries (elastic type) contain dense irregular connective tissue. Cartilage and bone are supportive tissue. Cartilage contains chondrocytes and is somewhat flexible. Hyaline cartilage is smooth and clear, covers joints, and is found in the growing portion of bones. Fibrocartilage is tough because of extra collagen fibers and forms, among other things, the intervertebral discs. Elastic cartilage can stretch and recoil to its original shape because of its high content of elastic fibers. The matrix contains very few blood vessels. Bones are made of a rigid, mineralized matrix containing calcium salts, crystals, and osteocytes lodged in lacunae. Bone tissue is highly vascularized. Cancellous bone is spongy and less solid than compact bone. Fluid tissue, for example blood and lymph, is characterized by a liquid matrix and no supporting fibers. Interactive Link Questions Exercise: Problem: Visit this link to test your connective tissue knowledge with this 10- question quiz. Can you name the 10 tissue types shown in the histology slides? Solution: Click at the bottom of the quiz for the answers. Review Questions Exercise: Problem: Connective tissue is made of which three essential components? a. cells, ground substance, and carbohydrate fibers b. cells, ground substance, and protein fibers c. collagen, ground substance, and protein fibers d. matrix, ground substance, and fluid Solution: B Exercise: Problem: Under the microscope, a tissue specimen shows cells located in spaces scattered in a transparent background. This is probably a. loose connective tissue b. a tendon c. bone d. hyaline cartilage Solution: D Exercise: Problem: Which connective tissue specializes in storage of fat? a. tendon b. adipose tissue c. reticular tissue d. dense connective tissue Solution: B Exercise: Problem: Ligaments connect bones together and withstand a lot of stress. What type of connective tissue should you expect ligaments to contain? a. areolar tissue b. adipose tissue c. dense regular connective tissue d. dense irregular connective tissue Solution: C Exercise: Problem: In adults, new connective tissue cells originate from the a. mesoderm b. mesenchyme c. ectoderm d. endoderm Solution: B Exercise: Problem:In bone, the main cells are a. fibroblasts b. chondrocytes c. lymphocytes d. osteocytes Solution: D Critical Thinking Questions Exercise: Problem: One of the main functions of connective tissue is to integrate organs and organ systems in the body. Discuss how blood fulfills this role. Solution: Blood is a fluid connective tissue, a variety of specialized cells that circulate in a watery fluid containing salts, nutrients, and dissolved proteins in a liquid extracellular matrix. Blood contains formed elements derived from bone marrow. Erythrocytes, or red blood cells, transport the gases oxygen and carbon dioxide. Leukocytes, or white blood cells, are responsible for the defense of the organism against potentially harmful microorganisms or molecules. Platelets are cell fragments involved in blood clotting. Some cells have the ability to cross the endothelial layer that lines vessels and enter adjacent tissues. Nutrients, salts, and waste are dissolved in the liquid matrix and transported through the body. Exercise: Problem: Why does an injury to cartilage, especially hyaline cartilage, heal much more slowly than a bone fracture? Solution: A layer of dense irregular connective tissue covers cartilage. No blood vessels supply cartilage tissue. Injuries to cartilage heal very slowly because cells and nutrients needed for repair diffuse slowly to the injury site. Glossary adipocytes lipid storage cells adipose tissue specialized areolar tissue rich in stored fat areolar tissue (also, loose connective tissue) a type of connective tissue proper that shows little specialization with cells dispersed in the matrix chondrocytes cells of the cartilage collagen fiber flexible fibrous proteins that give connective tissue tensile strength connective tissue proper connective tissue containing a viscous matrix, fibers, and cells. dense connective tissue connective tissue proper that contains many fibers that provide both elasticity and protection elastic cartilage type of cartilage, with elastin as the major protein, characterized by rigid support as well as elasticity elastic fiber fibrous protein within connective tissue that contains a high percentage of the protein elastin that allows the fibers to stretch and return to original size fibroblast most abundant cell type in connective tissue, secretes protein fibers and matrix into the extracellular space fibrocartilage tough form of cartilage, made of thick bundles of collagen fibers embedded in chondroitin sulfate ground substance fibrocyte less active form of fibroblast fluid connective tissue specialized cells that circulate in a watery fluid containing salts, nutrients, and dissolved proteins ground substance fluid or semi-fluid portion of the matrix hyaline cartilage most common type of cartilage, smooth and made of short collagen fibers embedded in a chondroitin sulfate ground substance lacunae (singular = lacuna) small spaces in bone or cartilage tissue that cells occupy loose connective tissue (also, areolar tissue) type of connective tissue proper that shows little specialization with cells dispersed in the matrix matrix extracellular material which is produced by the cells embedded in it, containing ground substance and fibers mesenchymal cell adult stem cell from which most connective tissue cells are derived mesenchyme embryonic tissue from which connective tissue cells derive mucous connective tissue specialized loose connective tissue present in the umbilical cord parenchyma functional cells of a gland or organ, in contrast with the supportive or connective tissue of a gland or organ reticular fiber fine fibrous protein, made of collagen subunits, which cross-link to form supporting “nets” within connective tissue reticular tissue type of loose connective tissue that provides a supportive framework to soft organs, such as lymphatic tissue, spleen, and the liver supportive connective tissue type of connective tissue that provides strength to the body and protects soft tissue Muscle Tissue and Motion By the end of this section, you will be able to: e Identify the three types of muscle tissue ¢ Compare and contrast the functions of each muscle tissue type e Explain how muscle tissue can enable motion Muscle tissue is characterized by properties that allow movement. Muscle cells are excitable; they respond to a stimulus. They are contractile, meaning they can shorten and generate a pulling force. When attached between two movable objects, in other words, bones, contractions of the muscles cause the bones to move. Some muscle movement is voluntary, which means it is under conscious control. For example, a person decides to open a book and read a chapter on anatomy. Other movements are involuntary, meaning they are not under conscious control, such as the contraction of your pupil in bright light. Muscle tissue is classified into three types according to structure and function: skeletal, cardiac, and smooth ({link]). Comparison of Structure and Properties of Muscle Tissue Types Tissue Skeletal Histology Long cylindrical fiber, Striated, many peripherally located nuclei Function Voluntary movement, produces heat, protects organs Location Attached to bones and around entrance points to body (e.g., mouth, anus) Comparison of Structure and Properties of Muscle Tissue Types Tissue Histology Function Location Short, branched, : rl ntr m aie st ated, Contracts to pump ean single blood central nucleus Involuntar Short, vey : movement, moves ae food, involuntar shaped, no ‘ y Walls of : control of : evident Pea major Smooth ihe respiration, moves Striation, organs and single secretions, regulates passageways F flow of blood in SSA eae arteries b each fiber y contraction Skeletal muscle is attached to bones and its contraction makes possible locomotion, facial expressions, posture, and other voluntary movements of the body. Forty percent of your body mass is made up of skeletal muscle. Skeletal muscles generate heat as a byproduct of their contraction and thus participate in thermal homeostasis. Shivering is an involuntary contraction of skeletal muscles in response to perceived lower than normal body temperature. The muscle cell, or myocyte, develops from myoblasts derived from the mesoderm. Myocytes and their numbers remain relatively constant throughout life. Skeletal muscle tissue is arranged in bundles surrounded by connective tissue. Under the light microscope, muscle cells appear striated with many nuclei squeezed along the membranes. The striation is due to the regular alternation of the contractile proteins actin and myosin, along with the structural proteins that couple the contractile proteins to connective tissues. The cells are multinucleated as a result of the fusion of the many myoblasts that fuse to form each long muscle fiber. Cardiac muscle forms the contractile walls of the heart. The cells of cardiac muscle, known as cardiomyocytes, also appear striated under the microscope. Unlike skeletal muscle fibers, cardiomyocytes are single cells typically with a single centrally located nucleus. A principal characteristic of cardiomyocytes is that they contract on their own intrinsic rhythms without any external stimulation. Cardiomyocyte attach to one another with specialized cell junctions called intercalated discs. Intercalated discs have both anchoring junctions and gap junctions. Attached cells form long, branching cardiac muscle fibers that are, essentially, a mechanical and electrochemical syncytium allowing the cells to synchronize their actions. The cardiac muscle pumps blood through the body and is under involuntary control. The attachment junctions hold adjacent cells together across the dynamic pressures changes of the cardiac cycle. Smooth muscle tissue contraction is responsible for involuntary movements in the internal organs. It forms the contractile component of the digestive, urinary, and reproductive systems as well as the airways and arteries. Each cell is spindle shaped with a single nucleus and no visible striations ((link]). Muscle Tissue (a) Skeletal muscle cells have prominent striation and nuclei on their periphery. (b) Smooth muscle cells have a single nucleus and no visible striations. (c) Cardiac muscle cells appear striated and have a single nucleus. From top, LM x 1600, LM x 1600, LM x 1600. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) Note: | acl ele, "a —: mess Openstax COLLEGE —— . . Watch this video to learn more about muscle tissue. In looking through a microscope how could you distinguish skeletal muscle tissue from smooth muscle? Chapter Review The three types of muscle cells are skeletal, cardiac, and smooth. Their morphologies match their specific functions in the body. Skeletal muscle is voluntary and responds to conscious stimuli. The cells are striated and multinucleated appearing as long, unbranched cylinders. Cardiac muscle is involuntary and found only in the heart. Each cell is striated with a single nucleus and they attach to one another to form long fibers. Cells are attached to one another at intercalated disks. The cells are interconnected physically and electrochemically to act as a syncytium. Cardiac muscle cells contract autonomously and involuntarily. Smooth muscle is involuntary. Each cell is a spindle-shaped fiber and contains a single nucleus. No striations are evident because the actin and myosin filaments do not align in the cytoplasm. Interactive Link Questions Exercise: Problem: Watch this video to learn more about muscle tissue. In looking through a microscope how could you distinguish skeletal muscle tissue from smooth muscle? Solution: Skeletal muscle cells are striated. Review Questions Exercise: Problem: Striations, cylindrical cells, and multiple nuclei are observed in a. Skeletal muscle only b. cardiac muscle only c. smooth muscle only d. skeletal and cardiac muscles Solution: A Exercise: Problem:The cells of muscles, myocytes, develop from a. myoblasts b. endoderm c. fibrocytes d. chondrocytes Solution: A Exercise: Problem: Skeletal muscle is composed of very hard working cells. Which organelles do you expect to find in abundance in skeletal muscle cell? a. nuclei b. striations c. golgi bodies d. mitochondria Solution: D Critical Thinking Questions Exercise: Problem: You are watching cells in a dish spontaneously contract. They are all contracting at different rates; some fast, some slow. After a while, several cells link up and they begin contracting in synchrony. Discuss what is going on and what type of cells you are looking at. Solution: The cells in the dish are cardiomyocytes, cardiac muscle cells. They have an intrinsic ability to contract. When they link up, they form intercalating discs that allow the cells to communicate with each other and begin contracting in synchrony. Exercise: Problem: Why does skeletal muscle look striated? Solution: Under the light microscope, cells appear striated due to the arrangement of the contractile proteins actin and myosin. Glossary cardiac muscle heart muscle, under involuntary control, composed of striated cells that attach to form fibers, each cell contains a single nucleus, contracts autonomously myocyte muscle cells skeletal muscle usually attached to bone, under voluntary control, each cell is a fiber that is multinucleated and striated smooth muscle under involuntary control, moves internal organs, cells contain a single nucleus, are spindle-shaped, and do not appear striated; each cell is a fiber striation alignment of parallel actin and myosin filaments which form a banded pattern Nervous Tissue Mediates Perception and Response By the end of this section, you will be able to: e Identify the classes of cells that make up nervous tissue e Discuss how nervous tissue mediates perception and response Nervous tissue is characterized as being excitable and capable of sending and receiving electrochemical signals that provide the body with information. Two main classes of cells make up nervous tissue: the neuron and neuroglia ([link]). Neurons propagate information via electrochemical impulses, called action potentials, which are biochemically linked to the release of chemical signals. Neuroglia play an essential role in supporting neurons and modulating their information propagation. The Neuron Contact with other cells a Nucleus Microfibrils and microtubules Axon Dendrites The cell body of a neuron, also called the soma, contains the nucleus and mitochondria. The dendrites transfer the nerve impulse to the soma. The axon carries the action potential away to another excitable cell. LM x 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: = mss Openstax COLLEGE Follow this link to learn more about nervous tissue. What are the main parts of a nerve cell? Neurons display distinctive morphology, well suited to their role as conducting cells, with three main parts. The cell body includes most of the cytoplasm, the organelles, and the nucleus. Dendrites branch off the cell body and appear as thin extensions. A long “tail,” the axon, extends from the neuron body and can be wrapped in an insulating layer known as myelin, which is formed by accessory cells. The synapse is the gap between nerve cells, or between a nerve cell and its target, for example, a muscle or a gland, across which the impulse is transmitted by chemical compounds known as neurotransmitters. Neurons categorized as multipolar neurons have several dendrites and a single prominent axon. Bipolar neurons possess a single dendrite and axon with the cell body, while unipolar neurons have only a single process extending out from the cell body, which divides into a functional dendrite and into a functional axon. When a neuron is sufficiently stimulated, it generates an action potential that propagates down the axon towards the synapse. If enough neurotransmitters are released at the synapse to stimulate the next neuron or target, a response is generated. The second class of neural cells comprises the neuroglia or glial cells, which have been characterized as having a simple support role. The word “glia” comes from the Greek word for glue. Recent research is shedding light on the more complex role of neuroglia in the function of the brain and nervous system. Astrocyte cells, named for their distinctive star shape, are abundant in the central nervous system. The astrocytes have many functions, including regulation of ion concentration in the intercellular space, uptake and/or breakdown of some neurotransmitters, and formation of the blood-brain barrier, the membrane that separates the circulatory system from the brain. Microglia protect the nervous system against infection but are not nervous tissue because they are related to macrophages. Oligodendrocyte cells produce myelin in the central nervous system (brain and spinal cord) while the Schwann cell produces myelin in the peripheral nervous system ([link]). Nervous Tissue Neurons Microglial cell Sa Astrocytes y ' “s aK. OZ Oligodendrocyte Nervous tissue is made up of neurons and neuroglia. The cells of nervous tissue are specialized to transmit and receive impulses. LM x 872. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Chapter Review The most prominent cell of the nervous tissue, the neuron, is characterized mainly by its ability to receive stimuli and respond by generating an electrical signal, known as an action potential, which can travel rapidly over great distances in the body. A typical neuron displays a distinctive morphology: a large cell body branches out into short extensions called dendrites, which receive chemical signals from other neurons, and a long tail called an axon, which relays signals away from the cell to other neurons, muscles, or glands. Many axons are wrapped by a myelin sheath, a lipid derivative that acts as an insulator and speeds up the transmission of the action potential. Other cells in the nervous tissue, the neuroglia, include the astrocytes, microglia, oligodendrocytes, and Schwann cells. Interactive Link Questions Exercise: Problem: Follow this link to learn more about nervous tissue. What are the main parts of a nerve cell? Solution: Dendrites, cell body, and the axon. Review Questions Exercise: Problem: The cells responsible for the transmission of the nerve impulse are a. Neurons b. oligodendrocytes c. astrocytes d. microglia Solution: A Exercise: Problem: The nerve impulse travels down a(n) , away from the cell body. a. dendrite b. axon c. microglia d. collagen fiber Solution: B Exercise: Problem: Which of the following central nervous system cells regulate ions, regulate the uptake and/or breakdown of some neurotransmitters, and contribute to the formation of the blood-brain barrier? a. microglia b. neuroglia c. oligodendrocytes d. astrocytes Solution: D Critical Thinking Questions Exercise: Problem: Which morphological adaptations of neurons make them suitable for the transmission of nerve impulse? Solution: Neurons are well suited for the transmission of nerve impulses because short extensions, dendrites, receive impulses from other neurons, while a long tail extension, an axon, carries electrical impulses away from the cell to other neurons. Exercise: Problem: What are the functions of astrocytes? Solution: Astrocytes regulate ions and uptake and/or breakdown of some neurotransmitters and contribute to the formation of the blood-brain- barrier. References Stern, P. Focus issue: getting excited about glia. Science [Internet]. 2010 [cited 2012 Dec 4]; 3(147):330-773. Available from: http://stke.sciencemag,org/cgi/content/abstract/sigtrans;3/147/eg11 Ming GL, Song H. Adult neurogenesis in the mammalian central nervous system. Annu. Rev. Neurosci. 2005 [cited 2012 Dec 4]; 28:223—250. Glossary astrocyte star-shaped cell in the central nervous system that regulates ions and uptake and/or breakdown of some neurotransmitters and contributes to the formation of the blood-brain barrier myelin layer of lipid inside some neuroglial cells that wraps around the axons of some neurons neuroglia supportive neural cells neuron excitable neural cell that transfer nerve impulses oligodendrocyte neuroglial cell that produces myelin in the brain Schwann cell neuroglial cell that produces myelin in the peripheral nervous system Tissue Injury and Aging By the end of this section, you will be able to: e Identify the cardinal signs of inflammation List the body’s response to tissue injury e Explain the process of tissue repair Discuss the progressive impact of aging on tissue e Describe cancerous mutations’ effect on tissue Tissues of all types are vulnerable to injury and, inevitably, aging. In the former case, understanding how tissues respond to damage can guide strategies to aid repair. In the latter case, understanding the impact of aging can help in the search for ways to diminish its effects. Tissue Injury and Repair Inflammation is the standard, initial response of the body to injury. Whether biological, chemical, physical, or radiation burns, all injuries lead to the same sequence of physiological events. Inflammation limits the extent of injury, partially or fully eliminates the cause of injury, and initiates repair and regeneration of damaged tissue. Necrosis, or accidental cell death, causes inflammation. Apoptosis is programmed cell death, a normal step-by-step process that destroys cells no longer needed by the body. By mechanisms still under investigation, apoptosis does not initiate the inflammatory response. Acute inflammation resolves over time by the healing of tissue. If inflammation persists, it becomes chronic and leads to diseased conditions. Arthritis and tuberculosis are examples of chronic inflammation. The suffix “-itis” denotes inflammation of a specific organ or type, for example, peritonitis is the inflammation of the peritoneum, and meningitis refers to the inflammation of the meninges, the tough membranes that surround the central nervous system The four cardinal signs of inflammation—redness, swelling, pain, and local heat—were first recorded in antiquity. Cornelius Celsus is credited with documenting these signs during the days of the Roman Empire, as early as the first century AD. A fifth sign, loss of function, may also accompany inflammation. Upon tissue injury, damaged cells release inflammatory chemical signals that evoke local vasodilation, the widening of the blood vessels. Increased blood flow results in apparent redness and heat. In response to injury, mast cells present in tissue degranulate, releasing the potent vasodilator histamine. Increased blood flow and inflammatory mediators recruit white blood cells to the site of inflammation. The endothelium lining the local blood vessel becomes “leaky” under the influence of histamine and other inflammatory mediators allowing neutrophils, macrophages, and fluid to move from the blood into the interstitial tissue spaces. The excess liquid in tissue causes swelling, more properly called edema. The swollen tissues squeezing pain receptors cause the sensation of pain. Prostaglandins released from injured cells also activate pain neurons. Non-steroidal anti- inflammatory drugs (NSAIDs) reduce pain because they inhibit the synthesis of prostaglandins. High levels of NSAIDs reduce inflammation. Antihistamines decrease allergies by blocking histamine receptors and as a result the histamine response. After containment of an injury, the tissue repair phase starts with removal of toxins and waste products. Clotting (coagulation) reduces blood loss from damaged blood vessels and forms a network of fibrin proteins that trap blood cells and bind the edges of the wound together. A scab forms when the clot dries, reducing the risk of infection. Sometimes a mixture of dead leukocytes and fluid called pus accumulates in the wound. As healing progresses, fibroblasts from the surrounding connective tissues replace the collagen and extracellular material lost by the injury. Angiogenesis, the growth of new blood vessels, results in vascularization of the new tissue known as granulation tissue. The clot retracts pulling the edges of the wound together, and it slowly dissolves as the tissue is repaired. When a large amount of granulation tissue forms and capillaries disappear, a pale scar is often visible in the healed area. A primary union describes the healing of a wound where the edges are close together. When there is a gaping wound, it takes longer to refill the area with cells and collagen. The process called secondary union occurs as the edges of the wound are pulled together by what is called wound contraction. When a wound is more than one quarter of an inch deep, sutures (stitches) are recommended to promote a primary union and avoid the formation of a disfiguring scar. Regeneration is the addition of new cells of the same type as the ones that were injured ((link]). Tissue Healing Clotting occurs, caused by clotting proteins Epithelial cells multiply and fill Restored epthelium thickens; the and plasma proteins, and a scab is formed in over the granulation tissue area matures and contracts Inflammatory chemicals White blood cells seep Granulation tissue restores Underlying area are released from injury _ into the injured area the vascular supply of scar tissue During wound repair, collagen fibers are laid down randomly by fibroblasts that move into repair the area. Note: Oa ec openstax COLLEGE” . 75 . O) a r Watch this video to see a hand heal. Over what period of time do you think these images were taken? Tissue and Aging According to poet Ralph Waldo Emerson, “The surest poison is time.” In fact, biology confirms that many functions of the body decline with age. All the cells, tissues, and organs are affected by senescence, with noticeable variability between individuals owing to different genetic makeup and lifestyles. The outward signs of aging are easily recognizable. The skin and other tissues become thinner and drier, reducing their elasticity, contributing to wrinkles and high blood pressure. Hair turns gray because follicles produce less melanin, the brown pigment of hair and the iris of the eye. The face looks flabby because elastic and collagen fibers decrease in connective tissue and muscle tone is lost. Glasses and hearing aids may become parts of life as the senses slowly deteriorate, all due to reduced elasticity. Overall height decreases as the bones lose calcium and other minerals. With age, fluid decreases in the fibrous cartilage disks intercalated between the vertebrae in the spine. Joints lose cartilage and stiffen. Many tissues, including those in muscles, lose mass through a process called atrophy. Lumps and rigidity become more widespread. As a consequence, the passageways, blood vessels, and airways become more rigid. The brain and spinal cord lose mass. Nerves do not transmit impulses with the same speed and frequency as in the past. Some loss of thought clarity and memory can accompany aging. More severe problems are not necessarily associated with the aging process and may be symptoms of underlying illness. As exterior signs of aging increase, so do the interior signs, which are not as noticeable. The incidence of heart diseases, respiratory syndromes, and type 2 diabetes increases with age, though these are not necessarily age- dependent effects. Wound healing is slower in the elderly, accompanied by a higher frequency of infection as the capacity of the immune system to fend off pathogen declines. Aging is also apparent at the cellular level because all cells experience changes with aging. Telomeres, regions of the chromosomes necessary for cell division, shorten each time cells divide. As they do, cells are less able to divide and regenerate. Because of alterations in cell membranes, transport of oxygen and nutrients into the cell and removal of carbon dioxide and waste products from the cell are not as efficient in the elderly. Cells may begin to function abnormally, which may lead to diseases associated with aging, including arthritis, memory issues, and some cancers. The progressive impact of aging on the body varies considerably among individuals, but Studies indicate, however, that exercise and healthy lifestyle choices can slow down the deterioration of the body that comes with old age. Note: Homeostatic Imbalances Tissues and Cancer Cancer is a generic term for many diseases in which cells escape regulatory signals. Uncontrolled growth, invasion into adjacent tissues, and colonization of other organs, if not treated early enough, are its hallmarks. Health suffers when tumors “rob” blood supply from the “normal” organs. A mutation is defined as a permanent change in the DNA of a cell. Epigenetic modifications, changes that do not affect the code of the DNA but alter how the DNA is decoded, are also known to generate abnormal cells. Alterations in the genetic material may be caused by environmental agents, infectious agents, or errors in the replication of DNA that accumulate with age. Many mutations do not cause any noticeable change in the functions of a cell. However, if the modification affects key proteins that have an impact on the cell’s ability to proliferate in an orderly fashion, the cell starts to divide abnormally. As changes in cells accumulate, they lose their ability to form regular tissues. A tumor, a mass of cells displaying abnormal architecture, forms in the tissue. Many tumors are benign, meaning they do not metastasize nor cause disease. A tumor becomes malignant, or cancerous, when it breaches the confines of its tissue, promotes angiogenesis, attracts the growth of capillaries, and metastasizes to other organs ((link]). The specific names of cancers reflect the tissue of origin. Cancers derived from epithelial cells are referred to as carcinomas. Cancer in myeloid tissue or blood cells form myelomas. Leukemias are cancers of white blood cells, whereas sarcomas derive from connective tissue. Cells in tumors differ both in structure and function. Some cells, called cancer stem cells, appear to be a subtype of cell responsible for uncontrolled growth. Recent research shows that contrary to what was previously assumed, tumors are not disorganized masses of cells, but have their own structures. Development of Cancer Cell division takes place to replace lost tissue Cell division accelerates .—\— WSS SS SW: nN Carcinoma breaks into underlying tissue Underlying tissue Note the change in cell size, nucleus size, and organization in the tissue. Note: Coker aa) ae — wm, OPENSTAX COLLEGE — . - tO) hs Watch this video to learn more about tumors. What is a tumor? Cancer treatments vary depending on the disease’s type and stage. Traditional approaches, including surgery, radiation, chemotherapy, and hormonal therapy, aim to remove or kill rapidly dividing cancer cells, but these strategies have their limitations. Depending on a tumor’s location, for example, cancer surgeons may be unable to remove it. Radiation and chemotherapy are difficult, and it is often impossible to target only the cancer cells. The treatments inevitably destroy healthy tissue as well. To address this, researchers are working on pharmaceuticals that can target specific proteins implicated in cancer-associated molecular pathways. Chapter Review Inflammation is the classic response of the body to injury and follows a common sequence of events. The area is red, feels warm to the touch, swells, and is painful. Injured cells, mast cells, and resident macrophages release chemical signals that cause vasodilation and fluid leakage in the surrounding tissue. The repair phase includes blood clotting, followed by regeneration of tissue as fibroblasts deposit collagen. Some tissues regenerate more readily than others. Epithelial and connective tissues replace damaged or dead cells from a supply of adult stem cells. Muscle and nervous tissues undergo either slow regeneration or do not repair at all. Age affects all the tissues and organs of the body. Damaged cells do not regenerate as rapidly as in younger people. Perception of sensation and effectiveness of response are lost in the nervous system. Muscles atrophy, and bones lose mass and become brittle. Collagen decreases in some connective tissue, and joints stiffen. Interactive Link Questions Exercise: Problem: Watch this video to see a hand heal. Over what period of time do you think these images were taken? Solution: Approximately one month. Exercise: Problem: Watch this video to learn more about tumors. What is a tumor? Solution: A mass of cancer cells that continue to grow and divide. Review Questions Exercise: Problem: Which of the following processes is not a cardinal sign of inflammation? a. redness b. heat c. fever d. swelling Solution: C Exercise: Problem: When a mast cell reacts to an irritation, which of the following chemicals does it release? a. collagen b. histamine c. hyaluronic acid d. meylin Solution: B Exercise: Problem: Atrophy refers to a. loss of elasticity b. loss of mass c. loss of rigidity d. loss of permeability Solution: B Exercise: Problem: Individuals can slow the rate of aging by modifying all of these lifestyle aspects except for a. diet b. exercise c. genetic factors d. stress Solution: C Critical Thinking Questions Exercise: Problem: Why is it important to watch for increased redness, swelling and pain after a cut or abrasion has been cleaned and bandaged? Solution: These symptoms would indicate that infection is present. Exercise: Problem: Aspirin is a non-steroidal anti-inflammatory drug (NSAID) that inhibits the formation of blood clots and is taken regularly by individuals with a heart condition. Steroids such as cortisol are used to control some autoimmune diseases and severe arthritis by down- regulating the inflammatory response. After reading the role of inflammation in the body’s response to infection, can you predict an undesirable consequence of taking anti-inflammatory drugs on a regular basis? Solution: Since NSAIDs or other anti-inflammatory drugs inhibit the formation of blood clots, regular and prolonged use of these drugs may promote internal bleeding, such as bleeding in the stomach. Excessive levels of cortisol would suppress inflammation, which could slow the wound healing process. Exercise: Problem: As an individual ages, a constellation of symptoms begins the decline to the point where an individual’s functioning is compromised. Identify and discuss two factors that have a role in factors leading to the compromised situation. Solution: The genetic makeup and the lifestyle of each individual are factors which determine the degree of decline in cells, tissues, and organs as an individual ages. Exercise: Problem: Discuss changes that occur in cells as a person ages. Solution: All cells experience changes with aging. They become larger, and many cannot divide and regenerate. Because of alterations in cell membranes, transport of oxygen and nutrients into the cell and removal of carbon dioxide and waste products are not as efficient in the elderly. Cells lose their ability to function, or they begin to function abnormally, leading to disease and cancer. References Emerson, RW. Old age. Atlantic. 1862 [cited 2012 Dec 4]; 9(51):134—140. Glossary apoptosis programmed cell death atrophy loss of mass and function clotting also called coagulation; complex process by which blood components form a plug to stop bleeding histamine chemical compound released by mast cells in response to injury that causes vasodilation and endothelium permeability inflammation response of tissue to injury necrosis accidental death of cells and tissues primary union condition of a wound where the wound edges are close enough to be brought together and fastened if necessary, allowing quicker and more thorough healing secondary union wound healing facilitated by wound contraction vasodilation widening of blood vessels wound contraction process whereby the borders of a wound are physically drawn together Layers of the Skin By the end of this section, you will be able to: e Identify the components of the integumentary system Describe the layers of the skin and the functions of each layer e Identify and describe the hypodermis and deep fascia Describe the role of keratinocytes and their life cycle e Describe the role of melanocytes in skin pigmentation Although you may not typically think of the skin as an organ, it is in fact made of tissues that work together as a single structure to perform unique and critical functions. The skin and its accessory structures make up the integumentary system, which provides the body with overall protection. The skin is made of multiple layers of cells and tissues, which are held to underlying structures by connective tissue ({link]). The deeper layer of skin is well vascularized (has numerous blood vessels). It also has numerous sensory, and autonomic and sympathetic nerve fibers ensuring communication to and from the brain. Layers of Skin Hair shaft Pore of sweat gland duct Epidermis | Arrector pili muscle Hair follicle Sebaceous (oil) gland Hypodermis Hair root Hair follicle receptor Eccrine sweat gland Pacinian corpuscle Cutaneous vascular plexus Adipose tissue Sensory nerve fiber The skin is composed of two main layers: the epidermis, made of closely packed epithelial cells, and the dermis, made of dense, irregular connective tissue that houses blood vessels, hair follicles, sweat glands, and other structures. Beneath the dermis lies the hypodermis, which is composed mainly of loose connective and fatty tissues. ee mess’ OPENStAX COLLEGE” i io ae T The skin consists of two main layers and a closely associated layer. View this animation to learn more about layers of the skin. What are the basic functions of each of these layers? The Epidermis The epidermis is composed of keratinized, stratified squamous epithelium. It is made of four or five layers of epithelial cells, depending on its location in the body. It does not have any blood vessels within it (i.e., it is avascular). Skin that has four layers of cells is referred to as “thin skin.” From deep to superficial, these layers are the stratum basale, stratum spinosum, stratum granulosum, and stratum corneum. Most of the skin can be classified as thin skin. “Thick skin” is found only on the palms of the hands and the soles of the feet. It has a fifth layer, called the stratum lucidum, located between the stratum corneum and the stratum granulosum (Llink]). Thin Skin versus Thick Skin These slides show cross- sections of the epidermis and dermis of (a) thin and (b) thick skin. Note the significant difference in the thickness of the epithelial layer of the thick skin. From top, LM x 40, LM ~x 40. (Micrographs provided by the Regents of University of Michigan Medical School © 2012) The cells in all of the layers except the stratum basale are called keratinocytes. A keratinocyte is a cell that manufactures and stores the protein keratin. Keratin is an intracellular fibrous protein that gives hair, nails, and skin their hardness and water-resistant properties. The keratinocytes in the stratum corneum are dead and regularly slough away, being replaced by cells from the deeper layers ({link]). Epidermis The epidermis is epithelium composed of multiple layers of cells. The basal layer consists of cuboidal cells, whereas the outer layers are squamous, keratinized cells, so the whole epithelium is often described as being keratinized stratified squamous epithelium. LM x 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: . . openstax COLLEGE View the University of Michigan WebScope to explore the tissue sample in greater detail. If you zoom on the cells at the outermost layer of this section of skin, what do you notice about the cells? Stratum Basale The stratum basale (also called the stratum germinativum) is the deepest epidermal layer and attaches the epidermis to the basal lamina, below which lie the layers of the dermis. The cells in the stratum basale bond to the dermis via intertwining collagen fibers, referred to as the basement membrane. A finger-like projection, or fold, known as the dermal papilla (plural = dermal papillae) is found in the superficial portion of the dermis. Dermal papillae increase the strength of the connection between the epidermis and dermis; the greater the folding, the stronger the connections made ([link]). Layers of the Epidermis Dead cells filled with keratin 5 = Stratum corneum = eS =o SSSqNqBPHOO Stratum lucidum ——_{_ Stratum granulosum : ——_,_ Stratum spinosum = Stratum basale — j Melanocyte Lamellar granules Keratinocyte Merkel cell : Sensory neuron Dermis The epidermis of thick skin has five layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum basale is a single layer of cells primarily made of basal cells. A basal cell is a cuboidal-shaped stem cell that is a precursor of the keratinocytes of the epidermis. All of the keratinocytes are produced from this single layer of cells, which are constantly going through mitosis to produce new cells. As new cells are formed, the existing cells are pushed superficially away from the stratum basale. Two other cell types are found dispersed among the basal cells in the stratum basale. The first is a Merkel cell, which functions as a receptor and is responsible for stimulating sensory nerves that the brain perceives as touch. These cells are especially abundant on the surfaces of the hands and feet. The second is a melanocyte, a cell that produces the pigment melanin. Melanin gives hair and skin its color, and also helps protect the living cells of the epidermis from ultraviolet (UV) radiation damage. In a growing fetus, fingerprints form where the cells of the stratum basale meet the papillae of the underlying dermal layer (papillary layer), resulting in the formation of the ridges on your fingers that you recognize as fingerprints. Fingerprints are unique to each individual and are used for forensic analyses because the patterns do not change with the growth and aging processes. Stratum Spinosum As the name suggests, the stratum spinosum is spiny in appearance due to the protruding cell processes that join the cells via a structure called a desmosome. The desmosomes interlock with each other and strengthen the bond between the cells. It is interesting to note that the “spiny” nature of this layer is an artifact of the staining process. Unstained epidermis samples do not exhibit this characteristic appearance. The stratum spinosum is composed of eight to 10 layers of keratinocytes, formed as a result of cell division in the stratum basale ([link]). Interspersed among the keratinocytes of this layer is a type of dendritic cell called the Langerhans cell, which functions as a macrophage by engulfing bacteria, foreign particles, and damaged cells that occur in this layer. Cells of the Epidermis 065 - Epidermis_001.svs WebScope 1 16013 x 16013 size 733.61MB mag 20X Return to image directory (© fale "SSB Oo Gm O OVE | Epiaermis Stratum ,, corneum i Stratum ~ granulosum Remnants of cross-sectioned Stratum shed hair and spinosum its follicle Stratum basal or germinativum Capillai Dermi poy EJM Mag. 2,700 X The cells in the different layers of the epidermis originate from basal cells located in the stratum basale, yet the cells of each layer are distinctively different. EM x 2700. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Note: poe openstax COLLEGE” View the University of Michigan WebScope to explore the tissue sample in greater detail. If you zoom on the cells at the outermost layer of this section of skin, what do you notice about the cells? The keratinocytes in the stratum spinosum begin the synthesis of keratin and release a water-repelling glycolipid that helps prevent water loss from the body, making the skin relatively waterproof. As new keratinocytes are produced atop the stratum basale, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum. Stratum Granulosum The stratum granulosum has a grainy appearance due to further changes to the keratinocytes as they are pushed from the stratum spinosum. The cells (three to five layers deep) become flatter, their cell membranes thicken, and they generate large amounts of the proteins keratin, which is fibrous, and keratohyalin, which accumulates as lamellar granules within the cells (see [link]). These two proteins make up the bulk of the keratinocyte mass in the stratum granulosum and give the layer its grainy appearance. The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that will form the stratum lucidum, the stratum comeum, and the accessory structures of hair and nails. Stratum Lucidum The stratum lucidum is a smooth, seemingly translucent layer of the epidermis located just above the stratum granulosum and below the stratum corneum. This thin layer of cells is found only in the thick skin of the palms, soles, and digits. The keratinocytes that compose the stratum lucidum are dead and flattened (see [link]). These cells are densely packed with eleiden, a clear protein rich in lipids, derived from keratohyalin, which gives these cells their transparent (i.e., lucid) appearance and provides a barrier to water. Stratum Corneum The stratum corneum is the most superficial layer of the epidermis and is the layer exposed to the outside environment (see [link]). The increased keratinization (also called cornification) of the cells in this layer gives it its name. There are usually 15 to 30 layers of cells in the stratum corneum. This dry, dead layer helps prevent the penetration of microbes and the dehydration of underlying tissues, and provides a mechanical protection against abrasion for the more delicate, underlying layers. Cells in this layer are shed periodically and are replaced by cells pushed up from the stratum granulosum (or stratum lucidum in the case of the palms and soles of feet). The entire layer is replaced during a period of about 4 weeks. Cosmetic procedures, such as microdermabrasion, help remove some of the dry, upper layer and aim to keep the skin looking “fresh” and healthy. Dermis The dermis might be considered the “core” of the integumentary system (derma- = “skin”), as distinct from the epidermis (epi- = “upon” or “over” and hypodermis (hypo- = “below”). It contains blood and lymph vessels, nerves, and other structures, such as hair follicles and sweat glands. The dermis is made of two layers of connective tissue that compose an interconnected mesh of elastin and collagenous fibers, produced by fibroblasts ({link]). Layers of the Dermis This stained slide shows the two components of the dermis—the papillary layer and the reticular layer. Both are made of connective tissue with fibers of collagen extending from one to the other, making the border between the two somewhat indistinct. The dermal papillae extending into the epidermis belong to the papillary layer, whereas the dense collagen fiber bundles below belong to the reticular layer. LM x 10. (credit: modification of work by “kilbad”/Wikimedia Commons) Papillary Layer The papillary layer is made of loose, areolar connective tissue, which means the collagen and elastin fibers of this layer form a loose mesh. This superficial layer of the dermis projects into the stratum basale of the epidermis to form finger-like dermal papillae (see [link]). Within the papillary layer are fibroblasts, a small number of fat cells (adipocytes), and an abundance of small blood vessels. In addition, the papillary layer contains phagocytes, defensive cells that help fight bacteria or other infections that have breached the skin. This layer also contains lymphatic capillaries, nerve fibers, and touch receptors called the Meissner corpuscles. Reticular Layer Underlying the papillary layer is the much thicker reticular layer, composed of dense, irregular connective tissue. This layer is well vascularized and has a rich sensory and sympathetic nerve supply. The reticular layer appears reticulated (net-like) due to a tight meshwork of fibers. Elastin fibers provide some elasticity to the skin, enabling movement. Collagen fibers provide structure and tensile strength, with strands of collagen extending into both the papillary layer and the hypodermis. In addition, collagen binds water to keep the skin hydrated. Collagen injections and Retin-A creams help restore skin turgor by either introducing collagen externally or stimulating blood flow and repair of the dermis, respectively. Hypodermis The hypodermis (also called the subcutaneous layer or superficial fascia) is a layer directly below the dermis and serves to connect the skin to the underlying fascia (fibrous tissue) of the bones and muscles. It is not strictly a part of the skin, although the border between the hypodermis and dermis can be difficult to distinguish. The hypodermis consists of well- vascularized, loose, areolar connective tissue and adipose tissue, which functions as a mode of fat storage and provides insulation and cushioning for the integument. Note: Everyday Connection Lipid Storage The hypodermis is home to most of the fat that concerns people when they are trying to keep their weight under control. Adipose tissue present in the hypodermis consists of fat-storing cells called adipocytes. This stored fat can serve as an energy reserve, insulate the body to prevent heat loss, and act as a cushion to protect underlying structures from trauma. Where the fat is deposited and accumulates within the hypodermis depends on hormones (testosterone, estrogen, insulin, glucagon, leptin, and others), as well as genetic factors. Fat distribution changes as our bodies mature and age. Men tend to accumulate fat in different areas (neck, arms, lower back, and abdomen) than do women (breasts, hips, thighs, and buttocks). The body mass index (BMI) is often used as a measure of fat, although this measure is, in fact, derived from a mathematical formula that compares body weight (mass) to height. Therefore, its accuracy as a health indicator can be called into question in individuals who are extremely physically fit. In many animals, there is a pattern of storing excess calories as fat to be used in times when food is not readily available. In much of the developed world, insufficient exercise coupled with the ready availability and consumption of high-calorie foods have resulted in unwanted accumulations of adipose tissue in many people. Although periodic accumulation of excess fat may have provided an evolutionary advantage to our ancestors, who experienced unpredictable bouts of famine, it is now becoming chronic and considered a major health threat. Recent studies indicate that a distressing percentage of our population is overweight and/or clinically obese. Not only is this a problem for the individuals affected, but it also has a severe impact on our healthcare system. Changes in lifestyle, specifically in diet and exercise, are the best ways to control body fat accumulation, especially when it reaches levels that increase the risk of heart disease and diabetes. Pigmentation The color of skin is influenced by a number of pigments, including melanin, carotene, and hemoglobin. Recall that melanin is produced by cells called melanocytes, which are found scattered throughout the stratum basale of the epidermis. The melanin is transferred into the keratinocytes via a cellular vesicle called a melanosome ((link]). Skin Pigmentation Surface Upper keratinocytes Melanosomes Basal keratinocytes Melanocytes The relative coloration of the skin depends of the amount of melanin produced by melanocytes in the stratum basale and taken up by keratinocytes. Melanin occurs in two primary forms. Eumelanin exists as black and brown, whereas pheomelanin provides a red color. Dark-skinned individuals produce more melanin than those with pale skin. Exposure to the UV rays of the sun or a tanning salon causes melanin to be manufactured and built up in keratinocytes, as sun exposure stimulates keratinocytes to secrete chemicals that stimulate melanocytes. The accumulation of melanin in keratinocytes results in the darkening of the skin, or a tan. This increased melanin accumulation protects the DNA of epidermal cells from UV ray damage and the breakdown of folic acid, a nutrient necessary for our health and well-being. In contrast, too much melanin can interfere with the production of vitamin D, an important nutrient involved in calcium absorption. Thus, the amount of melanin present in our skin is dependent on a balance between available sunlight and folic acid destruction, and protection from UV radiation and vitamin D production. It requires about 10 days after initial sun exposure for melanin synthesis to peak, which is why pale-skinned individuals tend to suffer sunburns of the epidermis initially. Dark-skinned individuals can also get sunburns, but are more protected than are pale-skinned individuals. Melanosomes are temporary structures that are eventually destroyed by fusion with lysosomes; this fact, along with melanin-filled keratinocytes in the stratum corneum sloughing off, makes tanning impermanent. Too much sun exposure can eventually lead to wrinkling due to the destruction of the cellular structure of the skin, and in severe cases, can cause sufficient DNA damage to result in skin cancer. When there is an irregular accumulation of melanocytes in the skin, freckles appear. Moles are larger masses of melanocytes, and although most are benign, they should be monitored for changes that might indicate the presence of cancer ({link]). Moles Moles range from benign accumulations of melanocytes to melanomas. These structures populate the landscape of our skin. (credit: the National Cancer Institute) Note: Disorders of the... Integumentary System The first thing a clinician sees is the skin, and so the examination of the skin should be part of any thorough physical examination. Most skin disorders are relatively benign, but a few, including melanomas, can be fatal if untreated. A couple of the more noticeable disorders, albinism and vitiligo, affect the appearance of the skin and its accessory organs. Although neither is fatal, it would be hard to claim that they are benign, at least to the individuals so afflicted. Albinism is a genetic disorder that affects (completely or partially) the coloring of skin, hair, and eyes. The defect is primarily due to the inability of melanocytes to produce melanin. Individuals with albinism tend to appear white or very pale due to the lack of melanin in their skin and hair. Recall that melanin helps protect the skin from the harmful effects of UV radiation. Individuals with albinism tend to need more protection from UV radiation, as they are more prone to sunburns and skin cancer. They also tend to be more sensitive to light and have vision problems due to the lack of pigmentation on the retinal wall. Treatment of this disorder usually involves addressing the symptoms, such as limiting UV light exposure to the skin and eyes. In vitiligo, the melanocytes in certain areas lose their ability to produce melanin, possibly due to an autoimmune reaction. This leads to a loss of color in patches ({link]). Neither albinism nor vitiligo directly affects the lifespan of an individual. Vitiligo Individuals with vitiligo experience depigmentation that results in lighter colored patches of skin. The condition is especially noticeable on darker skin. (credit: Klaus D. Peter) Other changes in the appearance of skin coloration can be indicative of diseases associated with other body systems. Liver disease or liver cancer can cause the accumulation of bile and the yellow pigment bilirubin, leading to the skin appearing yellow or jaundiced (jaune is the French word for “yellow”). Tumors of the pituitary gland can result in the secretion of large amounts of melanocyte-stimulating hormone (MSH), which results in a darkening of the skin. Similarly, Addison’s disease can stimulate the release of excess amounts of adrenocorticotropic hormone (ACTH), which can give the skin a deep bronze color. A sudden drop in oxygenation can affect skin color, causing the skin to initially turn ashen (white). With a prolonged reduction in oxygen levels, dark red deoxyhemoglobin becomes dominant in the blood, making the skin appear blue, a condition referred to as cyanosis (kyanos is the Greek word for “blue”). This happens when the oxygen supply is restricted, as when someone is experiencing difficulty in breathing because of asthma or a heart attack. However, in these cases the effect on skin color has nothing do with the skin’s pigmentation. mess’ OPENStAX COLLEGE” seh : T This ABC video follows the story of a pair of fraternal African-American twins, one of whom is albino. Watch this video to learn about the challenges these children and their family face. Which ethnicities do you think are exempt from the possibility of albinism? Chapter Review The skin is composed of two major layers: a superficial epidermis and a deeper dermis. The epidermis consists of several layers beginning with the innermost (deepest) stratum basale (germinatum), followed by the stratum spinosum, stratum granulosum, stratum lucidum (when present), and ending with the outermost layer, the stratum corneum. The topmost layer, the stratum corneum, consists of dead cells that shed periodically and is progressively replaced by cells formed from the basal layer. The stratum basale also contains melanocytes, cells that produce melanin, the pigment primarily responsible for giving skin its color. Melanin is transferred to keratinocytes in the stratum spinosum to protect cells from UV rays. The dermis connects the epidermis to the hypodermis, and provides strength and elasticity due to the presence of collagen and elastin fibers. It has only two layers: the papillary layer with papillae that extend into the epidermis and the lower, reticular layer composed of loose connective tissue. The hypodermis, deep to the dermis of skin, is the connective tissue that connects the dermis to underlying structures; it also harbors adipose tissue for fat storage and protection. Interactive Link Questions Exercise: Problem: The skin consists of two layers and a closely associated layer. View this animation to learn more about layers of the skin. What are the basic functions of each of these layers? Solution: The epidermis provides protection, the dermis provides support and flexibility, and the hypodermis (fat layer) provides insulation and padding. Exercise: Problem: [link] If you zoom on the cells at the outermost layer of this section of skin, what do you notice about the cells? Solution: [link] These cells do not have nuclei, so you can deduce that they are dead. They appear to be sloughing off. Exercise: Problem: [link] If you zoom on the cells of the stratum spinosum, what is distinctive about them? Solution: [link] These cells have desmosomes, which give the cells their spiny appearance. Exercise: Problem: This ABC video follows the story of a pair of fraternal African- American twins, one of whom is albino. Watch this video to learn about the challenges these children and their family face. Which ethnicities do you think are exempt from the possibility of albinism? Solution: There are none. Review Questions Exercise: Problem: The papillary layer of the dermis is most closely associated with which layer of the epidermis? a. stratum spinosum b. stratum corneum c. stratum granulosum d. stratum basale Solution: D Exercise: Problem:Langerhans cells are commonly found in the a. stratum spinosum b. stratum corneum c. stratum granulosum d. stratum basale Solution: A Exercise: Problem: The papillary and reticular layers of the dermis are composed mainly of a. melanocytes b. keratinocytes c. connective tissue d. adipose tissue Solution: C Exercise: Problem: Collagen lends to the skin. a. elasticity b. structure c. color d. UV protection Solution: B Exercise: Problem: Which of the following is not a function of the hypodermis? a. protects underlying organs b. helps maintain body temperature c. source of blood vessels in the epidermis d. a site to long-term energy storage Solution: C Critical Thinking Questions Exercise: Problem: What determines the color of skin, and what is the process that darkens skin when it is exposed to UV light? Solution: The pigment melanin, produced by melanocytes, is primarily responsible for skin color. Melanin comes in different shades of brown and black. Individuals with darker skin have darker, more abundant melanin, whereas fair-skinned individuals have a lighter shade of skin and less melanin. Exposure to UV irradiation stimulates the melanocytes to produce and secrete more melanin. Exercise: Problem: Cells of the epidermis derive from stem cells of the stratum basale. Describe how the cells change as they become integrated into the different layers of the epidermis. Solution: As the cells move into the stratum spinosum, they begin the synthesis of keratin and extend cell processes, desmosomes, which link the cells. As the stratum basale continues to produce new cells, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum. The cells become flatter, their cell membranes thicken, and they generate large amounts of the proteins keratin and keratohyalin. The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that form the stratum lucidum and the stratum corneum. The keratinocytes in these layers are mostly dead and flattened. Cells in the stratum corneum are periodically shed. Glossary albinism genetic disorder that affects the skin, in which there is no melanin production basal cell type of stem cell found in the stratum basale and in the hair matrix that continually undergoes cell division, producing the keratinocytes of the epidermis dermal papilla (plural = dermal papillae) extension of the papillary layer of the dermis that increases surface contact between the epidermis and dermis dermis layer of skin between the epidermis and hypodermis, composed mainly of connective tissue and containing blood vessels, hair follicles, sweat glands, and other structures desmosome structure that forms an impermeable junction between cells elastin fibers fibers made of the protein elastin that increase the elasticity of the dermis eleiden clear protein-bound lipid found in the stratum lucidum that is derived from keratohyalin and helps to prevent water loss epidermis outermost tissue layer of the skin hypodermis connective tissue connecting the integument to the underlying bone and muscle integumentary system skin and its accessory structures keratin type of structural protein that gives skin, hair, and nails its hard, water- resistant properties keratinocyte cell that produces keratin and is the most predominant type of cell found in the epidermis keratohyalin granulated protein found in the stratum granulosum Langerhans cell specialized dendritic cell found in the stratum spinosum that functions as a macrophage melanin pigment that determines the color of hair and skin melanocyte cell found in the stratum basale of the epidermis that produces the pigment melanin melanosome intercellular vesicle that transfers melanin from melanocytes into keratinocytes of the epidermis Merkel cell receptor cell in the stratum basale of the epidermis that responds to the sense of touch papillary layer superficial layer of the dermis, made of loose, areolar connective tissue reticular layer deeper layer of the dermis; it has a reticulated appearance due to the presence of abundant collagen and elastin fibers stratum basale deepest layer of the epidermis, made of epidermal stem cells stratum corneum most superficial layer of the epidermis stratum granulosum layer of the epidermis superficial to the stratum spinosum stratum lucidum layer of the epidermis between the stratum granulosum and stratum corneum, found only in thick skin covering the palms, soles of the feet, and digits stratum spinosum layer of the epidermis superficial to the stratum basale, characterized by the presence of desmosomes vitiligo skin condition in which melanocytes in certain areas lose the ability to produce melanin, possibly due an autoimmune reaction that leads to loss of color in patches Accessory Structures of the Skin By the end of this section, you will be able to: e Identify the accessory structures of the skin e Describe the structure and function of hair and nails e Describe the structure and function of sweat glands and sebaceous glands Accessory structures of the skin include hair, nails, sweat glands, and sebaceous glands. These structures embryologically originate from the epidermis and can extend down through the dermis into the hypodermis. Hair Hair is a keratinous filament growing out of the epidermis. It is primarily made of dead, keratinized cells. Strands of hair originate in an epidermal penetration of the dermis called the hair follicle. The hair shaft is the part of the hair not anchored to the follicle, and much of this is exposed at the skin’s surface. The rest of the hair, which is anchored in the follicle, lies below the surface of the skin and is referred to as the hair root. The hair root ends deep in the dermis at the hair bulb, and includes a layer of mitotically active basal cells called the hair matrix. The hair bulb surrounds the hair papilla, which is made of connective tissue and contains blood capillaries and nerve endings from the dermis ([link]). Hair Medulla Cortex Cuticle Sebaceous gland Inner root sheath Outer root sheath Hair matrix Hair papilla Hair follicles originate in the epidermis and have many different parts. Just as the basal layer of the epidermis forms the layers of epidermis that get pushed to the surface as the dead skin on the surface sheds, the basal cells of the hair bulb divide and push cells outward in the hair root and shaft as the hair grows. The medulla forms the central core of the hair, which is surrounded by the cortex, a layer of compressed, keratinized cells that is covered by an outer layer of very hard, keratinized cells known as the cuticle. These layers are depicted in a longitudinal cross-section of the hair follicle ([link]), although not all hair has a medullary layer. Hair texture (straight, curly) is determined by the shape and structure of the cortex, and to the extent that it is present, the medulla. The shape and structure of these layers are, in turn, determined by the shape of the hair follicle. Hair growth begins with the production of keratinocytes by the basal cells of the hair bulb. As new cells are deposited at the hair bulb, the hair shaft is pushed through the follicle toward the surface. Keratinization is completed as the cells are pushed to the skin surface to form the shaft of hair that is externally visible. The external hair is completely dead and composed entirely of keratin. For this reason, our hair does not have sensation. Furthermore, you can cut your hair or shave without damaging the hair structure because the cut is superficial. Most chemical hair removers also act superficially; however, electrolysis and yanking both attempt to destroy the hair bulb so hair cannot grow. Hair Follicle The slide shows a cross-section of a hair follicle. Basal cells of the hair matrix in the center differentiate into cells of the inner root sheath. Basal cells at the base of the hair root form the outer root sheath. LM x 4. (credit: modification of work by “kilbad”/Wikimedia Commons) The wall of the hair follicle is made of three concentric layers of cells. The cells of the internal root sheath surround the root of the growing hair and extend just up to the hair shaft. They are derived from the basal cells of the hair matrix. The external root sheath, which is an extension of the epidermis, encloses the hair root. It is made of basal cells at the base of the hair root and tends to be more keratinous in the upper regions. The glassy membrane is a thick, clear connective tissue sheath covering the hair root, connecting it to the tissue of the dermis. Note: Dasara — meee, OPENStAX COLLEGE —— . The hair follicle is made of multiple layers of cells that form from basal cells in the hair matrix and the hair root. Cells of the hair matrix divide and differentiate to form the layers of the hair. Watch this video to learn more about hair follicles. Hair serves a variety of functions, including protection, sensory input, thermoregulation, and communication. For example, hair on the head protects the skull from the sun. The hair in the nose and ears, and around the eyes (eyelashes) defends the body by trapping and excluding dust particles that may contain allergens and microbes. Hair of the eyebrows prevents sweat and other particles from dripping into and bothering the eyes. Hair also has a sensory function due to sensory innervation by a hair root plexus surrounding the base of each hair follicle. Hair is extremely sensitive to air movement or other disturbances in the environment, much more so than the skin surface. This feature is also useful for the detection of the presence of insects or other potentially damaging substances on the skin surface. Each hair root is connected to a smooth muscle called the arrector pili that contracts in response to nerve signals from the sympathetic nervous system, making the external hair shaft “stand up.” The primary purpose for this is to trap a layer of air to add insulation. This is visible in humans as goose bumps and even more obvious in animals, such as when a frightened cat raises its fur. Of course, this is much more obvious in organisms with a heavier coat than most humans, such as dogs and cats. Hair Growth Hair grows and is eventually shed and replaced by new hair. This occurs in three phases. The first is the anagen phase, during which cells divide rapidly at the root of the hair, pushing the hair shaft up and out. The length of this phase is measured in years, typically from 2 to 7 years. The catagen phase lasts only 2 to 3 weeks, and marks a transition from the hair follicle’s active growth. Finally, during the telogen phase, the hair follicle is at rest and no new growth occurs. At the end of this phase, which lasts about 2 to 4 months, another anagen phase begins. The basal cells in the hair matrix then produce a new hair follicle, which pushes the old hair out as the growth cycle repeats itself. Hair typically grows at the rate of 0.3 mm per day during the anagen phase. On average, 50 hairs are lost and replaced per day. Hair loss occurs if there is more hair shed than what is replaced and can happen due to hormonal or dietary changes. Hair loss can also result from the aging process, or the influence of hormones. Hair Color Similar to the skin, hair gets its color from the pigment melanin, produced by melanocytes in the hair papilla. Different hair color results from differences in the type of melanin, which is genetically determined. As a person ages, the melanin production decreases, and hair tends to lose its color and becomes gray and/or white. Nails The nail bed is a specialized structure of the epidermis that is found at the tips of our fingers and toes. The nail body is formed on the nail bed, and protects the tips of our fingers and toes as they are the farthest extremities and the parts of the body that experience the maximum mechanical stress ({link]). In addition, the nail body forms a back-support for picking up small objects with the fingers. The nail body is composed of densely packed dead keratinocytes. The epidermis in this part of the body has evolved a specialized structure upon which nails can form. The nail body forms at the nail root, which has a matrix of proliferating cells from the stratum basale that enables the nail to grow continuously. The lateral nail fold overlaps the nail on the sides, helping to anchor the nail body. The nail fold that meets the proximal end of the nail body forms the nail cuticle, also called the eponychium. The nail bed is rich in blood vessels, making it appear pink, except at the base, where a thick layer of epithelium over the nail matrix forms a crescent-shaped region called the lunula (the “little moon”). The area beneath the free edge of the nail, furthest from the cuticle, is called the hyponychium. It consists of a thickened layer of stratum corneum. Nails Free edge Eponychium a ee Proximal nail fold . Lunula Nail / Nail body Lateral nail fold Lunula Eponychium Proximal nail fold Epidermis Dermis Phalanx Hyponychium The nail is an accessory structure of the integumentary system. Note: — e meee <OPENStAX COLLEGE” — wy oe atten Nails are accessory structures of the integumentary system. Visit this link to learn more about the origin and growth of fingernails. Sweat Glands When the body becomes warm, sudoriferous glands produce sweat to cool the body. Sweat glands develop from epidermal projections into the dermis and are classified as merocrine glands; that is, the secretions are excreted by exocytosis through a duct without affecting the cells of the gland. There are two types of sweat glands, each secreting slightly different products. An eccrine sweat gland is type of gland that produces a hypotonic sweat for thermoregulation. These glands are found all over the skin’s surface, but are especially abundant on the palms of the hand, the soles of the feet, and the forehead ([link]). They are coiled glands lying deep in the dermis, with the duct rising up to a pore on the skin surface, where the sweat is released. This type of sweat, released by exocytosis, is hypotonic and composed mostly of water, with some salt, antibodies, traces of metabolic waste, and dermicidin, an antimicrobial peptide. Eccrine glands are a primary component of thermoregulation in humans and thus help to maintain homeostasis. Eccrine Gland Skin surface Eccrine sweat gland Eccrine glands are coiled glands in the dermis that release sweat that is mostly water. An apocrine sweat gland is usually associated with hair follicles in densely hairy areas, such as armpits and genital regions. Apocrine sweat glands are larger than eccrine sweat glands and lie deeper in the dermis, sometimes even reaching the hypodermis, with the duct normally emptying into the hair follicle. In addition to water and salts, apocrine sweat includes organic compounds that make the sweat thicker and subject to bacterial decomposition and subsequent smell. The release of this sweat is under both nervous and hormonal control, and plays a role in the poorly understood human pheromone response. Most commercial antiperspirants use an aluminum-based compound as their primary active ingredient to stop sweat. When the antiperspirant enters the sweat gland duct, the aluminum- based compounds precipitate due to a change in pH and form a physical block in the duct, which prevents sweat from coming out of the pore. — aa COLLEGE” Sweating regulates body temperature. The composition of the sweat determines whether body odor is a byproduct of sweating. Visit this link to learn more about sweating and body odor. Sebaceous Glands A sebaceous gland is a type of oil gland that is found all over the body and helps to lubricate and waterproof the skin and hair. Most sebaceous glands are associated with hair follicles. They generate and excrete sebum, a mixture of lipids, onto the skin surface, thereby naturally lubricating the dry and dead layer of keratinized cells of the stratum corneum, keeping it pliable. The fatty acids of sebum also have antibacterial properties, and prevent water loss from the skin in low-humidity environments. The secretion of sebum is stimulated by hormones, many of which do not become active until puberty. Thus, sebaceous glands are relatively inactive during childhood. Chapter Review Accessory structures of the skin include hair, nails, sweat glands, and sebaceous glands. Hair is made of dead keratinized cells, and gets its color from melanin pigments. Nails, also made of dead keratinized cells, protect the extremities of our fingers and toes from mechanical damage. Sweat glands and sebaceous glands produce sweat and sebum, respectively. Each of these fluids has a role to play in maintaining homeostasis. Sweat cools the body surface when it gets overheated and helps excrete small amounts of metabolic waste. Sebum acts as a natural moisturizer and keeps the dead, flaky, outer keratin layer healthy. Review Questions Exercise: Problem: In response to stimuli from the sympathetic nervous system, the arrector pili a. are glands on the skin surface b. can lead to excessive sweating c. are responsible for goose bumps d. secrete sebum Solution: C Exercise: Problem:The hair matrix contains a. the hair follicle b. the hair shaft c. the glassy membrane d. a layer of basal cells Solution: D Exercise: Problem:Eccrine sweat glands a. are present on hair b. are present in the skin throughout the body and produce watery Sweat c. produce sebum d. act as a moisturizer Solution: B Exercise: Problem: Sebaceous glands a. are a type of sweat gland b. are associated with hair follicles c. may function in response to touch d. release a watery solution of salt and metabolic waste Solution: B Exercise: Problem: Similar to the hair, nails grow continuously throughout our lives. Which of the following is furthest from the nail growth center? a. nail bed b. hyponychium c. nail root d. eponychium Solution: B Critical Thinking Questions Exercise: Problem: Explain the differences between eccrine and apocrine sweat glands. Solution: Eccrine sweat glands are all over the body, especially the forehead and palms of the hand. They release a watery sweat, mixed with some metabolic waste and antibodies. Apocrine glands are associated with hair follicles. They are larger than eccrine sweat glands and lie deeper in the dermis, sometimes even reaching the hypodermis. They release a thicker sweat that is often decomposed by bacteria on the skin, resulting in an unpleasant odor. Exercise: Problem: Describe the structure and composition of nails. Solution: Nails are composed of densely packed dead keratinocytes. They protect the fingers and toes from mechanical stress. The nail body is formed on the nail bed, which is at the nail root. Nail folds, folds of skin that overlap the nail on its side, secure the nail to the body. The crescent-shaped region at the base of the nail is the lunula. Glossary anagen active phase of the hair growth cycle apocrine sweat gland type of sweat gland that is associated with hair follicles in the armpits and genital regions arrector pili smooth muscle that is activated in response to external stimuli that pull on hair follicles and make the hair “stand up” catagen transitional phase marking the end of the anagen phase of the hair growth cycle cortex in hair, the second or middle layer of keratinocytes originating from the hair matrix, as seen in a cross-section of the hair bulb cuticle in hair, the outermost layer of keratinocytes originating from the hair matrix, as seen in a cross-section of the hair bulb eccrine sweat gland type of sweat gland that is common throughout the skin surface; it produces a hypotonic sweat for thermoregulation eponychium nail fold that meets the proximal end of the nail body, also called the cuticle external root sheath outer layer of the hair follicle that is an extension of the epidermis, which encloses the hair root glassy membrane layer of connective tissue that surrounds the base of the hair follicle, connecting it to the dermis hair keratinous filament growing out of the epidermis hair bulb structure at the base of the hair root that surrounds the dermal papilla hair follicle cavity or sac from which hair originates hair matrix layer of basal cells from which a strand of hair grows hair papilla mass of connective tissue, blood capillaries, and nerve endings at the base of the hair follicle hair root part of hair that is below the epidermis anchored to the follicle hair shaft part of hair that is above the epidermis but is not anchored to the follicle hyponychium thickened layer of stratum corneum that lies below the free edge of the nail internal root sheath innermost layer of keratinocytes in the hair follicle that surround the hair root up to the hair shaft lunula basal part of the nail body that consists of a crescent-shaped layer of thick epithelium medulla in hair, the innermost layer of keratinocytes originating from the hair matrix nail bed layer of epidermis upon which the nail body forms nail body main keratinous plate that forms the nail nail cuticle fold of epithelium that extends over the nail bed, also called the eponychium nail fold fold of epithelium at that extend over the sides of the nail body, holding it in place nail root part of the nail that is lodged deep in the epidermis from which the nail gTOWS sebaceous gland type of oil gland found in the dermis all over the body and helps to lubricate and waterproof the skin and hair by secreting sebum sebum oily substance that is composed of a mixture of lipids that lubricates the skin and hair sudoriferous gland sweat gland telogen resting phase of the hair growth cycle initiated with catagen and terminated by the beginning of a new anagen phase of hair growth Diseases, Disorders, and Injuries of the Integumentary System By the end of this section, you will be able to: e Describe several different diseases and disorders of the skin e Describe the effect of injury to the skin and the process of healing The integumentary system is susceptible to a variety of diseases, disorders, and injuries. These range from annoying but relatively benign bacterial or fungal infections that are categorized as disorders, to skin cancer and severe burns, which can be fatal. In this section, you will learn several of the most common skin conditions. Diseases One of the most talked about diseases is skin cancer. Cancer is a broad term that describes diseases caused by abnormal cells in the body dividing uncontrollably. Most cancers are identified by the organ or tissue in which the cancer originates. One common form of cancer is skin cancer. The Skin Cancer Foundation reports that one in five Americans will experience some type of skin cancer in their lifetime. The degradation of the ozone layer in the atmosphere and the resulting increase in exposure to UV radiation has contributed to its rise. Overexposure to UV radiation damages DNA, which can lead to the formation of cancerous lesions. Although melanin offers some protection against DNA damage from the sun, often it is not enough. The fact that cancers can also occur on areas of the body that are normally not exposed to UV radiation suggests that there are additional factors that can lead to cancerous lesions. In general, cancers result from an accumulation of DNA mutations. These mutations can result in cell populations that do not die when they should and uncontrolled cell proliferation that leads to tumors. Although many tumors are benign (harmless), some produce cells that can mobilize and establish tumors in other organs of the body; this process is referred to as metastasis. Cancers are characterized by their ability to metastasize. Basal Cell Carcinoma Basal cell carcinoma is a form of cancer that affects the mitotically active stem cells in the stratum basale of the epidermis. It is the most common of all cancers that occur in the United States and is frequently found on the head, neck, arms, and back, which are areas that are most susceptible to long-term sun exposure. Although UV rays are the main culprit, exposure to other agents, such as radiation and arsenic, can also lead to this type of cancer. Wounds on the skin due to open sores, tattoos, burns, etc. may be predisposing factors as well. Basal cell carcinomas start in the stratum basale and usually spread along this boundary. At some point, they begin to grow toward the surface and become an uneven patch, bump, growth, or scar on the skin surface ([link]). Like most cancers, basal cell carcinomas respond best to treatment when caught early. Treatment options include surgery, freezing (cryosurgery), and topical ointments (Mayo Clinic 2012). Basal Cell Carcinoma Basal cell carcinoma can take several different forms. Similar to other forms of skin cancer, it is readily cured if caught early and treated. (credit: John Hendrix, MD) Squamous Cell Carcinoma Squamous cell carcinoma is a cancer that affects the keratinocytes of the stratum spinosum and presents as lesions commonly found on the scalp, ears, and hands ([link]). It is the second most common skin cancer. The American Cancer Society reports that two of 10 skin cancers are squamous cell carcinomas, and it is more aggressive than basal cell carcinoma. If not removed, these carcinomas can metastasize. Surgery and radiation are used to cure squamous cell carcinoma. Squamous Cell Carcinoma Squamous cell carcinoma presents here as a lesion on an individual’s nose. (credit: the National Cancer Institute) Melanoma A melanoma is a cancer characterized by the uncontrolled growth of melanocytes, the pigment-producing cells in the epidermis. Typically, a melanoma develops from a mole. It is the most fatal of all skin cancers, as it is highly metastatic and can be difficult to detect before it has spread to other organs. Melanomas usually appear as asymmetrical brown and black patches with uneven borders and a raised surface ([{link]). Treatment typically involves surgical excision and immunotherapy. Melanoma Melanomas typically present as large brown or black patches with uneven borders and a raised surface. (credit: the National Cancer Institute) Doctors often give their patients the following ABCDE mnemonic to help with the diagnosis of early-stage melanoma. If you observe a mole on your body displaying these signs, consult a doctor. e Asymmetry — the two sides are not symmetrical ¢ Borders — the edges are irregular in shape e Color — the color is varied shades of brown or black e Diameter — it is larger than 6 mm (0.24 in) e Evolving — its shape has changed Some specialists cite the following additional signs for the most serious form, nodular melanoma: e Elevated — it is raised on the skin surface e Firm — it feels hard to the touch ¢ Growing — it is getting larger Skin Disorders Two common skin disorders are eczema and acne. Eczema is an inflammatory condition and occurs in individuals of all ages. Acne involves the clogging of pores, which can lead to infection and inflammation, and is often seen in adolescents. Other disorders, not discussed here, include seborrheic dermatitis (on the scalp), psoriasis, cold sores, impetigo, scabies, hives, and warts. Eczema Eczema is an allergic reaction that manifests as dry, itchy patches of skin that resemble rashes ([link]). It may be accompanied by swelling of the skin, flaking, and in severe cases, bleeding. Many who suffer from eczema have antibodies against dust mites in their blood, but the link between eczema and allergy to dust mites has not been proven. Symptoms are usually managed with moisturizers, corticosteroid creams, and immunosuppressants. Eczema Eczema is a common skin disorder that presents as a red, flaky rash. (credit: “Jambula”/Wikimedia Commons) Acne Acne is a skin disturbance that typically occurs on areas of the skin that are rich in sebaceous glands (face and back). It is most common along with the onset of puberty due to associated hormonal changes, but can also occur in infants and continue into adulthood. Hormones, such as androgens, stimulate the release of sebum. An overproduction and accumulation of sebum along with keratin can block hair follicles. This plug is initially white. The sebum, when oxidized by exposure to air, turns black. Acne results from infection by acne-causing bacteria (Propionibacterium and Staphylococcus), which can lead to redness and potential scarring due to the natural wound healing process ((Link]). Acne Epidermis Plugged follicle Mild inflammation Marked inflammation Sebaceous Accumulation of shed Bacteria proliferate keratin and sebum Acne is a result of over-productive sebaceous glands, which leads to formation of blackheads and inflammation of the skin. Note: Career Connection Dermatologist Have you ever had a skin rash that did not respond to over-the-counter creams, or a mole that you were concerned about? Dermatologists help patients with these types of problems and more, on a daily basis. Dermatologists are medical doctors who specialize in diagnosing and treating skin disorders. Like all medical doctors, dermatologists earn a medical degree and then complete several years of residency training. In addition, dermatologists may then participate in a dermatology fellowship or complete additional, specialized training in a dermatology practice. If practicing in the United States, dermatologists must pass the United States Medical Licensing Exam (USMLE), become licensed in their state of practice, and be certified by the American Board of Dermatology. Most dermatologists work in a medical office or private-practice setting. They diagnose skin conditions and rashes, prescribe oral and topical medications to treat skin conditions, and may perform simple procedures, such as mole or wart removal. In addition, they may refer patients to an oncologist if skin cancer that has metastasized is suspected. Recently, cosmetic procedures have also become a prominent part of dermatology. Botox injections, laser treatments, and collagen and dermal filler injections are popular among patients, hoping to reduce the appearance of skin aging. Dermatology is a competitive specialty in medicine. Limited openings in dermatology residency programs mean that many medical students compete for a few select spots. Dermatology is an appealing specialty to many prospective doctors, because unlike emergency room physicians or surgeons, dermatologists generally do not have to work excessive hours or be “on-call” weekends and holidays. Moreover, the popularity of cosmetic dermatology has made it a growing field with many lucrative opportunities. It is not unusual for dermatology clinics to market themselves exclusively as cosmetic dermatology centers, and for dermatologists to specialize exclusively in these procedures. Consider visiting a dermatologist to talk about why he or she entered the field and what the field of dermatology is like. Visit this site for additional information. Injuries Because the skin is the part of our bodies that meets the world most directly, it is especially vulnerable to injury. Injuries include burns and wounds, as well as scars and calluses. They can be caused by sharp objects, heat, or excessive pressure or friction to the skin. Skin injuries set off a healing process that occurs in several overlapping stages. The first step to repairing damaged skin is the formation of a blood clot that helps stop the flow of blood and scabs over with time. Many different types of cells are involved in wound repair, especially if the surface area that needs repair is extensive. Before the basal stem cells of the stratum basale can recreate the epidermis, fibroblasts mobilize and divide rapidly to repair the damaged tissue by collagen deposition, forming granulation tissue. Blood capillaries follow the fibroblasts and help increase blood circulation and oxygen supply to the area. Immune cells, such as macrophages, roam the area and engulf any foreign matter to reduce the chance of infection. Burns A burn results when the skin is damaged by intense heat, radiation, electricity, or chemicals. The damage results in the death of skin cells, which can lead to a massive loss of fluid. Dehydration, electrolyte imbalance, and renal and circulatory failure follow, which can be fatal. Burn patients are treated with intravenous fluids to offset dehydration, as well as intravenous nutrients that enable the body to repair tissues and replace lost proteins. Another serious threat to the lives of burn patients is infection. Burned skin is extremely susceptible to bacteria and other pathogens, due to the loss of protection by intact layers of skin. Burns are sometimes measured in terms of the size of the total surface area affected. This is referred to as the “rule of nines,” which associates specific anatomical areas with a percentage that is a factor of nine ([link]). Burns are also classified by the degree of their severity. A first-degree burn is a superficial burn that affects only the epidermis. Although the skin may be painful and swollen, these burns typically heal on their own within a few days. Mild sunburn fits into the category of a first-degree burn. A second- degree burn goes deeper and affects both the epidermis and a portion of the dermis. These burns result in swelling and a painful blistering of the skin. It is important to keep the burn site clean and sterile to prevent infection. If this is done, the burn will heal within several weeks. A third- degree burn fully extends into the epidermis and dermis, destroying the tissue and affecting the nerve endings and sensory function. These are serious burns that may appear white, red, or black; they require medical attention and will heal slowly without it. A fourth-degree burn is even more severe, affecting the underlying muscle and bone. Oddly, third and fourth-degree burns are usually not as painful because the nerve endings themselves are damaged. Full-thickness burns cannot be repaired by the body, because the local tissues used for repair are damaged and require excision (debridement), or amputation in severe cases, followed by grafting of the skin from an unaffected part of the body, or from skin grown in tissue culture for grafting purposes. Calculating the Size of a Burn Head and neck 9% Upper limbs 9% each Trunk 36% Genitalia 1% Lower limbs 18% each The size of a burn will guide decisions made about the need for specialized treatment. Specific parts of the body are associated with a percentage of body area. me oS = openstax COLLEGE” r 4 abe Skin grafts are required when the damage from trauma or infection cannot be closed with sutures or staples. Watch this video to learn more about skin grafting procedures. Scars and Keloids Most cuts or wounds, with the exception of ones that only scratch the surface (the epidermis), lead to scar formation. A scar is collagen-rich skin formed after the process of wound healing that differs from normal skin. Scarring occurs in cases in which there is repair of skin damage, but the skin fails to regenerate the original skin structure. Fibroblasts generate scar tissue in the form of collagen, and the bulk of repair is due to the basket- weave pattern generated by collagen fibers and does not result in regeneration of the typical cellular structure of skin. Instead, the tissue is fibrous in nature and does not allow for the regeneration of accessory structures, such as hair follicles, sweat glands, or sebaceous glands. Sometimes, there is an overproduction of scar tissue, because the process of collagen formation does not stop when the wound is healed; this results in the formation of a raised or hypertrophic scar called a keloid. In contrast, scars that result from acne and chickenpox have a sunken appearance and are called atrophic scars. Scarring of skin after wound healing is a natural process and does not need to be treated further. Application of mineral oil and lotions may reduce the formation of scar tissue. However, modern cosmetic procedures, such as dermabrasion, laser treatments, and filler injections have been invented as remedies for severe scarring. All of these procedures try to reorganize the structure of the epidermis and underlying collagen tissue to make it look more natural. Bedsores and Stretch Marks Skin and its underlying tissue can be affected by excessive pressure. One example of this is called a bedsore. Bedsores, also called decubitis ulcers, are caused by constant, long-term, unrelieved pressure on certain body parts that are bony, reducing blood flow to the area and leading to necrosis (tissue death). Bedsores are most common in elderly patients who have debilitating conditions that cause them to be immobile. Most hospitals and long-term care facilities have the practice of turning the patients every few hours to prevent the incidence of bedsores. If left untreated by removal of necrotized tissue, bedsores can be fatal if they become infected. The skin can also be affected by pressure associated with rapid growth. A stretch mark results when the dermis is stretched beyond its limits of elasticity, as the skin stretches to accommodate the excess pressure. Stretch marks usually accompany rapid weight gain during puberty and pregnancy. They initially have a reddish hue, but lighten over time. Other than for cosmetic reasons, treatment of stretch marks is not required. They occur most commonly over the hips and abdomen. Calluses When you wear shoes that do not fit well and are a constant source of abrasion on your toes, you tend to form a callus at the point of contact. This occurs because the basal stem cells in the stratum basale are triggered to divide more often to increase the thickness of the skin at the point of abrasion to protect the rest of the body from further damage. This is an example of a minor or local injury, and the skin manages to react and treat the problem independent of the rest of the body. Calluses can also form on your fingers if they are subject to constant mechanical stress, such as long periods of writing, playing string instruments, or video games. A corn is a specialized form of callus. Corns form from abrasions on the skin that result from an elliptical-type motion. Chapter Review Skin cancer is a result of damage to the DNA of skin cells, often due to excessive exposure to UV radiation. Basal cell carcinoma and squamous cell carcinoma are highly curable, and arise from cells in the stratum basale and stratum spinosum, respectively. Melanoma is the most dangerous form of skin cancer, affecting melanocytes, which can spread/metastasize to other organs. Burns are an injury to the skin that occur as a result of exposure to extreme heat, radiation, or chemicals. First-degree and second-degree burns usually heal quickly, but third-degree burns can be fatal because they penetrate the full thickness of the skin. Scars occur when there is repair of skin damage. Fibroblasts generate scar tissue in the form of collagen, which forms a basket-weave pattern that looks different from normal skin. Bedsores and stretch marks are the result of excessive pressure on the skin and underlying tissue. Bedsores are characterized by necrosis of tissue due to immobility, whereas stretch marks result from rapid growth. Eczema is an allergic reaction that manifests as a rash, and acne results from clogged sebaceous glands. Eczema and acne are usually long-term skin conditions that may be treated successfully in mild cases. Calluses and corns are the result of abrasive pressure on the skin. Review Questions Exercise: Problem:In general, skin cancers a. are easily treatable and not a major health concern b. occur due to poor hygiene c. can be reduced by limiting exposure to the sun d. affect only the epidermis Solution: C Exercise: Problem: Bedsores a. can be treated with topical moisturizers b. can result from deep massages c. are preventable by eliminating pressure points d. are caused by dry skin Solution: C Exercise: Problem: An individual has spent too much time sun bathing. Not only is his skin painful to touch, but small blisters have appeared in the affected area. This indicates that he has damaged which layers of his skin? a. epidermis only b. hypodermis only c. epidermis and hypodermis d. epidermis and dermis Solution: D Exercise: Problem: After a skin injury, the body initiates a wound-healing response. The first step of this response is the formation of a blood clot to stop bleeding. Which of the following would be the next response? a. increased production of melanin by melanocytes b. increased production of connective tissue c. an increase in Pacinian corpuscles around the wound d. an increased activity in the stratum lucidum Solution: B Exercise: Problem: Squamous cell carcinomas are the second most common of the skin cancers and are capable of metastasizing if not treated. This cancer affects which cells? a. basal cells of the stratum basale b. melanocytes of the stratum basale c. keratinocytes of the stratum spinosum d. Langerhans cells of the stratum lucidum Solution: C Critical Thinking Questions Exercise: Problem:Why do teenagers often experience acne? Solution: Acne results from a blockage of sebaceous glands by sebum. The blockage causes blackheads to form, which are susceptible to infection. The infected tissue then becomes red and inflamed. Teenagers experience this at high rates because the sebaceous glands become active during puberty. Hormones that are especially active during puberty stimulate the release of sebum, leading in many cases to blockages. Exercise: Problem:Why do scars look different from surrounding skin? Solution: Scars are made of collagen and do not have the cellular structure of normal skin. The tissue is fibrous and does not allow for the regeneration of accessory structures, such as hair follicles, and sweat or sebaceous glands. References American Cancer Society (US). Skin cancer: basal and squamous cell [Internet]. c2013 [cited 2012 Nov 1]. Available from: http://www.cancer.org/acs/groups/cid/documents/webcontent/003139- pdt.pdf. Lucile Packard Children’s Hospital at Stanford (US). Classification and treatment of burns [Internet]. Palo Alto (CA). c2012 [cited 2012 Nov 1]. Available from: Mayo Clinic (US). Basal cell carcinoma [Internet]. Scottsdale (AZ); c2012 [cited 2012 Nov 1]. Available from: http://www.mayoclinic.com/health/basal-cell- carcinoma/ds00925/dsection=treatments-and-drugs. Beck, J. FYI: how much can a human body sweat before it runs out? Popular Science [Internet]. New York (NY); c2012 [cited 2012 Nov 1]. much-can-human-body-sweat-it-runs-out. Skin Cancer Foundation (US). Skin cancer facts [Internet]. New York (NY); c2013 [cited 2012 Nov 1]. Available from: http://www.skincancer.org/skin- cancer-information/skin-cancer-facts#top. Glossary acne skin condition due to infected sebaceous glands basal cell carcinoma cancer that originates from basal cells in the epidermis of the skin bedsore sore on the skin that develops when regions of the body start necrotizing due to constant pressure and lack of blood supply; also called decubitis ulcers callus thickened area of skin that arises due to constant abrasion corm type of callus that is named for its shape and the elliptical motion of the abrasive force eczema skin condition due to an allergic reaction, which resembles a rash first-degree burn superficial burn that injures only the epidermis fourth-degree burn burn in which full thickness of the skin and underlying muscle and bone is damaged keloid type of scar that has layers raised above the skin surface melanoma type of skin cancer that originates from the melanocytes of the skin metastasis spread of cancer cells from a source to other parts of the body scar collagen-rich skin formed after the process of wound healing that is different from normal skin second-degree burn partial-thickness burn that injures the epidermis and a portion of the dermis squamous cell carcinoma type of skin cancer that originates from the stratum spinosum of the epidermis stretch mark mark formed on the skin due to a sudden growth spurt and expansion of the dermis beyond its elastic limits third-degree burn burn that penetrates and destroys the full thickness of the skin (epidermis and dermis) The Functions of the Skeletal System By the end of this section, you will be able to: ¢ Define bone, cartilage, and the skeletal system e List and describe the functions of the skeletal system Bone, or osseous tissue, is a hard, dense connective tissue that forms most of the adult skeleton, the support structure of the body. In the areas of the skeleton where bones move (for example, the ribcage and joints), cartilage, a semi-rigid form of connective tissue, provides flexibility and smooth surfaces for movement. The skeletal system is the body system composed of bones and cartilage and performs the following critical functions for the human body: e supports the body e facilitates movement ¢ protects internal organs e produces blood cells e stores and releases minerals and fat Support, Movement, and Protection The most apparent functions of the skeletal system are the gross functions —those visible by observation. Simply by looking at a person, you can see how the bones support, facilitate movement, and protect the human body. Just as the steel beams of a building provide a scaffold to support its weight, the bones and cartilage of your skeletal system compose the scaffold that supports the rest of your body. Without the skeletal system, you would be a limp mass of organs, muscle, and skin. Bones also facilitate movement by serving as points of attachment for your muscles. While some bones only serve as a support for the muscles, others also transmit the forces produced when your muscles contract. From a mechanical point of view, bones act as levers and joints serve as fulcrums ({link]). Unless a muscle spans a joint and contracts, a bone is not going to move. For information on the interaction of the skeletal and muscular systems, that is, the musculoskeletal system, seek additional content. Bones Support Movement Bones act as levers when muscles span a joint and contract. (credit: Benjamin J. DeLong) Bones also protect internal organs from injury by covering or surrounding them. For example, your ribs protect your lungs and heart, the bones of your vertebral column (spine) protect your spinal cord, and the bones of your cranium (skull) protect your brain ({link]). Bones Protect Brain The cranium completely surrounds and protects the brain from non- traumatic injury. Note: Career Connection Orthopedist An orthopedist is a doctor who specializes in diagnosing and treating disorders and injuries related to the musculoskeletal system. Some orthopedic problems can be treated with medications, exercises, braces, and other devices, but others may be best treated with surgery ([link]). Arm Brace An orthopedist will sometimes prescribe the use of a brace that reinforces the underlying bone structure it is being used to support. (credit: Juhan Sonin) While the origin of the word “orthopedics” (ortho- = “straight”; paed- = “child”), literally means “straightening of the child,” orthopedists can have patients who range from pediatric to geriatric. In recent years, orthopedists have even performed prenatal surgery to correct spina bifida, a congenital defect in which the neural canal in the spine of the fetus fails to close completely during embryologic development. Orthopedists commonly treat bone and joint injuries but they also treat other bone conditions including curvature of the spine. Lateral curvatures (scoliosis) can be severe enough to slip under the shoulder blade (scapula) forcing it up as a hump. Spinal curvatures can also be excessive dorsoventrally (kyphosis) causing a hunch back and thoracic compression. These curvatures often appear in preteens as the result of poor posture, abnormal growth, or indeterminate causes. Mostly, they are readily treated by orthopedists. As people age, accumulated spinal column injuries and diseases like osteoporosis can also lead to curvatures of the spine, hence the stooping you sometimes see in the elderly. Some orthopedists sub-specialize in sports medicine, which addresses both simple injuries, such as a sprained ankle, and complex injuries, such as a torn rotator cuff in the shoulder. Treatment can range from exercise to surgery. Mineral Storage, Energy Storage, and Hematopoiesis On a metabolic level, bone tissue performs several critical functions. For one, the bone matrix acts as a reservoir for a number of minerals important to the functioning of the body, especially calcium, and phosphorus. These minerals, incorporated into bone tissue, can be released back into the bloodstream to maintain levels needed to support physiological processes. Calcium ions, for example, are essential for muscle contractions and controlling the flow of other ions involved in the transmission of nerve impulses. Bone also serves as a site for fat storage and blood cell production. The softer connective tissue that fills the interior of most bone is referred to as bone marrow ((link]). There are two types of bone marrow: yellow marrow and red marrow. Yellow marrow contains adipose tissue; the triglycerides stored in the adipocytes of the tissue can serve as a source of energy. Red marrow is where hematopoiesis—the production of blood cells—takes place. Red blood cells, white blood cells, and platelets are all produced in the red marrow. Head of Femur Showing Red and Yellow Marrow Outer surface of bone Red marrow Yellow marrow The head of the femur contains both yellow and red marrow. Yellow marrow stores fat. Red marrow is responsible for hematopoiesis. (credit: modification of work by “stevenfruitsmaak”/Wikimedia Commons) Chapter Review The major functions of the bones are body support, facilitation of movement, protection of internal organs, storage of minerals and fat, and hematopoiesis. Together, the muscular system and skeletal system are known as the musculoskeletal system. Review Questions Exercise: Problem: Which function of the skeletal system would be especially important if you were in a car accident? a. storage of minerals b. protection of internal organs c. facilitation of movement d. fat storage Solution: B Exercise: Problem:Bone tissue can be described as a. dead calcified tissue b. cartilage c. the skeletal system d. dense, hard connective tissue Solution: D Exercise: Problem: Without red marrow, bones would not be able to a. store phosphate b. store calcium c. make blood cells d. move like levers Solution: C Exercise: Problem: Yellow marrow has been identified as a. an area of fat storage b. a point of attachment for muscles c. the hard portion of bone d. the cause of kyphosis Solution: A Exercise: Problem: Which of the following can be found in areas of movement? a. hematopoiesis b. cartilage c. yellow marrow d. red marrow Solution: B Exercise: Problem:The skeletal system is made of a. muscles and tendons b. bones and cartilage c. vitreous humor d. minerals and fat Solution: B Critical Thinking Questions Exercise: Problem: The skeletal system is composed of bone and cartilage and has many functions. Choose three of these functions and discuss what features of the skeletal system allow it to accomplish these functions. Solution: It supports the body. The rigid, yet flexible skeleton acts as a framework to support the other organs of the body. It facilitates movement. The movable joints allow the skeleton to change shape and positions; that is, move. It protects internal organs. Parts of the skeleton enclose or partly enclose various organs of the body including our brain, ears, heart, and lungs. Any trauma to these organs has to be mediated through the skeletal system. It produces blood cells. The central cavity of long bones is filled with marrow. The red marrow is responsible for forming red and white blood cells. It stores and releases minerals and fat. The mineral component of bone, in addition to providing hardness to bone, provides a mineral reservoir that can be tapped as needed. Additionally, the yellow marrow, which is found in the central cavity of long bones along with red marrow, serves as a Storage site for fat. Glossary bone hard, dense connective tissue that forms the structural elements of the skeleton cartilage semi-rigid connective tissue found on the skeleton in areas where flexibility and smooth surfaces support movement hematopoiesis production of blood cells, which occurs in the red marrow of the bones orthopedist doctor who specializes in diagnosing and treating musculoskeletal disorders and injuries osseous tissue bone tissue; a hard, dense connective tissue that forms the structural elements of the skeleton red marrow connective tissue in the interior cavity of a bone where hematopoiesis takes place skeletal system organ system composed of bones and cartilage that provides for movement, support, and protection yellow marrow connective tissue in the interior cavity of a bone where fat is stored Bone Classification By the end of this section, you will be able to: ¢ Classify bones according to their shapes e Describe the function of each category of bones The 206 bones that compose the adult skeleton are divided into five categories based on their shapes ((link]). Their shapes and their functions are related such that each categorical shape of bone has a distinct function. Classifications of Bones Irregular bone Flat bone Vertebra Sternum Long bone Short bones Lateral cuneiform Sesamoid bone Intermediate cuneiform = - Medial - y cuneiform Patella ne - Bones are classified according to their shape. Long Bones A long bone is one that is cylindrical in shape, being longer than it is wide. Keep in mind, however, that the term describes the shape of a bone, not its size. Long bones are found in the arms (humerus, ulna, radius) and legs (femur, tibia, fibula), as well as in the fingers (metacarpals, phalanges) and toes (metatarsals, phalanges). Long bones function as levers; they move when muscles contract. Short Bones A short bone is one that is cube-like in shape, being approximately equal in length, width, and thickness. The only short bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles. Short bones provide stability and support as well as some limited motion. Flat Bones The term “flat bone” is somewhat of a misnomer because, although a flat bone is typically thin, it is also often curved. Examples include the cranial (skull) bones, the scapulae (shoulder blades), the sternum (breastbone), and the ribs. Flat bones serve as points of attachment for muscles and often protect internal organs. Irregular Bones An irregular bone is one that does not have any easily characterized shape and therefore does not fit any other classification. These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. Many facial bones, particularly the ones containing sinuses, are classified as irregular bones. Sesamoid Bones A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form in tendons (the sheaths of tissue that connect bones to muscles) where a great deal of pressure is generated in a joint. The sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and placement from person to person but are typically found in tendons associated with the feet, hands, and knees. The patellae (singular = patella) are the only sesamoid bones found in common with every person. [link] reviews bone classifications with their associated features, functions, and examples. Bone Classifications Bone classification Long Short Features Cylinder-like shape, longer than it is wide Cube-like shape, approximately equal in length, width, and thickness Function(s) Leverage Provide stability, support, while allowing for some motion Examples Femur, tibia, fibula, metatarsals, humerus, ulna, radius, metacarpals, phalanges Carpals, tarsals Bone Classifications Bone classification Features Function(s) Examples Points of attachment Sternum, Flat Thin and for muscles; ribs, curved protectors scapulae, of internal cranial bones organs Pr Complex — Vertebrae, Irregular internal shape facial bones organs Pr Small and oe fount: tendons i : : from Patell Sesamoid anbeddedan 0) atellae compressive tendons forces Chapter Review Bones can be classified according to their shapes. Long bones, such as the femur, are longer than they are wide. Short bones, such as the carpals, are approximately equal in length, width, and thickness. Flat bones are thin, but are often curved, such as the ribs. Irregular bones such as those of the face have no characteristic shape. Sesamoid bones, such as the patellae, are small and round, and are located in tendons. Review Questions Exercise: Problem: Most of the bones of the arms and hands are long bones; however, the bones in the wrist are categorized as a. flat bones b. short bones c. sesamoid bones d. irregular bones Solution: B Exercise: Problem:Sesamoid bones are found embedded in a. joints b. muscles c. ligaments d. tendons Solution: D Exercise: Problem: Bones that surround the spinal cord are classified as bones. a. irregular b. sesamoid c. flat d. short Solution: A Exercise: Problem: Which category of bone is among the most numerous in the skeleton? a. long bone b. sesamoid bone c. short bone d. flat bone Solution: A Exercise: Problem:Long bones enable body movement by acting as a a. counterweight b. resistive force c. lever d. fulcrum Solution: GC Critical Thinking Questions Exercise: Problem: What are the structural and functional differences between a tarsal and a metatarsal? Solution: Structurally, a tarsal is a short bone, meaning its length, width, and thickness are about equal, while a metatarsal is a long bone whose length is greater than its width. Functionally, the tarsal provides limited motion, while the metatarsal acts as a lever. Exercise: Problem: What are the structural and functional differences between the femur and the patella? Solution: Structurally, the femur is a long bone, meaning its length is greater than its width, while the patella, a sesamoid bone, is small and round. Functionally, the femur acts as a lever, while the patella protects the patellar tendon from compressive forces. Glossary flat bone thin and curved bone; serves as a point of attachment for muscles and protects internal organs irregular bone bone of complex shape; protects internal organs from compressive forces long bone cylinder-shaped bone that is longer than it is wide; functions as a lever sesamoid bone small, round bone embedded in a tendon; protects the tendon from compressive forces short bone cube-shaped bone that is approximately equal in length, width, and thickness; provides limited motion Bone Structure By the end of this section, you will be able to: e Identify the anatomical features of a bone ¢ Define and list examples of bone markings ¢ Describe the histology of bone tissue e Compare and contrast compact and spongy bone e Identify the structures that compose compact and spongy bone e Describe how bones are nourished and innervated Bone tissue (osseous tissue) differs greatly from other tissues in the body. Bone is hard and many of its functions depend on that characteristic hardness. Later discussions in this chapter will show that bone is also dynamic in that its shape adjusts to accommodate stresses. This section will examine the gross anatomy of bone first and then move on to its histology. Gross Anatomy of Bone The structure of a long bone allows for the best visualization of all of the parts of a bone ([link]). A long bone has two parts: the diaphysis and the epiphysis. The diaphysis is the tubular shaft that runs between the proximal and distal ends of the bone. The hollow region in the diaphysis is called the medullary cavity, which is filled with yellow marrow. The walls of the diaphysis are composed of dense and hard compact bone. Anatomy of a Long Bone Articular cartilage Proximal epiphysis Metaphysis Spongy bone Epiphyseal line Red bone marrow Endosteum Compact bone Medullary cavity Diaphysis Yellow bone marrow Periosteum Nutrient artery Distal epiphysis Articular cartilage A typical long bone shows the gross anatomical characteristics of bone. The wider section at each end of the bone is called the epiphysis (plural = epiphyses), which is filled with spongy bone. Red marrow fills the spaces in the spongy bone. Each epiphysis meets the diaphysis at the metaphysis, the narrow area that contains the epiphyseal plate (growth plate), a layer of hyaline (transparent) cartilage in a growing bone. When the bone stops growing in early adulthood (approximately 18-21 years), the cartilage is replaced by osseous tissue and the epiphyseal plate becomes an epiphyseal line. The medullary cavity has a delicate membranous lining called the endosteum (end- = “inside”; oste- = “bone”), where bone growth, repair, and remodeling occur. The outer surface of the bone is covered with a fibrous membrane called the periosteum (peri- = “around” or “surrounding”). The periosteum contains blood vessels, nerves, and lymphatic vessels that nourish compact bone. Tendons and ligaments also attach to bones at the periosteum. The periosteum covers the entire outer surface except where the epiphyses meet other bones to form joints ([link]). In this region, the epiphyses are covered with articular cartilage, a thin layer of cartilage that reduces friction and acts as a shock absorber. Periosteum and Endosteum Periosteum / Endosteum Periosteum Osteoclast (fibrous layer) Osteocyte in lacuna Bone matrix Osteocyte Periosteum Osteogenic cell (cellular layer) Osteoblast The periosteum forms the outer surface of bone, and the endosteum lines the medullary cavity. Flat bones, like those of the cranium, consist of a layer of diploé (spongy bone), lined on either side by a layer of compact bone ([link]). The two layers of compact bone and the interior spongy bone work together to protect the internal organs. If the outer layer of a cranial bone fractures, the brain is still protected by the intact inner layer. Anatomy of a Flat Bone | \i 4 ‘i ten as Periosteum RN Ce I ee ~ ° Dee | c=! OSC” bone (diploé) This cross-section of a flat bone shows the spongy bone (diploé) lined on either side by a layer of compact bone. Bone Markings The surface features of bones vary considerably, depending on the function and location in the body. [link] describes the bone markings, which are illustrated in ([link]). There are three general classes of bone markings: (1) articulations, (2) projections, and (3) holes. As the name implies, an articulation is where two bone surfaces come together (articulus = “joint”). These surfaces tend to conform to one another, such as one being rounded and the other cupped, to facilitate the function of the articulation. A projection is an area of a bone that projects above the surface of the bone. These are the attachment points for tendons and ligaments. In general, their size and shape is an indication of the forces exerted through the attachment to the bone. A hole is an opening or groove in the bone that allows blood vessels and nerves to enter the bone. As with the other markings, their size and shape reflect the size of the vessels and nerves that penetrate the bone at these points. Bone Markings Marking Articulations Head Facet Condyle Projections Protuberance Process Spine Tubercle Tuberosity Description Where two bones meet Prominent rounded surface Flat surface Rounded surface Raised markings Protruding Prominence feature Sharp process Small, rounded process Rough surface Example Knee joint Head of femur Vertebrae Occipital condyles Spinous process of the vertebrae Chin Transverse process of vertebra Ischial spine Tubercle of humerus Deltoid tuberosity Bone Markings Marking Line Crest Holes Fossa Fovea Sulcus Canal Fissure Foramen Meatus Description Slight, elongated ridge Ridge Holes and depressions Elongated basin Small pit Groove Passage in bone Slit through bone Hole through bone Opening into canal Example Temporal lines of the parietal bones Iliac crest Foramen (holes through which blood vessels can pass through) Mandibular fossa Fovea capitis on the head of the femur Sigmoid sulcus of the temporal bones Auditory canal Auricular fissure Foramen magnum in the occipital bone External auditory meatus Bone Markings Marking Description Example Air-filled Sinus space in Nasal sinus bone Bone Features Examples of processes formed where Examples of an elevation or depression tendons or ligaments attach Fovea capitis 17 Sulcus Head Tubercle Pay \ Tuberosity Pelvis Fossa Humerus Examples of openings Tubercle Condyle Facet Foramen Condyles Examples of processes formed to Canal , \ \ Fissure articulate with adjacent bones \ \ Protuberance Skull The surface features of bones depend on their function, location, attachment of ligaments and tendons, or the penetration of blood vessels and nerves. Bone Cells and Tissue Bone contains a relatively small number of cells entrenched in a matrix of collagen fibers that provide a surface for inorganic salt crystals to adhere. These salt crystals form when calcium phosphate and calcium carbonate combine to create hydroxyapatite, which incorporates other inorganic salts like magnesium hydroxide, fluoride, and sulfate as it crystallizes, or calcifies, on the collagen fibers. The hydroxyapatite crystals give bones their hardness and strength, while the collagen fibers give them flexibility so that they are not brittle. Although bone cells compose a small amount of the bone volume, they are crucial to the function of bones. Four types of cells are found within bone tissue: osteoblasts, osteocytes, osteogenic cells, and osteoclasts ([link]). Bone Cells wet &. Osteocyte Osteoblast Osteogenic cell Osteoclast (maintains (forms bone matrix) (stem cell) (resorbs bone) bone tissue) Four types of cells are found within bone tissue. Osteogenic cells are undifferentiated and develop into osteoblasts. When osteoblasts get trapped within the calcified matrix, their structure and function changes, and they become osteocytes. Osteoclasts develop from monocytes and macrophages and differ in appearance from other bone cells. The osteoblast is the bone cell responsible for forming new bone and is found in the growing portions of bone, including the periosteum and endosteum. Osteoblasts, which do not divide, synthesize and secrete the collagen matrix and calcium salts. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast become trapped within it; as a result, it changes in structure and becomes an osteocyte, the primary cell of mature bone and the most common type of bone cell. Each osteocyte is located in a space called a lacuna and is surrounded by bone tissue. Osteocytes maintain the mineral concentration of the matrix via the secretion of enzymes. Like osteoblasts, osteocytes lack mitotic activity. They can communicate with each other and receive nutrients via long cytoplasmic processes that extend through canaliculi (singular = canaliculus), channels within the bone matrix. If osteoblasts and osteocytes are incapable of mitosis, then how are they replenished when old ones die? The answer lies in the properties of a third category of bone cells—the osteogenic cell. These osteogenic cells are undifferentiated with high mitotic activity and they are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. They differentiate and develop into osteoblasts. The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release. The cell responsible for bone resorption, or breakdown, is the osteoclast. They are found on bone surfaces, are multinucleated, and originate from monocytes and macrophages, two types of white blood cells, not from osteogenic cells. Osteoclasts are continually breaking down old bone while osteoblasts are continually forming new bone. The ongoing balance between osteoblasts and osteoclasts is responsible for the constant but subtle reshaping of bone. [link] reviews the bone cells, their functions, and locations. Bone Cells Cell type Function Osteogenic Develop into cells osteoblasts Osteoblasts Bone formation Maintain mineral Osteocytes concentration of matrix Osteoclasts Bone resorption Compact and Spongy Bone Location Deep layers of the periosteum and the marrow Growing portions of bone, including periosteum and endosteum Entrapped in matrix Bone surfaces and at sites of old, injured, or unneeded bone The differences between compact and spongy bone are best explored via their histology. Most bones contain compact and spongy osseous tissue, but their distribution and concentration vary based on the bone’s overall function. Compact bone is dense so that it can withstand compressive forces, while spongy (cancellous) bone has open spaces and supports shifts in weight distribution. Compact Bone Compact bone is the denser, stronger of the two types of bone tissue ({link]). It can be found under the periosteum and in the diaphyses of long bones, where it provides support and protection. Diagram of Compact Bone (a) This cross-sectional view of compact bone shows the basic structural unit, the osteon. (b) In this micrograph of the osteon, you can clearly see the concentric lamellae and central canals. LM x 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Compact bone Spongy bone Medullary cavity Periosteum Concentric lamellae Osteon Lymphatic vessel Circumferential lamellae Nerve Periosteal artery Blood vessels Periosteal vein rabeéulae Periosteum: Outer fibrous layer Inner osteogenic layer Interstitial lamellae Medullar cavity Perforating canal Spongy bone Central canal Blood vessels Lymphatic vessel Nerve Compact bone The microscopic structural unit of compact bone is called an osteon, or Haversian system. Each osteon is composed of concentric rings of calcified matrix called lamellae (singular = lamella). Running down the center of each osteon is the central canal, or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels. These vessels and nerves branch off at right angles through a perforating canal, also known as Volkmann’s canals, to extend to the periosteum and endosteum. The osteocytes are located inside spaces called lacunae (singular = lacuna), found at the borders of adjacent lamellae. As described earlier, canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This system allows nutrients to be transported to the osteocytes and wastes to be removed from them. Spongy (Cancellous) Bone Like compact bone, spongy bone, also known as cancellous bone, contains osteocytes housed in lacunae, but they are not arranged in concentric circles. Instead, the lacunae and osteocytes are found in a lattice-like network of matrix spikes called trabeculae (singular = trabecula) ([link]). The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone. The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where hematopoiesis occurs. Osteoclast Osteoblasts aligned along trabeculae of new bone Canaliculi Endosteum _Lamellae openings : Ge Canaliculi on surface aay Lamellae Spongy bone is composed of trabeculae that contain the osteocytes. Red marrow fills the spaces in some bones. Note: Aging and the... Skeletal System: Paget’s Disease Paget’s disease usually occurs in adults over age 40. It is a disorder of the bone remodeling process that begins with overactive osteoclasts. This means more bone is resorbed than is laid down. The osteoblasts try to compensate but the new bone they lay down is weak and brittle and therefore prone to fracture. While some people with Paget’s disease have no symptoms, others experience pain, bone fractures, and bone deformities ([link]). Bones of the pelvis, skull, spine, and legs are the most commonly affected. When occurring in the skull, Paget’s disease can cause headaches and hearing loss. Paget's Disease Normal Paget’s disease Normal leg bones are relatively Straight, but those affected by Paget’s disease are porous and curved. What causes the osteoclasts to become overactive? The answer is still unknown, but hereditary factors seem to play a role. Some scientists believe Paget’s disease is due to an as-yet-unidentified virus. Paget’s disease is diagnosed via imaging studies and lab tests. X-rays may show bone deformities or areas of bone resorption. Bone scans are also useful. In these studies, a dye containing a radioactive ion is injected into the body. Areas of bone resorption have an affinity for the ion, so they will light up on the scan if the ions are absorbed. In addition, blood levels of an enzyme called alkaline phosphatase are typically elevated in people with Paget’s disease. Bisphosphonates, drugs that decrease the activity of osteoclasts, are often used in the treatment of Paget’s disease. However, in a small percentage of cases, bisphosphonates themselves have been linked to an increased risk of fractures because the old bone that is left after bisphosphonates are administered becomes worn out and brittle. Still, most doctors feel that the benefits of bisphosphonates more than outweigh the risk; the medical professional has to weigh the benefits and risks on a case-by-case basis. Bisphosphonate treatment can reduce the overall risk of deformities or fractures, which in turn reduces the risk of surgical repair and its associated risks and complications. Blood and Nerve Supply The spongy bone and medullary cavity receive nourishment from arteries that pass through the compact bone. The arteries enter through the nutrient foramen (plural = foramina), small openings in the diaphysis ((link]). The osteocytes in spongy bone are nourished by blood vessels of the periosteum that penetrate spongy bone and blood that circulates in the marrow cavities. As the blood passes through the marrow cavities, it is collected by veins, which then pass out of the bone through the foramina. In addition to the blood vessels, nerves follow the same paths into the bone where they tend to concentrate in the more metabolically active regions of the bone. The nerves sense pain, and it appears the nerves also play roles in regulating blood supplies and in bone growth, hence their concentrations in metabolically active sites of the bone. Diagram of Blood and Nerve Supply to Bone ere Articular —_ ae zm) cartilage Epiphyseal artery and vein = Metaphyseal artery and vein Periosteum Compact bone Nutrient artery and vein Nutrient foramen Medullary cavity Metaphyseal artery and vein Metaphysis —|| Epiphyseal line Blood vessels and nerves enter the bone through the nutrient foramen. Note: Watch this video to see the microscopic features of a bone. Chapter Review A hollow medullary cavity filled with yellow marrow runs the length of the diaphysis of a long bone. The walls of the diaphysis are compact bone. The epiphyses, which are wider sections at each end of a long bone, are filled with spongy bone and red marrow. The epiphyseal plate, a layer of hyaline cartilage, is replaced by osseous tissue as the organ grows in length. The medullary cavity has a delicate membranous lining called the endosteum. The outer surface of bone, except in regions covered with articular cartilage, is covered with a fibrous membrane called the periosteum. Flat bones consist of two layers of compact bone surrounding a layer of spongy bone. Bone markings depend on the function and location of bones. Articulations are places where two bones meet. Projections stick out from the surface of the bone and provide attachment points for tendons and ligaments. Holes are openings or depressions in the bones. Bone matrix consists of collagen fibers and organic ground substance, primarily hydroxyapatite formed from calcium salts. Osteogenic cells develop into osteoblasts. Osteoblasts are cells that make new bone. They become osteocytes, the cells of mature bone, when they get trapped in the matrix. Osteoclasts engage in bone resorption. Compact bone is dense and composed of osteons, while spongy bone is less dense and made up of trabeculae. Blood vessels and nerves enter the bone through the nutrient foramina to nourish and innervate bones. Review Questions Exercise: Problem: Which of the following occurs in the spongy bone of the epiphysis? a. bone growth b. bone remodeling c. hematopoiesis d. shock absorption Solution: C Exercise: Problem:The diaphysis contains a. the metaphysis b. fat stores c. spongy bone d. compact bone Solution: B Exercise: Problem: The fibrous membrane covering the outer surface of the bone is the a. periosteum b. epiphysis c. endosteum d. diaphysis Solution: A Exercise: Problem: Which of the following are incapable of undergoing mitosis? a. osteoblasts and osteoclasts b. osteocytes and osteoclasts c. osteoblasts and osteocytes d. osteogenic cells and osteoclasts Solution: ‘§ Exercise: Problem: Which cells do not originate from osteogenic cells? a. osteoblasts b. osteoclasts c. osteocytes d. osteoprogenitor cells Solution: D Exercise: Problem: Which of the following are found in compact bone and cancellous bone? a. Haversian systems b. Haversian canals c. lamellae d. lacunae Solution: C Exercise: Problem: Which of the following are only found in cancellous bone? a. canaliculi b. Volkmann’s canals c. trabeculae d. calcium salts Solution: C Exercise: Problem: The area of a bone where the nutrient foramen passes forms what kind of bone marking? a. a hole b. a facet c. a canal d. a fissure Solution: A Critical Thinking Questions Exercise: Problem: If the articular cartilage at the end of one of your long bones were to degenerate, what symptoms do you think you would experience? Why? Solution: If the articular cartilage at the end of one of your long bones were to deteriorate, which is actually what happens in osteoarthritis, you would experience joint pain at the end of that bone and limitation of motion at that joint because there would be no cartilage to reduce friction between adjacent bones and there would be no cartilage to act as a shock absorber. Exercise: Problem: In what ways is the structural makeup of compact and spongy bone well suited to their respective functions? Solution: The densely packed concentric rings of matrix in compact bone are ideal for resisting compressive forces, which is the function of compact bone. The open spaces of the trabeculated network of spongy bone allow spongy bone to support shifts in weight distribution, which is the function of spongy bone. Glossary articular cartilage thin layer of cartilage covering an epiphysis; reduces friction and acts as a shock absorber articulation where two bone surfaces meet canaliculi (singular = canaliculus) channels within the bone matrix that house one of an osteocyte’s many cytoplasmic extensions that it uses to communicate and receive nutrients central canal longitudinal channel in the center of each osteon; contains blood vessels, nerves, and lymphatic vessels; also known as the Haversian canal compact bone dense osseous tissue that can withstand compressive forces diaphysis tubular shaft that runs between the proximal and distal ends of a long bone diploé layer of spongy bone, that is sandwiched between two the layers of compact bone found in flat bones endosteum delicate membranous lining of a bone’s medullary cavity epiphyseal plate (also, growth plate) sheet of hyaline cartilage in the metaphysis of an immature bone; replaced by bone tissue as the organ grows in length epiphysis wide section at each end of a long bone; filled with spongy bone and red marrow hole opening or depression in a bone lacunae (singular = lacuna) spaces in a bone that house an osteocyte medullary cavity hollow region of the diaphysis; filled with yellow marrow nutrient foramen small opening in the middle of the external surface of the diaphysis, through which an artery enters the bone to provide nourishment osteoblast cell responsible for forming new bone osteoclast cell responsible for resorbing bone osteocyte primary cell in mature bone; responsible for maintaining the matrix osteogenic cell undifferentiated cell with high mitotic activity; the only bone cells that divide; they differentiate and develop into osteoblasts osteon (also, Haversian system) basic structural unit of compact bone; made of concentric layers of calcified matrix perforating canal (also, Volkmann’s canal) channel that branches off from the central canal and houses vessels and nerves that extend to the periosteum and endosteum periosteum fibrous membrane covering the outer surface of bone and continuous with ligaments projection bone markings where part of the surface sticks out above the rest of the surface, where tendons and ligaments attach spongy bone (also, cancellous bone) trabeculated osseous tissue that supports shifts in weight distribution trabeculae (singular = trabecula) spikes or sections of the lattice-like matrix in spongy bone Bone Formation and Development By the end of this section, you will be able to: e Explain the function of cartilage e List the steps of intramembranous ossification e List the steps of endochondral ossification e Explain the growth activity at the epiphyseal plate e Compare and contrast the processes of modeling and remodeling In the early stages of embryonic development, the embryo’s skeleton consists of fibrous membranes and hyaline cartilage. By the sixth or seventh week of embryonic life, the actual process of bone development, ossification (osteogenesis), begins. There are two osteogenic pathways— intramembranous ossification and endochondral ossification—but bone is the same regardless of the pathway that produces it. Cartilage Templates Bone is a replacement tissue; that is, it uses a model tissue on which to lay down its mineral matrix. For skeletal development, the most common template is cartilage. During fetal development, a framework is laid down that determines where bones will form. This framework is a flexible, semi- solid matrix produced by chondroblasts and consists of hyaluronic acid, chondroitin sulfate, collagen fibers, and water. As the matrix surrounds and isolates chondroblasts, they are called chondrocytes. Unlike most connective tissues, cartilage is avascular, meaning that it has no blood vessels supplying nutrients and removing metabolic wastes. All of these functions are carried on by diffusion through the matrix. This is why damaged cartilage does not repair itself as readily as most tissues do. Throughout fetal development and into childhood growth and development, bone forms on the cartilaginous matrix. By the time a fetus is born, most of the cartilage has been replaced with bone. Some additional cartilage will be replaced throughout childhood, and some cartilage remains in the adult skeleton. Intramembranous Ossification During intramembranous ossification, compact and spongy bone develops directly from sheets of mesenchymal (undifferentiated) connective tissue. The flat bones of the face, most of the cranial bones, and the clavicles (collarbones) are formed via intramembranous ossification. The process begins when mesenchymal cells in the embryonic skeleton gather together and begin to differentiate into specialized cells ({link]a). Some of these cells will differentiate into capillaries, while others will become osteogenic cells and then osteoblasts. Although they will ultimately be spread out by the formation of bone tissue, early osteoblasts appear in a cluster called an ossification center. The osteoblasts secrete osteoid, uncalcified matrix, which calcifies (hardens) within a few days as mineral salts are deposited on it, thereby entrapping the osteoblasts within. Once entrapped, the osteoblasts become osteocytes ([{link]b). As osteoblasts transform into osteocytes, osteogenic cells in the surrounding connective tissue differentiate into new osteoblasts. Osteoid (unmineralized bone matrix) secreted around the capillaries results in a trabecular matrix, while osteoblasts on the surface of the spongy bone become the periosteum ([link]c). The periosteum then creates a protective layer of compact bone superficial to the trabecular bone. The trabecular bone crowds nearby blood vessels, which eventually condense into red marrow ((link]d). Intramembranous Ossification Mesenchymal cells Osteoid Osteoblast Ossification center Osteocyte New bone matrix Osteoid Osteoblast Mesenchyme —= SS forms the ES ——— 6. y -——_ Fibrous periosteum periosteum Osteoblast Compact bone Trabeculae Spongy bone (cavities contain Zz = — = = red marrow) Blood vessel Intramembranous ossification follows four steps. (a) Mesenchymal cells group into clusters, and ossification centers form. (b) Secreted osteoid traps osteoblasts, which then become osteocytes. (c) Trabecular matrix and periosteum form. (d) Compact bone develops superficial to the trabecular bone, and crowded blood vessels condense into red marrow. Intramembranous ossification begins in utero during fetal development and continues on into adolescence. At birth, the skull and clavicles are not fully ossified nor are the sutures of the skull closed. This allows the skull and shoulders to deform during passage through the birth canal. The last bones to ossify via intramembranous ossification are the flat bones of the face, which reach their adult size at the end of the adolescent growth spurt. Endochondral Ossification In endochondral ossification, bone develops by replacing hyaline cartilage. Cartilage does not become bone. Instead, cartilage serves as a template to be completely replaced by new bone. Endochondral ossification takes much longer than intramembranous ossification. Bones at the base of the skull and long bones form via endochondral ossification. In a long bone, for example, at about 6 to 8 weeks after conception, some of the mesenchymal cells differentiate into chondrocytes (cartilage cells) that form the cartilaginous skeletal precursor of the bones ([link]a). Soon after, the perichondrium, a membrane that covers the cartilage, appears [link]b). Endochondral Ossification Perichondrium Primary ossification center Hyaline cartilage Calcified matrix (c) Periosteum (covers compact bone) Medullary cavity Artery and vein (provide nutrients to bone) (d) Secondary ossification center Articular cartilage _—— Artery and vein Artery and vein (provide nutrients to bone) (e) Endochondral ossification follows five steps. (a) Mesenchymal cells differentiate into chondrocytes. (b) The cartilage model of the future bony skeleton and the perichondrium form. (c) Capillaries penetrate cartilage. Perichondrium transforms into periosteum. Periosteal collar develops. Primary ossification center develops. (d) Cartilage and chondrocytes continue to grow at ends of the bone. (e) Secondary ossification centers develop. (f) Cartilage remains at epiphyseal (growth) plate and at joint surface as articular cartilage. As more matrix is produced, the chondrocytes in the center of the cartilaginous model grow in size. As the matrix calcifies, nutrients can no longer reach the chondrocytes. This results in their death and the disintegration of the surrounding cartilage. Blood vessels invade the resulting spaces, not only enlarging the cavities but also carrying osteogenic cells with them, many of which will become osteoblasts. These enlarging spaces eventually combine to become the medullary cavity. As the cartilage grows, capillaries penetrate it. This penetration initiates the transformation of the perichondrium into the bone-producing periosteum. Here, the osteoblasts form a periosteal collar of compact bone around the cartilage of the diaphysis. By the second or third month of fetal life, bone cell development and ossification ramps up and creates the primary ossification center, a region deep in the periosteal collar where ossification begins ({link]c). While these deep changes are occurring, chondrocytes and cartilage continue to grow at the ends of the bone (the future epiphyses), which increases the bone’s length at the same time bone is replacing cartilage in the diaphyses. By the time the fetal skeleton is fully formed, cartilage only remains at the joint surface as articular cartilage and between the diaphysis and epiphysis as the epiphyseal plate, the latter of which is responsible for the longitudinal growth of bones. After birth, this same sequence of events (matrix mineralization, death of chondrocytes, invasion of blood vessels from the periosteum, and seeding with osteogenic cells that become osteoblasts) occurs in the epiphyseal regions, and each of these centers of activity is referred to as a secondary ossification center ((link]e). How Bones Grow in Length The epiphyseal plate is the area of growth in a long bone. It is a layer of hyaline cartilage where ossification occurs in immature bones. On the epiphyseal side of the epiphyseal plate, cartilage is formed. On the diaphyseal side, cartilage is ossified, and the diaphysis grows in length. The epiphyseal plate is composed of four zones of cells and activity ((link]). The reserve zone is the region closest to the epiphyseal end of the plate and contains small chondrocytes within the matrix. These chondrocytes do not participate in bone growth but secure the epiphyseal plate to the osseous tissue of the epiphysis. Longitudinal Bone Growth Changes in _— Co Q Reserve zone fo) (6) Matrix production Growth plate zones ere = | Proliferative zone Mitosis _ | Lipids, glycogen, and alkaline | phosphatase accumulate; matrix calcifies. Maturation and nail hypertrophy <P Calcified matrix = \— Celldeath = Primary spongiosa Secondary spongiosa Zone of ossification Metaphysis The epiphyseal plate is responsible for longitudinal bone growth. The proliferative zone is the next layer toward the diaphysis and contains stacks of slightly larger chondrocytes. It makes new chondrocytes (via mitosis) to replace those that die at the diaphyseal end of the plate. Chondrocytes in the next layer, the zone of maturation and hypertrophy, are older and larger than those in the proliferative zone. The more mature cells are situated closer to the diaphyseal end of the plate. The longitudinal growth of bone is a result of cellular division in the proliferative zone and the maturation of cells in the zone of maturation and hypertrophy. Most of the chondrocytes in the zone of calcified matrix, the zone closest to the diaphysis, are dead because the matrix around them has calcified. Capillaries and osteoblasts from the diaphysis penetrate this zone, and the osteoblasts secrete bone tissue on the remaining calcified cartilage. Thus, the zone of calcified matrix connects the epiphyseal plate to the diaphysis. A bone grows in length when osseous tissue is added to the diaphysis. Bones continue to grow in length until early adulthood. The rate of growth is controlled by hormones, which will be discussed later. When the chondrocytes in the epiphyseal plate cease their proliferation and bone replaces the cartilage, longitudinal growth stops. All that remains of the epiphyseal plate is the epiphyseal line ([link]). Progression from Epiphyseal Plate to Epiphyseal Line Epiphysis ee WV Metaphysis Diaphysis Epiphyseal Epiphyseal line plate (growth plate) Ee Metaphysis — J __ |} —— Eriptysis ——{__U Femur (thighbone) (a) Growing long bone (b) Mature long bone As a bone matures, the epiphyseal plate progresses to an epiphyseal line. (a) Epiphyseal plates are visible in a growing bone. (b) Epiphyseal lines are the remnants of epiphyseal plates in a mature bone. How Bones Grow in Diameter While bones are increasing in length, they are also increasing in diameter; growth in diameter can continue even after longitudinal growth ceases. This is called appositional growth. Osteoclasts resorb old bone that lines the medullary cavity, while osteoblasts, via intramembranous ossification, produce new bone tissue beneath the periosteum. The erosion of old bone along the medullary cavity and the deposition of new bone beneath the periosteum not only increase the diameter of the diaphysis but also increase the diameter of the medullary cavity. This process is called modeling. Bone Remodeling The process in which matrix is resorbed on one surface of a bone and deposited on another is known as bone modeling. Modeling primarily takes place during a bone’s growth. However, in adult life, bone undergoes remodeling, in which resorption of old or damaged bone takes place on the same surface where osteoblasts lay new bone to replace that which is resorbed. Injury, exercise, and other activities lead to remodeling. Those influences are discussed later in the chapter, but even without injury or exercise, about 5 to 10 percent of the skeleton is remodeled annually just by destroying old bone and renewing it with fresh bone. Note: Diseases of the... Skeletal System Osteogenesis imperfecta (OI) is a genetic disease in which bones do not form properly and therefore are fragile and break easily. It is also called brittle bone disease. The disease is present from birth and affects a person throughout life. The genetic mutation that causes OI affects the body’s production of collagen, one of the critical components of bone matrix. The severity of the disease can range from mild to severe. Those with the most severe forms of the disease sustain many more fractures than those with a mild form. Frequent and multiple fractures typically lead to bone deformities and short stature. Bowing of the long bones and curvature of the spine are also common in people afflicted with OI. Curvature of the spine makes breathing difficult because the lungs are compressed. Because collagen is such an important structural protein in many parts of the body, people with OI may also experience fragile skin, weak muscles, loose joints, easy bruising, frequent nosebleeds, brittle teeth, blue sclera, and hearing loss. There is no known cure for OI. Treatment focuses on helping the person retain as much independence as possible while minimizing fractures and maximizing mobility. Toward that end, safe exercises, like swimming, in which the body is less likely to experience collisions or compressive forces, are recommended. Braces to support legs, ankles, knees, and wrists are used as needed. Canes, walkers, or wheelchairs can also help compensate for weaknesses. When bones do break, casts, splints, or wraps are used. In some cases, metal rods may be surgically implanted into the long bones of the arms and legs. Research is currently being conducted on using bisphosphonates to treat OI. Smoking and being overweight are especially risky in people with OI, since smoking is known to weaken bones, and extra body weight puts additional stress on the bones. Note: eee openstax COLLEGE lll oF 1s Watch this video to see how a bone grows. Chapter Review All bone formation is a replacement process. Embryos develop a cartilaginous skeleton and various membranes. During development, these are replaced by bone during the ossification process. In intramembranous ossification, bone develops directly from sheets of mesenchymal connective tissue. In endochondral ossification, bone develops by replacing hyaline cartilage. Activity in the epiphyseal plate enables bones to grow in length. Modeling allows bones to grow in diameter. Remodeling occurs as bone is resorbed and replaced by new bone. Osteogenesis imperfecta is a genetic disease in which collagen production is altered, resulting in fragile, brittle bones. Review Questions Exercise: Problem: Why is cartilage slow to heal? a. because it eventually develops into bone b. because it is semi-solid and flexible c. because it does not have a blood supply d. because endochondral ossification replaces all cartilage with bone Solution: C Exercise: Problem: Why are osteocytes spread out in bone tissue? a. They develop from mesenchymal cells. b. They are surrounded by osteoid. c. They travel through the capillaries. d. Formation of osteoid spreads out the osteoblasts that formed the ossification centers. Solution: D Exercise: Problem: In endochondral ossification, what happens to the chondrocytes? a. They develop into osteocytes. b. They die in the calcified matrix that surrounds them and form the medullary cavity. c. They grow and form the periosteum. d. They group together to form the primary ossification center. Solution: B Exercise: Problem: Which of the following bones is (are) formed by intramembranous ossification? a. the metatarsals b. the femur c. the ribs d. the flat bones of the cranium Solution: D Exercise: Problem:Bones grow in length due to activity in the a. epiphyseal plate b. perichondrium Cc. periosteum d. medullary cavity Solution: A Exercise: Problem:Bones grow in diameter due to bone formation a. in the medullary cavity b. beneath the periosteum c. in the epiphyseal plate d. within the metaphysis Solution: B Exercise: Problem: Which of the following represents the correct sequence of zones in the epiphyseal plate? a. proliferation, reserved, maturation, calcification b. maturation, proliferation, reserved, calcification c. calcification, maturation, proliferation, reserved d. calcification, reserved, proliferation, maturation Solution: C Critical Thinking Questions Exercise: Problem: In what ways do intramembranous and endochondral ossification differ? Solution: In intramembranous ossification, bone develops directly from sheets of mesenchymal connective tissue, but in endochondral ossification, bone develops by replacing hyaline cartilage. Intramembranous ossification is complete by the end of the adolescent growth spurt, while endochondral ossification lasts into young adulthood. The flat bones of the face, most of the cranial bones, and a good deal of the clavicles (collarbones) are formed via intramembranous ossification, while bones at the base of the skull and the long bones form via endochondral ossification. Exercise: Problem: Considering how a long bone develops, what are the similarities and differences between a primary and a secondary ossification center? Solution: A single primary ossification center is present, during endochondral ossification, deep in the periosteal collar. Like the primary ossification center, secondary ossification centers are present during endochondral ossification, but they form later, and there are two of them, one in each epiphysis. Glossary endochondral ossification process in which bone forms by replacing hyaline cartilage epiphyseal line completely ossified remnant of the epiphyseal plate intramembranous ossification process by which bone forms directly from mesenchymal tissue modeling process, during bone growth, by which bone is resorbed on one surface of a bone and deposited on another ossification (also, osteogenesis) bone formation ossification center cluster of osteoblasts found in the early stages of intramembranous ossification osteoid uncalcified bone matrix secreted by osteoblasts perichondrium membrane that covers cartilage primary ossification center region, deep in the periosteal collar, where bone development starts during endochondral ossification proliferative zone region of the epiphyseal plate that makes new chondrocytes to replace those that die at the diaphyseal end of the plate and contributes to longitudinal growth of the epiphyseal plate remodeling process by which osteoclasts resorb old or damaged bone at the same time as and on the same surface where osteoblasts form new bone to replace that which is resorbed reserve zone region of the epiphyseal plate that anchors the plate to the osseous tissue of the epiphysis secondary ossification center region of bone development in the epiphyses zone of calcified matrix region of the epiphyseal plate closest to the diaphyseal end; functions to connect the epiphyseal plate to the diaphysis zone of maturation and hypertrophy region of the epiphyseal plate where chondrocytes from the proliferative zone grow and mature and contribute to the longitudinal growth of the epiphyseal plate Fractures: Bone Repair By the end of this section, you will be able to: e Differentiate among the different types of fractures e Describe the steps involved in bone repair A fracture is a broken bone. It will heal whether or not a physician resets it in its anatomical position. If the bone is not reset correctly, the healing process will keep the bone in its deformed position. When a broken bone is manipulated and set into its natural position without surgery, the procedure is called a closed reduction. Open reduction requires surgery to expose the fracture and reset the bone. While some fractures can be minor, others are quite severe and result in grave complications. For example, a fractured diaphysis of the femur has the potential to release fat globules into the bloodstream. These can become lodged in the capillary beds of the lungs, leading to respiratory distress and if not treated quickly, death. Types of Fractures Fractures are classified by their complexity, location, and other features ({link]). [link] outlines common types of fractures. Some fractures may be described using more than one term because it may have the features of more than one type (e.g., an open transverse fracture). Types of Fractures Open Transverse Compare healthy bone with different types of fractures: (a) closed fracture, (b) open fracture, (c) transverse fracture, (d) spiral fracture, (e) comminuted fracture, (f) impacted fracture, (g) greenstick fracture, and (h) oblique fracture. Types of Fractures Type of fracture Transverse Oblique Spiral Comminuted Impacted Greenstick Open (or compound) Description Occurs straight across the long axis of the bone Occurs at an angle that is not 90 degrees Bone segments are pulled apart as a result of a twisting motion Several breaks result in many small pieces between two large segments One fragment is driven into the other, usually as a result of compression A partial fracture in which only one side of the bone is broken A fracture in which at least one end of the broken bone tears through the skin; carries a high risk of infection Types of Fractures Type of fracture Description oe kon A fracture in which the skin remains intact simple) Bone Repair When a bone breaks, blood flows from any vessel torn by the fracture. These vessels could be in the periosteum, osteons, and/or medullary cavity. The blood begins to clot, and about six to eight hours after the fracture, the clotting blood has formed a fracture hematoma ([link]a). The disruption of blood flow to the bone results in the death of bone cells around the fracture. Stages in Fracture Repair Hematoma New blood vessels cae eres Bony callus Spongy bone trabecula of spongy bone (b) (c) (d) The healing of a bone fracture follows a series of progressive steps: (a) A fracture hematoma forms. (b) Internal and external calli form. (c) Cartilage of the calli is replaced by trabecular bone. (d) Remodeling occurs. Within about 48 hours after the fracture, chondrocytes from the endosteum have created an internal callus (plural = calli) by secreting a fibrocartilaginous matrix between the two ends of the broken bone, while the periosteal chondrocytes and osteoblasts create an external callus of hyaline cartilage and bone, respectively, around the outside of the break ({link]b). This stabilizes the fracture. Over the next several weeks, osteoclasts resorb the dead bone; osteogenic cells become active, divide, and differentiate into osteoblasts. The cartilage in the calli is replaced by trabecular bone via endochondral ossification ((link]c). Eventually, the internal and external calli unite, compact bone replaces spongy bone at the outer margins of the fracture, and healing is complete. A slight swelling may remain on the outer surface of the bone, but quite often, that region undergoes remodeling ([link]d), and no external evidence of the fracture remains. Note: 40 = mess’ OPENStAX COLLEGE” — iL —_ 'T . ol r H Visit this website to review different types of fractures and then take a short self-assessment quiz. Chapter Review Fractured bones may be repaired by closed reduction or open reduction. Fractures are classified by their complexity, location, and other features. Common types of fractures are transverse, oblique, spiral, comminuted, impacted, greenstick, open (or compound), and closed (or simple). Healing of fractures begins with the formation of a hematoma, followed by internal and external calli. Osteoclasts resorb dead bone, while osteoblasts create new bone that replaces the cartilage in the calli. The calli eventually unite, remodeling occurs, and healing is complete. Review Questions Exercise: Problem:A fracture can be both a. open and closed b. open and transverse c. transverse and greenstick d. greenstick and comminuted Solution: B Exercise: Problem: How can a fractured diaphysis release fat globules into the bloodstream? a. The bone pierces fat stores in the skin. b. The yellow marrow in the diaphysis is exposed and damaged. c. The injury triggers the body to release fat from healthy bones. d. The red marrow in the fractured bone releases fat to heal the fracture. Solution: B Exercise: Problem:In a compound fracture, a. the break occurs at an angle to the bone b. the broken bone does not tear the skin c. one fragment of broken bone is compressed into the other d. broken bone pierces the skin Solution: D Exercise: Problem:The internal and external calli are replaced by a. hyaline cartilage b. trabecular bone c. osteogenic cells d. osteoclasts Solution: B Exercise: Problem: The first type of bone to form during fracture repair is bone. a. compact b. lamellar c. spongy d. dense Solution: C Critical Thinking Questions Exercise: Problem: What is the difference between closed reduction and open reduction? In what type of fracture would closed reduction most likely occur? In what type of fracture would open reduction most likely occur? Solution: In closed reduction, the broken ends of a fractured bone can be reset without surgery. Open reduction requires surgery to return the broken ends of the bone to their correct anatomical position. A partial fracture would likely require closed reduction. A compound fracture would require open reduction. Exercise: Problem: In terms of origin and composition, what are the differences between an internal callus and an external callus? Solution: The internal callus is produced by cells in the endosteum and is composed of a fibrocartilaginous matrix. The external callus is produced by cells in the periosteum and consists of hyaline cartilage and bone. Glossary closed reduction manual manipulation of a broken bone to set it into its natural position without surgery external callus collar of hyaline cartilage and bone that forms around the outside of a fracture fracture broken bone fracture hematoma blood clot that forms at the site of a broken bone internal callus fibrocartilaginous matrix, in the endosteal region, between the two ends of a broken bone open reduction surgical exposure of a bone to reset a fracture Divisions of the Skeletal System By the end of this section, you will be able to: e Discuss the functions of the skeletal system e Distinguish between the axial skeleton and appendicular skeleton ¢ Define the axial skeleton and its components ¢ Define the appendicular skeleton and its components The skeletal system includes all of the bones, cartilages, and ligaments of the body that support and give shape to the body and body structures. The skeleton consists of the bones of the body. For adults, there are 206 bones in the skeleton. Younger individuals have higher numbers of bones because some bones fuse together during childhood and adolescence to form an adult bone. The primary functions of the skeleton are to provide a rigid, internal structure that can support the weight of the body against the force of gravity, and to provide a structure upon which muscles can act to produce movements of the body. The lower portion of the skeleton is specialized for stability during walking or running. In contrast, the upper skeleton has greater mobility and ranges of motion, features that allow you to lift and carry objects or turn your head and trunk. In addition to providing for support and movements of the body, the skeleton has protective and storage functions. It protects the internal organs, including the brain, spinal cord, heart, lungs, and pelvic organs. The bones of the skeleton serve as the primary storage site for important minerals such as calcium and phosphate. The bone marrow found within bones stores fat and houses the blood-cell producing tissue of the body. The skeleton is subdivided into two major divisions—the axial and appendicular. The Axial Skeleton The skeleton is subdivided into two major divisions—the axial and appendicular. The axial skeleton forms the vertical, central axis of the body and includes all bones of the head, neck, chest, and back ((link]). It serves to protect the brain, spinal cord, heart, and lungs. It also serves as the attachment site for muscles that move the head, neck, and back, and for muscles that act across the shoulder and hip joints to move their corresponding limbs. The axial skeleton of the adult consists of 80 bones, including the skull, the vertebral column, and the thoracic cage. The skull is formed by 22 bones. Also associated with the head are an additional seven bones, including the hyoid bone and the ear ossicles (three small bones found in each middle ear). The vertebral column consists of 24 bones, each called a vertebra, plus the sacrum and coccyx. The thoracic cage includes the 12 pairs of ribs, and the sternum, the flattened bone of the anterior chest. Axial and Appendicular Skeleton Skull Cranial portion 6 ) Facial portion ee, ey ee Pectoral (shoulder) girdle ft} = ae S (FRI mn Clavicle E =n) SS eS) Scapula (ee == | Thoracic cage —— Sternum a Ly ‘Z pnt Ribs : Vertebral yw ZEN y) _ Uppeeilins = Vertebral column Gor . Humerus column Pelvic J er. = | one fe. h Pelvic girdle WZ | Radius wey, girdle (hip bones) | é \ Carpals < OC (hip bones) fie oT Met Is foe Oe g yi ff iN etacarpals /) N | fiw f AY | of \ Phalanges y iy) i 7h i Lower limb ———————- Femur i G&G EE Patella C) Key Axial skeleton Appendicular skeleton Tarsals \_}— Metatarsals Phalanges Anterior view Posterior view The axial skeleton supports the head, neck, back, and chest and thus forms the vertical axis of the body. It consists of the skull, vertebral column (including the sacrum and coccyx), and the thoracic cage, formed by the ribs and sternum. The appendicular skeleton is made up of all bones of the upper and lower limbs. The Appendicular Skeleton The appendicular skeleton includes all bones of the upper and lower limbs, plus the bones that attach each limb to the axial skeleton. There are 126 bones in the appendicular skeleton of an adult. The bones of the appendicular skeleton are covered in a separate chapter. Chapter Review The skeletal system includes all of the bones, cartilages, and ligaments of the body. It serves to support the body, protect the brain and other internal organs, and provides a rigid structure upon which muscles can pull to generate body movements. It also stores fat and the tissue responsible for the production of blood cells. The skeleton is subdivided into two parts. The axial skeleton forms a vertical axis that includes the head, neck, back, and chest. It has 80 bones and consists of the skull, vertebral column, and thoracic cage. The adult vertebral column consists of 24 vertebrae plus the sacrum and coccyx. The thoracic cage is formed by 12 pairs of ribs and the sternum. The appendicular skeleton consists of 126 bones in the adult and includes all of the bones of the upper and lower limbs plus the bones that anchor each limb to the axial skeleton. Review Questions Exercise: Problem: Which of the following is part of the axial skeleton? a. shoulder bones b. thigh bone c. foot bones d. vertebral column Solution: D Exercise: Problem: Which of the following is a function of the axial skeleton? a. allows for movement of the wrist and hand b. protects nerves and blood vessels at the elbow c. supports trunk of body d. allows for movements of the ankle and foot Solution: C Exercise: Problem:The axial skeleton a. consists of 126 bones b. forms the vertical axis of the body c. includes all bones of the body trunk and limbs d. includes only the bones of the lower limbs Solution: B Critical Thinking Question Exercise: Problem: Define the two divisions of the skeleton. Solution: The axial skeleton forms the vertical axis of the body and includes the bones of the head, neck, back, and chest of the body. It consists of 80 bones that include the skull, vertebral column, and thoracic cage. The appendicular skeleton consists of 126 bones and includes all bones of the upper and lower limbs. Exercise: Problem: Discuss the functions of the axial skeleton. Solution: The axial skeleton supports the head, neck, back, and chest of the body and allows for movements of these body regions. It also gives bony protections for the brain, spinal cord, heart, and lungs; stores fat and minerals; and houses the blood-cell producing tissue. Glossary appendicular skeleton all bones of the upper and lower limbs, plus the girdle bones that attach each limb to the axial skeleton axial skeleton central, vertical axis of the body, including the skull, vertebral column, and thoracic cage coccyx small bone located at inferior end of the adult vertebral column that is formed by the fusion of four coccygeal vertebrae; also referred to as the “tailbone” ear ossicles three small bones located in the middle ear cavity that serve to transmit sound vibrations to the inner ear hyoid bone small, U-shaped bone located in upper neck that does not contact any other bone ribs thin, curved bones of the chest wall sacrum single bone located near the inferior end of the adult vertebral column that is formed by the fusion of five sacral vertebrae; forms the posterior portion of the pelvis skeleton bones of the body skull bony structure that forms the head, face, and jaws, and protects the brain; consists of 22 bones sternum flattened bone located at the center of the anterior chest thoracic cage consists of 12 pairs of ribs and sternum vertebra individual bone in the neck and back regions of the vertebral column vertebral column entire sequence of bones that extend from the skull to the tailbone The Skull By the end of this section, you will be able to: List and identify the bones of the brain case and face Locate the major suture lines of the skull and name the bones associated with each Locate and define the boundaries of the anterior, middle, and posterior cranial fossae, the temporal fossa, and infratemporal fossa Define the paranasal sinuses and identify the location of each Name the bones that make up the walls of the orbit and identify the openings associated with the orbit Identify the bones and structures that form the nasal septum and nasal conchae, and locate the hyoid bone Identify the bony openings of the skull The cranium (skull) is the skeletal structure of the head that supports the face and protects the brain. It is subdivided into the facial bones and the brain case, or cranial vault ([link]). The facial bones underlie the facial structures, form the nasal cavity, enclose the eyeballs, and support the teeth of the upper and lower jaws. The rounded brain case surrounds and protects the brain and houses the middle and inner ear structures. In the adult, the skull consists of 22 individual bones, 21 of which are immobile and united into a single unit. The 22nd bone is the mandible (lower jaw), which is the only moveable bone of the skull. Parts of the Skull Brain case Facial bones The skull consists of the rounded brain case that houses the brain and the facial bones that form the upper and lower jaws, nose, orbits, and other facial structures. Watch this video to view a rotating and exploded skull, with color-coded bones. Which bone (yellow) is centrally located and joins with most of the other bones of the skull? Anterior View of Skull The anterior skull consists of the facial bones and provides the bony support for the eyes and structures of the face. This view of the skull is dominated by the openings of the orbits and the nasal cavity. Also seen are the upper and lower jaws, with their respective teeth ([link]). The orbit is the bony socket that houses the eyeball and muscles that move the eyeball or open the upper eyelid. The upper margin of the anterior orbit is the supraorbital margin. Located near the midpoint of the supraorbital margin is a small opening called the supraorbital foramen. This provides for passage of a sensory nerve to the skin of the forehead. Below the orbit is the infraorbital foramen, which is the point of emergence for a sensory nerve that supplies the anterior face below the orbit. Anterior View of Skull Coronal suture Glabella Frontal bone Parietal bone Supraorbital foramen Supraorbital margin . Orbit Sphenoid bone : Optic canal Temporal bone Superior orbital fissure Ethmoid bone Lacrimal bone Nasal bone . Inferior orbital fissure Palatine bone Zygomatic bone Nasal septum: Perpendicular plate Infraorbital foramen of ethmoid bone Vomer bone Middle nasal concha Inferior nasal concha Maxilla Alveolar process of maxilla Alveolar process . / of mandible Mental foramen ——————__>) € y —_—_ Mandible Anterior view An anterior view of the skull shows the bones that form the forehead, orbits (eye sockets), nasal cavity, nasal septum, and upper and lower jaws. Inside the nasal area of the skull, the nasal cavity is divided into halves by the nasal septum. The upper portion of the nasal septum is formed by the perpendicular plate of the ethmoid bone and the lower portion is the vomer bone. Each side of the nasal cavity is triangular in shape, with a broad inferior space that narrows superiorly. When looking into the nasal cavity from the front of the skull, two bony plates are seen projecting from each lateral wall. The larger of these is the inferior nasal concha, an independent bone of the skull. Located just above the inferior concha is the middle nasal concha, which is part of the ethmoid bone. A third bony plate, also part of the ethmoid bone, is the superior nasal concha. It is much smaller and out of sight, above the middle concha. The superior nasal concha is located just lateral to the perpendicular plate, in the upper nasal cavity. Lateral View of Skull A view of the lateral skull is dominated by the large, rounded brain case above and the upper and lower jaws with their teeth below ([link]). Separating these areas is the bridge of bone called the zygomatic arch. The zygomatic arch is the bony arch on the side of skull that spans from the area of the cheek to just above the ear canal. It is formed by the junction of two bony processes: a short anterior component, the temporal process of the zygomatic bone (the cheekbone) and a longer posterior portion, the zygomatic process of the temporal bone, extending forward from the temporal bone. Thus the temporal process (anteriorly) and the zygomatic process (posteriorly) join together, like the two ends of a drawbridge, to form the zygomatic arch. One of the major muscles that pulls the mandible upward during biting and chewing arises from the zygomatic arch. On the lateral side of the brain case, above the level of the zygomatic arch, is a Shallow space called the temporal fossa. Below the level of the zygomatic arch and deep to the vertical portion of the mandible is another space called the infratemporal fossa. Both the temporal fossa and infratemporal fossa contain muscles that act on the mandible during chewing. Lateral View of Skull Zygomatic arch Coronal suture Parietal bone Frontal bone Greater wing of sphenoid bone Si tl quamous suture Ethmoid bone Temporal bone Lacrimal bone Squamous ‘ temporal Lacrimal fossa Zygomatic process ~L_ External acoustic meatus Lambdoid suture Nasal bone Zygomatic bone Mastoid portion Temporal process Styloid process Mastoid process Maxilla Articular tubercle Occipital bone Mandibular fossa Mandible Mental protuberance of mandible Right lateral view The lateral skull shows the large rounded brain case, zygomatic arch, and the upper and lower jaws. The zygomatic arch is formed jointly by the zygomatic process of the temporal bone and the temporal process of the zygomatic bone. The shallow space above the zygomatic arch is the temporal fossa. The space inferior to the zygomatic arch and deep to the posterior mandible is the infratemporal fossa. Bones of the Brain Case The brain case contains and protects the brain. The interior space that is almost completely occupied by the brain is called the cranial cavity. This cavity is bounded superiorly by the rounded top of the skull, which is called the calvaria (skullcap), and the lateral and posterior sides of the skull. The bones that form the top and sides of the brain case are usually referred to as the “flat” bones of the skull. The floor of the brain case is referred to as the base of the skull. This is a complex area that varies in depth and has numerous openings for the passage of cranial nerves, blood vessels, and the spinal cord. Inside the skull, the base is subdivided into three large spaces, called the anterior cranial fossa, middle cranial fossa, and posterior cranial fossa (fossa = “trench or ditch”) ([link]). From anterior to posterior, the fossae increase in depth. The shape and depth of each fossa corresponds to the shape and size of the brain region that each houses. The boundaries and openings of the cranial fossae (singular = fossa) will be described in a later section. Cranial Fossae Anterior cranial fossa Middle cranial fossa Posterior cranial fossa Brain within —____. cranial cavity ; Lateral view The bones of the brain case surround and protect the brain, which occupies the cranial cavity. The base of the brain case, which forms the floor of cranial cavity, is subdivided into the shallow anterior cranial fossa, the middle cranial fossa, and the deep posterior cranial fossa. The brain case consists of eight bones. These include the paired parietal and temporal bones, plus the unpaired frontal, occipital, sphenoid, and ethmoid bones. Parietal Bone The parietal bone forms most of the upper lateral side of the skull (see [link]). These are paired bones, with the right and left parietal bones joining together at the top of the skull. Each parietal bone is also bounded anteriorly by the frontal bone, inferiorly by the temporal bone, and posteriorly by the occipital bone. Temporal Bone The temporal bone forms the lower lateral side of the skull (see [link]). Common wisdom has it that the temporal bone (temporal = “time”’) is so named because this area of the head (the temple) is where hair typically first turns gray, indicating the passage of time. The temporal bone is subdivided into several regions ({link]). The flattened, upper portion is the squamous portion of the temporal bone. Below this area and projecting anteriorly is the zygomatic process of the temporal bone, which forms the posterior portion of the zygomatic arch. Posteriorly is the mastoid portion of the temporal bone. Projecting inferiorly from this region is a large prominence, the mastoid process, which serves as a muscle attachment site. The mastoid process can easily be felt on the side of the head just behind your earlobe. On the interior of the skull, the petrous portion of each temporal bone forms the prominent, diagonally oriented petrous ridge in the floor of the cranial cavity. Located inside each petrous ridge are small cavities that house the structures of the middle and inner ears. Temporal Bone External acoustic Squamous meatus portion Zygomatic process Articular Masicid tubercle pomiat Mandibular fossa Mastoid process Styloid process A lateral view of the isolated temporal bone shows the squamous, mastoid, and zygomatic portions of the temporal bone. Important landmarks of the temporal bone, as shown in [link], include the following: e External acoustic meatus (ear canal)—This is the large opening on the lateral side of the skull that is associated with the ear. ¢ Internal acoustic meatus—This opening is located inside the cranial cavity, on the medial side of the petrous ridge. It connects to the middle and inner ear cavities of the temporal bone. e Mandibular fossa—This is the deep, oval-shaped depression located on the external base of the skull, just in front of the external acoustic meatus. The mandible (lower jaw) joins with the skull at this site as part of the temporomandibular joint, which allows for movements of the mandible during opening and closing of the mouth. e Articular tubercle—The smooth ridge located immediately anterior to the mandibular fossa. Both the articular tubercle and mandibular fossa contribute to the temporomandibular joint, the joint that provides for movements between the temporal bone of the skull and the mandible. e Styloid process—Posterior to the mandibular fossa on the external base of the skull is an elongated, downward bony projection called the styloid process, so named because of its resemblance to a stylus (a pen or writing tool). This structure serves as an attachment site for several small muscles and for a ligament that supports the hyoid bone of the neck. (See also [link].) ¢ Stylomastoid foramen—This small opening is located between the styloid process and mastoid process. This is the point of exit for the cranial nerve that supplies the facial muscles. ¢ Carotid canal—tThe carotid canal is a zig-zag shaped tunnel that provides passage through the base of the skull for one of the major arteries that supplies the brain. Its entrance is located on the outside base of the skull, anteromedial to the styloid process. The canal then runs anteromedially within the bony base of the skull, and then turns upward to its exit in the floor of the middle cranial cavity, above the foramen lacerum. External and Internal Views of Base of Skull Maxilla: Palatine process Zygomatic bone Palatine bone (horizontal plate) Zygomatic arch Medial and lateral pterygoid plates Articular tubercle Sphenoid bone Foramen ovale Foramen spinosum Mandibular fossa Foramen lacerum External auditory meatus Jugular foramen Mastoid process Occipital condyle Styloid process Temporal bone Stylomastoid foramen Foramen magnum F g Entrance to carotid canal Occipital bone Superior nuchal line External occipital protuberance (a) Inferior view Transverse plane Frontal bone Ethmoid bone: Crista galli Superior orbital Cribriform plate fissure Sphenoid bone: Lesser wing Hypophyseal eit eels Foramen rotundum turcica Foramen lacerum and exit of carotid canal Foramen ovale Foramen spinosum Internal acoustic meatus Temporal bone Hypoglossal canal Petrous portion Foramen magnum (petrous ridge) Occipital bone Jugular foramen Parietal bone (b) Superior view (a) The hard palate is formed anteriorly by the palatine processes of the maxilla bones and posteriorly by the horizontal plate of the palatine bones. (b) The complex floor of the cranial cavity is formed by the frontal, ethmoid, sphenoid, temporal, and occipital bones. The lesser wing of the sphenoid bone separates the anterior and middle cranial fossae. The petrous ridge (petrous portion of temporal bone) separates the middle and posterior cranial fossae. Frontal Bone The frontal bone is the single bone that forms the forehead. At its anterior midline, between the eyebrows, there is a slight depression called the glabella (see [link]). The frontal bone also forms the supraorbital margin of the orbit. Near the middle of this margin, is the supraorbital foramen, the opening that provides passage for a sensory nerve to the forehead. The frontal bone is thickened just above each supraorbital margin, forming rounded brow ridges. These are located just behind your eyebrows and vary in size among individuals, although they are generally larger in males. Inside the cranial cavity, the frontal bone extends posteriorly. This flattened region forms both the roof of the orbit below and the floor of the anterior cranial cavity above (see [link]b). Occipital Bone The occipital bone is the single bone that forms the posterior skull and posterior base of the cranial cavity ([link]; see also [link]). On its outside surface, at the posterior midline, is a small protrusion called the external occipital protuberance, which serves as an attachment site for a ligament of the posterior neck. Lateral to either side of this bump is a superior nuchal line (nuchal = “nape” or “posterior neck”). The nuchal lines represent the most superior point at which muscles of the neck attach to the skull, with only the scalp covering the skull above these lines. On the base of the skull, the occipital bone contains the large opening of the foramen magnum, which allows for passage of the spinal cord as it exits the skull. On either side of the foramen magnum is an oval-shaped occipital condyle. These condyles form joints with the first cervical vertebra and thus support the skull on top of the vertebral column. Posterior View of Skull Parietal bones Sagittal suture Lambdoid suture Occipital bone External occipital protuberance Temporal bone Superior nuchal line Mastoid process Occipital condyle Foramen magnum Zygomatic bone Posterior view This view of the posterior skull shows attachment sites for muscles and joints that support the skull. Sphenoid Bone The sphenoid bone is a single, complex bone of the central skull ([{link]). It serves as a “keystone” bone, because it joins with almost every other bone of the skull. The sphenoid forms much of the base of the central skull (see [link]) and also extends laterally to contribute to the sides of the skull (see [link]). Inside the cranial cavity, the right and left lesser wings of the sphenoid bone, which resemble the wings of a flying bird, form the lip of a prominent ridge that marks the boundary between the anterior and middle cranial fossae. The sella turcica (“Turkish saddle”) is located at the midline of the middle cranial fossa. This bony region of the sphenoid bone is named for its resemblance to the horse saddles used by the Ottoman Turks, with a high back and a tall front. The rounded depression in the floor of the sella turcica is the hypophyseal (pituitary) fossa, which houses the pea-sized pituitary (hypophyseal) gland. The greater wings of the sphenoid bone extend laterally to either side away from the sella turcica, where they form the anterior floor of the middle cranial fossa. The greater wing is best seen on the outside of the lateral skull, where it forms a rectangular area immediately anterior to the squamous portion of the temporal bone. On the inferior aspect of the skull, each half of the sphenoid bone forms two thin, vertically oriented bony plates. These are the medial pterygoid plate and lateral pterygoid plate (pterygoid = “wing-shaped”). The right and left medial pterygoid plates form the posterior, lateral walls of the nasal cavity. The somewhat larger lateral pterygoid plates serve as attachment sites for chewing muscles that fill the infratemporal space and act on the mandible. Sphenoid Bone Superior orbital fissure Foramen rotundum < 'y Foramen ovale Ww “| Foramen spinosum Hypophyseal fossa of sella turcica Body of sphenoid (a) Superior view Body of sphenoid Lesser wing Superior orbital fissure Pterygoid plates (b) Posterior view Shown in isolation in (a) superior and (b) posterior views, the sphenoid bone is a single midline bone that forms the anterior walls and floor of the middle cranial fossa. It has a pair of lesser wings and a pair of greater wings. The sella turcica surrounds the hypophyseal fossa. Projecting downward are the medial and lateral pterygoid plates. The sphenoid has multiple openings for the passage of nerves and blood vessels, including the optic canal, superior orbital fissure, foramen rotundum, foramen ovale, and foramen spinosum. Ethmoid Bone The ethmoid bone is a single, midline bone that forms the roof and lateral walls of the upper nasal cavity, the upper portion of the nasal septum, and contributes to the medial wall of the orbit ({link] and [link]). On the interior of the skull, the ethmoid also forms a portion of the floor of the anterior cranial cavity (see [link |b). Within the nasal cavity, the perpendicular plate of the ethmoid bone forms the upper portion of the nasal septum. The ethmoid bone also forms the lateral walls of the upper nasal cavity. Extending from each lateral wall are the superior nasal concha and middle nasal concha, which are thin, curved projections that extend into the nasal cavity ({link]). In the cranial cavity, the ethmoid bone forms a small area at the midline in the floor of the anterior cranial fossa. This region also forms the narrow roof of the underlying nasal cavity. This portion of the ethmoid bone consists of two parts, the crista galli and cribriform plates. The crista galli (“rooster’s comb or crest”) is a small upward bony projection located at the midline. It functions as an anterior attachment point for one of the covering layers of the brain. To either side of the crista galli is the cribriform plate (cribrum = “sieve”), a small, flattened area with numerous small openings termed olfactory foramina. Small nerve branches from the olfactory areas of the nasal cavity pass through these openings to enter the brain. The lateral portions of the ethmoid bone are located between the orbit and upper nasal cavity, and thus form the lateral nasal cavity wall and a portion of the medial orbit wall. Located inside this portion of the ethmoid bone are several small, air-filled spaces that are part of the paranasal sinus system of the skull. Sagittal Section of Skull Sella turcica: Hypophyseal fossa Parietal bone Crista galli Frontal sinus Cribriform plate Perpendicular plate Temporal bone Nasal bone F Sphenoid bone Internal acoustic meatus Sphenoid sinus Hypoglossal canal Inferior nasal concha Occipital bone Vomer Styloid process bs a Maxilla Medial and lateral pterygoid plates Palatine bone Mandibular foramen Mylohyoid line Mandible Hyoid bone This midline view of the sagittally sectioned skull shows the nasal septum. Ethmoid Bone Superior Crista galli Cribriform plate Ethmoid air cells Superior nasal ; concha Nasal cavity Middle nasal concha Medial wall of orbit Perpendicular plate Inferior The unpaired ethmoid bone is located at the midline within the central skull. It has an upward projection, the crista galli, and a downward projection, the perpendicular plate, which forms the upper nasal septum. The cribriform plates form both the roof of the nasal cavity and a portion of the anterior cranial fossa floor. The lateral sides of the ethmoid bone form the lateral walls of the upper nasal cavity, part of the medial orbit wall, and give rise to the superior and middle nasal conchae. The ethmoid bone also contains the ethmoid air cells. Lateral Wall of Nasal Cavity Ethmoid bone: Superior nasal concha Middle nasal concha Inferior nasal concha Sphenoidal sinus Medial view The three nasal conchae are curved bones that project from the lateral walls of the nasal cavity. The superior nasal concha and middle nasal concha are parts of the ethmoid bone. The inferior nasal concha is an independent bone of the skull. Sutures of the Skull A suture is an immobile joint between adjacent bones of the skull. The narrow gap between the bones is filled with dense, fibrous connective tissue that unites the bones. The long sutures located between the bones of the brain case are not straight, but instead follow irregular, tightly twisting paths. These twisting lines serve to tightly interlock the adjacent bones, thus adding strength to the skull for brain protection. The two suture lines seen on the top of the skull are the coronal and sagittal sutures. The coronal suture runs from side to side across the skull, within the coronal plane of section (see [link]). It joins the frontal bone to the right and left parietal bones. The sagittal suture extends posteriorly from the coronal suture, running along the midline at the top of the skull in the Sagittal plane of section (see [link]). It unites the right and left parietal bones. On the posterior skull, the sagittal suture terminates by joining the lambdoid suture. The lambdoid suture extends downward and laterally to either side away from its junction with the sagittal suture. The lambdoid suture joins the occipital bone to the right and left parietal and temporal bones. This suture is named for its upside-down "V" shape, which resembles the capital letter version of the Greek letter lambda (A). The squamous suture is located on the lateral skull. It unites the squamous portion of the temporal bone with the parietal bone (see [link]). At the intersection of four bones is the pterion, a small, capital-H-shaped suture line region that unites the frontal bone, parietal bone, squamous portion of the temporal bone, and greater wing of the sphenoid bone. It is the weakest part of the skull. The pterion is located approximately two finger widths above the zygomatic arch and a thumb’s width posterior to the upward portion of the zygomatic bone. Note: Disorders of the... Skeletal System Head and traumatic brain injuries are major causes of immediate death and disability, with bleeding and infections as possible additional complications. According to the Centers for Disease Control and Prevention (2010), approximately 30 percent of all injury-related deaths in the United States are caused by head injuries. The majority of head injuries involve falls. They are most common among young children (ages 0-4 years), adolescents (15-19 years), and the elderly (over 65 years). Additional causes vary, but prominent among these are automobile and motorcycle accidents. Strong blows to the brain-case portion of the skull can produce fractures. These may result in bleeding inside the skull with subsequent injury to the brain. The most common is a linear skull fracture, in which fracture lines radiate from the point of impact. Other fracture types include a comminuted fracture, in which the bone is broken into several pieces at the point of impact, or a depressed fracture, in which the fractured bone is pushed inward. In a contrecoup (counterblow) fracture, the bone at the point of impact is not broken, but instead a fracture occurs on the opposite side of the skull. Fractures of the occipital bone at the base of the skull can occur in this manner, producing a basilar fracture that can damage the artery that passes through the carotid canal. A blow to the lateral side of the head may fracture the bones of the pterion. The pterion is an important clinical landmark because located immediately deep to it on the inside of the skull is a major branch of an artery that supplies the skull and covering layers of the brain. A strong blow to this region can fracture the bones around the pterion. If the underlying artery is damaged, bleeding can cause the formation of a hematoma (collection of blood) between the brain and interior of the skull. As blood accumulates, it will put pressure on the brain. Symptoms associated with a hematoma may not be apparent immediately following the injury, but if untreated, blood accumulation will exert increasing pressure on the brain and can result in death within a few hours. Note: [= [=] ‘all “eat a ee 5 openstax COLLEGE” View this animation to see how a blow to the head may produce a contrecoup (counterblow) fracture of the basilar portion of the occipital bone on the base of the skull. Why may a basilar fracture be life threatening? Facial Bones of the Skull The facial bones of the skull form the upper and lower jaws, the nose, nasal cavity and nasal septum, and the orbit. The facial bones include 14 bones, with six paired bones and two unpaired bones. The paired bones are the maxilla, palatine, zygomatic, nasal, lacrimal, and inferior nasal conchae bones. The unpaired bones are the vomer and mandible bones. Although classified with the brain-case bones, the ethmoid bone also contributes to the nasal septum and the walls of the nasal cavity and orbit. Maxillary Bone The maxillary bone, often referred to simply as the maxilla (plural = maxillae), is one of a pair that together form the upper jaw, much of the hard palate, the medial floor of the orbit, and the lateral base of the nose (see [link]). The curved, inferior margin of the maxillary bone that forms the upper jaw and contains the upper teeth is the alveolar process of the maxilla ({link]). Each tooth is anchored into a deep socket called an alveolus. On the anterior maxilla, just below the orbit, is the infraorbital foramen. This is the point of exit for a sensory nerve that supplies the nose, upper lip, and anterior cheek. On the inferior skull, the palatine process from each maxillary bone can be seen joining together at the midline to form the anterior three-quarters of the hard palate (see [link]a). The hard palate is the bony plate that forms the roof of the mouth and floor of the nasal cavity, separating the oral and nasal cavities. Maxillary Bone Articulates with frontal bone Infraorbital foramen Alveolar process Right lateral view The maxillary bone forms the upper jaw and supports the upper teeth. Each maxilla also forms the lateral floor of each orbit and the majority of the hard palate. Palatine Bone The palatine bone is one of a pair of irregularly shaped bones that contribute small areas to the lateral walls of the nasal cavity and the medial wall of each orbit. The largest region of each of the palatine bone is the horizontal plate. The plates from the right and left palatine bones join together at the midline to form the posterior quarter of the hard palate (see [link]a). Thus, the palatine bones are best seen in an inferior view of the skull and hard palate. Note: Homeostatic Imbalances Cleft Lip and Cleft Palate During embryonic development, the right and left maxilla bones come together at the midline to form the upper jaw. At the same time, the muscle and skin overlying these bones join together to form the upper lip. Inside the mouth, the palatine processes of the maxilla bones, along with the horizontal plates of the right and left palatine bones, join together to form the hard palate. If an error occurs in these developmental processes, a birth defect of cleft lip or cleft palate may result. Cleft lip is a common development defect that affects approximately 1:1000 births, most of which are male. This defect involves a partial or complete failure of the right and left portions of the upper lip to fuse together, leaving a cleft (gap). A more severe developmental defect is cleft palate, which affects the hard palate. The hard palate is the bony structure that separates the nasal cavity from the oral cavity. It is formed during embryonic development by the midline fusion of the horizontal plates from the right and left palatine bones and the palatine processes of the maxilla bones. Cleft palate affects approximately 1:2500 births and is more common in females. It results from a failure of the two halves of the hard palate to completely come together and fuse at the midline, thus leaving a gap between them. This gap allows for communication between the nasal and oral cavities. In severe cases, the bony gap continues into the anterior upper jaw where the alveolar processes of the maxilla bones also do not properly join together above the front teeth. If this occurs, a cleft lip will also be seen. Because of the communication between the oral and nasal cavities, a cleft palate makes it very difficult for an infant to generate the suckling needed for nursing, thus leaving the infant at risk for malnutrition. Surgical repair is required to correct cleft palate defects. Zygomatic Bone The zygomatic bone is also known as the cheekbone. Each of the paired zygomatic bones forms much of the lateral wall of the orbit and the lateral- inferior margins of the anterior orbital opening (see [link]). The short temporal process of the zygomatic bone projects posteriorly, where it forms the anterior portion of the zygomatic arch (see [link]). Nasal Bone The nasal bone is one of two small bones that articulate (join) with each other to form the bony base (bridge) of the nose. They also support the cartilages that form the lateral walls of the nose (see [link]). These are the bones that are damaged when the nose is broken. Lacrimal Bone Each lacrimal bone is a small, rectangular bone that forms the anterior, medial wall of the orbit (see [link] and [link]). The anterior portion of the lacrimal bone forms a shallow depression called the lacrimal fossa, and extending inferiorly from this is the nasolacrimal canal. The lacrimal fluid (tears of the eye), which serves to maintain the moist surface of the eye, drains at the medial corner of the eye into the nasolacrimal canal. This duct then extends downward to open into the nasal cavity, behind the inferior nasal concha. In the nasal cavity, the lacrimal fluid normally drains posteriorly, but with an increased flow of tears due to crying or eye irritation, some fluid will also drain anteriorly, thus causing a runny nose. Inferior Nasal Conchae The right and left inferior nasal conchae form a curved bony plate that projects into the nasal cavity space from the lower lateral wall (see [link]). The inferior concha is the largest of the nasal conchae and can easily be seen when looking into the anterior opening of the nasal cavity. Vomer Bone The unpaired vomer bone, often referred to simply as the vomer, is triangular-shaped and forms the posterior-inferior part of the nasal septum (see [link]). The vomer is best seen when looking from behind into the posterior openings of the nasal cavity (see [link]a). In this view, the vomer is seen to form the entire height of the nasal septum. A much smaller portion of the vomer can also be seen when looking into the anterior opening of the nasal cavity. Mandible The mandible forms the lower jaw and is the only moveable bone of the skull. At the time of birth, the mandible consists of paired right and left bones, but these fuse together during the first year to form the single U- shaped mandible of the adult skull. Each side of the mandible consists of a horizontal body and posteriorly, a vertically oriented ramus of the mandible (ramus = “branch’). The outside margin of the mandible, where the body and ramus come together is called the angle of the mandible ({link]). The ramus on each side of the mandible has two upward-going bony projections. The more anterior projection is the flattened coronoid process of the mandible, which provides attachment for one of the biting muscles. The posterior projection is the condylar process of the mandible, which is topped by the oval-shaped condyle. The condyle of the mandible articulates (joins) with the mandibular fossa and articular tubercle of the temporal bone. Together these articulations form the temporomandibular joint, which allows for opening and closing of the mouth (see [link]). The broad U- shaped curve located between the coronoid and condylar processes is the mandibular notch. Important landmarks for the mandible include the following: e Alveolar process of the mandible—This is the upper border of the mandibular body and serves to anchor the lower teeth. ¢ Mental protuberance—The forward projection from the inferior margin of the anterior mandible that forms the chin (mental = “chin’”). ¢ Mental foramen—The opening located on each side of the anterior- lateral mandible, which is the exit site for a sensory nerve that supplies the chin. e Mylohyoid line—This bony ridge extends along the inner aspect of the mandibular body (see [link]). The muscle that forms the floor of the oral cavity attaches to the mylohyoid lines on both sides of the mandible. e Mandibular foramen—This opening is located on the medial side of the ramus of the mandible. The opening leads into a tunnel that runs down the length of the mandibular body. The sensory nerve and blood vessels that supply the lower teeth enter the mandibular foramen and then follow this tunnel. Thus, to numb the lower teeth prior to dental work, the dentist must inject anesthesia into the lateral wall of the oral cavity at a point prior to where this sensory nerve enters the mandibular foramen. ¢ Lingula—tThis small flap of bone is named for its shape (lingula = “little tongue”). It is located immediately next to the mandibular foramen, on the medial side of the ramus. A ligament that anchors the mandible during opening and closing of the mouth extends down from the base of the skull and attaches to the lingula. Isolated Mandible Condylar process ; Coronoid process Lingula Mandibular notch Mandibular condyle Mandibular foramen Mylohyoid line Alveolar process Ramus of mandible Mental protuberance Mental foramen Mandibular angle Body of mandible Right lateral view The mandible is the only moveable bone of the skull. The Orbit The orbit is the bony socket that houses the eyeball and contains the muscles that move the eyeball or open the upper eyelid. Each orbit is cone- shaped, with a narrow posterior region that widens toward the large anterior opening. To help protect the eye, the bony margins of the anterior opening are thickened and somewhat constricted. The medial walls of the two orbits are parallel to each other but each lateral wall diverges away from the midline at a 45° angle. This divergence provides greater lateral peripheral vision. The walls of each orbit include contributions from seven skull bones ({link]). The frontal bone forms the roof and the zygomatic bone forms the lateral wall and lateral floor. The medial floor is primarily formed by the maxilla, with a small contribution from the palatine bone. The ethmoid bone and lacrimal bone make up much of the medial wall and the sphenoid bone forms the posterior orbit. At the posterior apex of the orbit is the opening of the optic canal, which allows for passage of the optic nerve from the retina to the brain. Lateral to this is the elongated and irregularly shaped superior orbital fissure, which provides passage for the artery that supplies the eyeball, sensory nerves, and the nerves that supply the muscles involved in eye movements. Bones of the Orbit Frontal bone Supraorbital foramen Supraorbital margin Sphenoid bone Nasal bone Lacrimal bone Ethmoid bone Lacrimal fossa Optic canal Superior orbital fissure Zygomatic bone Palatine bone Infraorbital foramen Seven skull bones contribute to the walls of the orbit. Opening into the posterior orbit from the cranial cavity are the optic canal and superior orbital fissure. The Nasal Septum and Nasal Conchae The nasal septum consists of both bone and cartilage components ([Link]; see also [link]). The upper portion of the septum is formed by the perpendicular plate of the ethmoid bone. The lower and posterior parts of the septum are formed by the triangular-shaped vomer bone. In an anterior view of the skull, the perpendicular plate of the ethmoid bone is easily seen inside the nasal opening as the upper nasal septum, but only a small portion of the vomer is seen as the inferior septum. A better view of the vomer bone is seen when looking into the posterior nasal cavity with an inferior view of the skull, where the vomer forms the full height of the nasal septum. The anterior nasal septum is formed by the septal cartilage, a flexible plate that fills in the gap between the perpendicular plate of the ethmoid and vomer bones. This cartilage also extends outward into the nose where it separates the right and left nostrils. The septal cartilage is not found in the dry skull. Attached to the lateral wall on each side of the nasal cavity are the superior, middle, and inferior nasal conchae (singular = concha), which are named for their positions (see [link]). These are bony plates that curve downward as they project into the space of the nasal cavity. They serve to swirl the incoming air, which helps to warm and moisturize it before the air moves into the delicate air sacs of the lungs. This also allows mucus, secreted by the tissue lining the nasal cavity, to trap incoming dust, pollen, bacteria, and viruses. The largest of the conchae is the inferior nasal concha, which is an independent bone of the skull. The middle concha and the superior conchae, which is the smallest, are both formed by the ethmoid bone. When looking into the anterior nasal opening of the skull, only the inferior and middle conchae can be seen. The small superior nasal concha is well hidden above and behind the middle concha. Nasal Septum Frontal bone Crista galli Frontal sinus Sphenoid sinus Nasal bone Nasal septum: Perpendicular plate of ethmoid bone Vomer bone Septal cartilage Sphenoid bone Qy Palatine process Horizontal plate — of maxilla of palatine bone Sagittal section The nasal septum is formed by the perpendicular plate of the ethmoid bone and the vomer bone. The septal cartilage fills the gap between these bones and extends into the nose. Cranial Fossae Inside the skull, the floor of the cranial cavity is subdivided into three cranial fossae (spaces), which increase in depth from anterior to posterior (see [link], [link]b, and [link]). Since the brain occupies these areas, the shape of each conforms to the shape of the brain regions that it contains. Each cranial fossa has anterior and posterior boundaries and is divided at the midline into right and left areas by a significant bony structure or opening. Anterior Cranial Fossa The anterior cranial fossa is the most anterior and the shallowest of the three cranial fossae. It overlies the orbits and contains the frontal lobes of the brain. Anteriorly, the anterior fossa is bounded by the frontal bone, which also forms the majority of the floor for this space. The lesser wings of the sphenoid bone form the prominent ledge that marks the boundary between the anterior and middle cranial fossae. Located in the floor of the anterior cranial fossa at the midline is a portion of the ethmoid bone, consisting of the upward projecting crista galli and to either side of this, the cribriform plates. Middle Cranial Fossa The middle cranial fossa is deeper and situated posterior to the anterior fossa. It extends from the lesser wings of the sphenoid bone anteriorly, to the petrous ridges (petrous portion of the temporal bones) posteriorly. The large, diagonally positioned petrous ridges give the middle cranial fossa a butterfly shape, making it narrow at the midline and broad laterally. The temporal lobes of the brain occupy this fossa. The middle cranial fossa is divided at the midline by the upward bony prominence of the sella turcica, a part of the sphenoid bone. The middle cranial fossa has several openings for the passage of blood vessels and cranial nerves (see [link]). Openings in the middle cranial fossa are as follows: Optic canal—This opening is located at the anterior lateral comer of the sella turcica. It provides for passage of the optic nerve into the orbit. Superior orbital fissure—This large, irregular opening into the posterior orbit is located on the anterior wall of the middle cranial fossa, lateral to the optic canal and under the projecting margin of the lesser wing of the sphenoid bone. Nerves to the eyeball and associated muscles, and sensory nerves to the forehead pass through this opening. Foramen rotundum—This rounded opening (rotundum = “round”) is located in the floor of the middle cranial fossa, just inferior to the superior orbital fissure. It is the exit point for a major sensory nerve that supplies the cheek, nose, and upper teeth. Foramen ovale of the middle cranial fossa—This large, oval-shaped opening in the floor of the middle cranial fossa provides passage for a major sensory nerve to the lateral head, cheek, chin, and lower teeth. Foramen spinosum—This small opening, located posterior-lateral to the foramen ovale, is the entry point for an important artery that supplies the covering layers surrounding the brain. The branching pattern of this artery forms readily visible grooves on the internal surface of the skull and these grooves can be traced back to their origin at the foramen spinosum. Carotid canal—This is the zig-zag passageway through which a major artery to the brain enters the skull. The entrance to the carotid canal is located on the inferior aspect of the skull, anteromedial to the styloid process (see [link]a). From here, the canal runs anteromedially within the bony base of the skull. Just above the foramen lacerum, the carotid canal opens into the middle cranial cavity, near the posterior- lateral base of the sella turcica. Foramen lacerum—This irregular opening is located in the base of the skull, immediately inferior to the exit of the carotid canal. This opening is an artifact of the dry skull, because in life it is completely filled with cartilage. All the openings of the skull that provide for passage of nerves or blood vessels have smooth margins; the word lacerum (“ragged” or “torn’”) tells us that this opening has ragged edges and thus nothing passes through it. Posterior Cranial Fossa The posterior cranial fossa is the most posterior and deepest portion of the cranial cavity. It contains the cerebellum of the brain. The posterior fossa is bounded anteriorly by the petrous ridges, while the occipital bone forms the floor and posterior wall. It is divided at the midline by the large foramen magnum (“great aperture”), the opening that provides for passage of the spinal cord. Located on the medial wall of the petrous ridge in the posterior cranial fossa is the internal acoustic meatus (see [link]). This opening provides for passage of the nerve from the hearing and equilibrium organs of the inner ear, and the nerve that supplies the muscles of the face. Located at the anterior-lateral margin of the foramen magnum is the hypoglossal canal. These emerge on the inferior aspect of the skull at the base of the occipital condyle and provide passage for an important nerve to the tongue. Immediately inferior to the internal acoustic meatus is the large, irregularly shaped jugular foramen (see [link]a). Several cranial nerves from the brain exit the skull via this opening. It is also the exit point through the base of the skull for all the venous return blood leaving the brain. The venous structures that carry blood inside the skull form large, curved grooves on the inner walls of the posterior cranial fossa, which terminate at each jugular foramen. Paranasal Sinuses The paranasal sinuses are hollow, air-filled spaces located within certain bones of the skull ([link]). All of the sinuses communicate with the nasal cavity (paranasal = “next to nasal cavity”) and are lined with nasal mucosa. They serve to reduce bone mass and thus lighten the skull, and they also add resonance to the voice. This second feature is most obvious when you have a cold or sinus congestion. These produce swelling of the mucosa and excess mucus production, which can obstruct the narrow passageways between the sinuses and the nasal cavity, causing your voice to sound different to yourself and others. This blockage can also allow the sinuses to fill with fluid, with the resulting pressure producing pain and discomfort. The paranasal sinuses are named for the skull bone that each occupies. The frontal sinus is located just above the eyebrows, within the frontal bone (see [link]). This irregular space may be divided at the midline into bilateral spaces, or these may be fused into a single sinus space. The frontal sinus is the most anterior of the paranasal sinuses. The largest sinus is the maxillary sinus. These are paired and located within the right and left maxillary bones, where they occupy the area just below the orbits. The maxillary sinuses are most commonly involved during sinus infections. Because their connection to the nasal cavity is located high on their medial wall, they are difficult to drain. The sphenoid sinus is a single, midline sinus. It is located within the body of the sphenoid bone, just anterior and inferior to the sella turcica, thus making it the most posterior of the paranasal sinuses. The lateral aspects of the ethmoid bone contain multiple small spaces separated by very thin bony walls. Each of these spaces is called an ethmoid air cell. These are located on both sides of the ethmoid bone, between the upper nasal cavity and medial orbit, just behind the superior nasal conchae. Paranasal Sinuses Anterior Lateral The paranasal sinuses are hollow, air-filled spaces named for the skull bone that each occupies. The most anterior is the frontal sinus, located in the frontal bone above the eyebrows. The largest are the maxillary sinuses, located in the right and left maxillary bones below the orbits. The most posterior is the sphenoid sinus, located in the body of the sphenoid bone, under the sella turcica. The ethmoid air cells are multiple small spaces located in the right and left sides of the ethmoid bone, between the medial wall of the orbit and lateral wall of the upper nasal cavity. Hyoid Bone The hyoid bone is an independent bone that does not contact any other bone and thus is not part of the skull ({link]). It is a small U-shaped bone located in the upper neck near the level of the inferior mandible, with the tips of the “U” pointing posteriorly. The hyoid serves as the base for the tongue above, and is attached to the larynx below and the pharynx posteriorly. The hyoid is held in position by a series of small muscles that attach to it either from above or below. These muscles act to move the hyoid up/down or forward/back. Movements of the hyoid are coordinated with movements of the tongue, larynx, and pharynx during swallowing and speaking. Hyoid Bone Mandible Hyoid bone Larynx Greater horn Lesser horn Body Greater horn Lesser horn Right lateral view The hyoid bone is located in the upper neck and does not join with any other bone. It provides attachments for muscles that act on the tongue, larynx, and pharynx. Chapter Review The skull consists of the brain case and the facial bones. The brain case surrounds and protects the brain, which occupies the cranial cavity inside the skull. It consists of the rounded calvaria and a complex base. The brain case is formed by eight bones, the paired parietal and temporal bones plus the unpaired frontal, occipital, sphenoid, and ethmoid bones. The narrow gap between the bones is filled with dense, fibrous connective tissue that unites the bones. The sagittal suture joins the right and left parietal bones. The coronal suture joins the parietal bones to the frontal bone, the lamboid suture joins them to the occipital bone, and the squamous suture joins them to the temporal bone. The facial bones support the facial structures and form the upper and lower jaws. These consist of 14 bones, with the paired maxillary, palatine, zygomatic, nasal, lacrimal, and inferior conchae bones and the unpaired vomer and mandible bones. The ethmoid bone also contributes to the formation of facial structures. The maxilla forms the upper jaw and the mandible forms the lower jaw. The maxilla also forms the larger anterior portion of the hard palate, which is completed by the smaller palatine bones that form the posterior portion of the hard palate. The floor of the cranial cavity increases in depth from front to back and is divided into three cranial fossae. The anterior cranial fossa is located between the frontal bone and lesser wing of the sphenoid bone. A small area of the ethmoid bone, consisting of the crista galli and cribriform plates, is located at the midline of this fossa. The middle cranial fossa extends from the lesser wing of the sphenoid bone to the petrous ridge (petrous portion of temporal bone). The right and left sides are separated at the midline by the sella turcica, which surrounds the shallow hypophyseal fossa. Openings through the skull in the floor of the middle fossa include the optic canal and superior orbital fissure, which open into the posterior orbit, the foramen rotundum, foramen ovale, and foramen spinosum, and the exit of the carotid canal with its underlying foramen lacerum. The deep posterior cranial fossa extends from the petrous ridge to the occipital bone. Openings here include the large foramen magnum, plus the internal acoustic meatus, jugular foramina, and hypoglossal canals. Additional openings located on the external base of the skull include the stylomastoid foramen and the entrance to the carotid canal. The anterior skull has the orbits that house the eyeballs and associated muscles. The walls of the orbit are formed by contributions from seven bones: the frontal, zygomatic, maxillary, palatine, ethmoid, lacrimal, and sphenoid. Located at the superior margin of the orbit is the supraorbital foramen, and below the orbit is the infraorbital foramen. The mandible has two openings, the mandibular foramen on its inner surface and the mental foramen on its external surface near the chin. The nasal conchae are bony projections from the lateral walls of the nasal cavity. The large inferior nasal concha is an independent bone, while the middle and superior conchae are parts of the ethmoid bone. The nasal septum is formed by the perpendicular plate of the ethmoid bone, the vomer bone, and the septal cartilage. The paranasal sinuses are air-filled spaces located within the frontal, maxillary, sphenoid, and ethmoid bones. On the lateral skull, the zygomatic arch consists of two parts, the temporal process of the zygomatic bone anteriorly and the zygomatic process of the temporal bone posteriorly. The temporal fossa is the shallow space located on the lateral skull above the level of the zygomatic arch. The infratemporal fossa is located below the zygomatic arch and deep to the ramus of the mandible. The hyoid bone is located in the upper neck and does not join with any other bone. It is held in position by muscles and serves to support the tongue above, the larynx below, and the pharynx posteriorly. Interactive Link Questions Exercise: Problem: Watch this video to view a rotating and exploded skull with color- coded bones. Which bone (yellow) is centrally located and joins with most of the other bones of the skull? Solution: The sphenoid bone joins with most other bones of the skull. It is centrally located, where it forms portions of the rounded brain case and cranial base. Exercise: Problem: View this animation to see how a blow to the head may produce a contrecoup (counterblow) fracture of the basilar portion of the occipital bone on the base of the skull. Why may a basilar fracture be life threatening? Solution: A basilar fracture may damage an artery entering the skull, causing bleeding in the brain. Review Questions Exercise: Problem: Which of the following is a bone of the brain case? a. parietal bone b. zygomatic bone c. maxillary bone d. lacrimal bone Solution: A Exercise: Problem:The lambdoid suture joins the parietal bone to the a. frontal bone b. occipital bone c. other parietal bone d. temporal bone Solution: B Exercise: Problem:The middle cranial fossa a. is bounded anteriorly by the petrous ridge b. is bounded posteriorly by the lesser wing of the sphenoid bone c. is divided at the midline by a small area of the ethmoid bone d. has the foramen rotundum, foramen ovale, and foramen spinosum Solution: D Exercise: Problem:The paranasal sinuses are a. air-filled spaces found within the frontal, maxilla, sphenoid, and ethmoid bones only b. air-filled spaces found within all bones of the skull c. not connected to the nasal cavity d. divided at the midline by the nasal septum Solution: A Exercise: Problem: Parts of the sphenoid bone include the a. sella turcica b. squamous portion c. glabella d. zygomatic process Solution: A Exercise: Problem:The bony openings of the skull include the a. carotid canal, which is located in the anterior cranial fossa b. superior orbital fissure, which is located at the superior margin of the anterior orbit c. mental foramen, which is located just below the orbit d. hypoglossal canal, which is located in the posterior cranial fossa Solution: D Critical Thinking Questions Exercise: Problem: Define and list the bones that form the brain case or support the facial structures. Solution: The brain case is that portion of the skull that surrounds and protects the brain. It is subdivided into the rounded top of the skull, called the calvaria, and the base of the skull. There are eight bones that form the brain case. These are the paired parietal and temporal bones, plus the unpaired frontal, occipital, sphenoid, and ethmoid bones. The facial bones support the facial structures, and form the upper and lower jaws, nasal cavity, nasal septum, and orbit. There are 14 facial bones. These are the paired maxillary, palatine, zygomatic, nasal, lacrimal, and inferior nasal conchae bones, and the unpaired vomer and mandible bones. Exercise: Problem: Identify the major sutures of the skull, their locations, and the bones united by each. Solution: The coronal suture passes across the top of the anterior skull. It unites the frontal bone anteriorly with the right and left parietal bones. The sagittal suture runs at the midline on the top of the skull. It unites the right and left parietal bones with each other. The squamous suture is a curved suture located on the lateral side of the skull. It unites the squamous portion of the temporal bone to the parietal bone. The lambdoid suture is located on the posterior skull and has an inverted V- shape. It unites the occipital bone with the right and left parietal bones. Exercise: Problem: Describe the anterior, middle, and posterior cranial fossae and their boundaries, and give the midline structure that divides each into right and left areas. Solution: The anterior cranial fossa is the shallowest of the three cranial fossae. It extends from the frontal bone anteriorly to the lesser wing of the sphenoid bone posteriorly. It is divided at the midline by the crista galli and cribriform plates of the ethmoid bone. The middle cranial fossa is located in the central skull, and is deeper than the anterior fossa. The middle fossa extends from the lesser wing of the sphenoid bone anteriorly to the petrous ridge posteriorly. It is divided at the midline by the sella turcica. The posterior cranial fossa is the deepest fossa. It extends from the petrous ridge anteriorly to the occipital bone posteriorly. The large foramen magnum is located at the midline of the posterior fossa. Exercise: Problem: Describe the parts of the nasal septum in both the dry and living skull. Solution: There are two bony parts of the nasal septum in the dry skull. The perpendicular plate of the ethmoid bone forms the superior part of the septum. The vomer bone forms the inferior and posterior parts of the septum. In the living skull, the septal cartilage completes the septum by filling in the anterior area between the bony components and extending outward into the nose. References Centers for Disease Control and Prevention (US). Injury prevention and control: traumatic brain injury [Internet]. Atlanta, GA; [cited 2013 Mar 18]. Glossary alveolar process of the mandible upper border of mandibular body that contains the lower teeth alveolar process of the maxilla curved, inferior margin of the maxilla that supports and anchors the upper teeth angle of the mandible rounded corner located at outside margin of the body and ramus junction anterior cranial fossa shallowest and most anterior cranial fossa of the cranial base that extends from the frontal bone to the lesser wing of the sphenoid bone articular tubercle smooth ridge located on the inferior skull, immediately anterior to the mandibular fossa brain case portion of the skull that contains and protects the brain, consisting of the eight bones that form the cranial base and rounded upper skull calvaria (also, skullcap) rounded top of the skull carotid canal zig-zag tunnel providing passage through the base of the skull for the internal carotid artery to the brain; begins anteromedial to the styloid process and terminates in the middle cranial cavity, near the posterior- lateral base of the sella turcica condylar process of the mandible thickened upward projection from posterior margin of mandibular ramus condyle oval-shaped process located at the top of the condylar process of the mandible coronal suture joint that unites the frontal bone to the right and left parietal bones across the top of the skull coronoid process of the mandible flattened upward projection from the anterior margin of the mandibular ramus cranial cavity interior space of the skull that houses the brain cranium skull cribriform plate small, flattened areas with numerous small openings, located to either side of the midline in the floor of the anterior cranial fossa; formed by the ethmoid bone crista galli small upward projection located at the midline in the floor of the anterior cranial fossa; formed by the ethmoid bone ethmoid air cell one of several small, air-filled spaces located within the lateral sides of the ethmoid bone, between the orbit and upper nasal cavity ethmoid bone unpaired bone that forms the roof and upper, lateral walls of the nasal cavity, portions of the floor of the anterior cranial fossa and medial wall of orbit, and the upper portion of the nasal septum external acoustic meatus ear canal opening located on the lateral side of the skull external occipital protuberance small bump located at the midline on the posterior skull facial bones fourteen bones that support the facial structures and form the upper and lower jaws and the hard palate foramen lacerum irregular opening in the base of the skull, located inferior to the exit of carotid canal foramen magnum large opening in the occipital bone of the skull through which the spinal cord emerges and the vertebral arteries enter the cranium foramen ovale of the middle cranial fossa oval-shaped opening in the floor of the middle cranial fossa foramen rotundum round opening in the floor of the middle cranial fossa, located between the superior orbital fissure and foramen ovale foramen spinosum small opening in the floor of the middle cranial fossa, located lateral to the foramen ovale frontal bone unpaired bone that forms forehead, roof of orbit, and floor of anterior cranial fossa frontal sinus air-filled space within the frontal bone; most anterior of the paranasal sinuses glabella slight depression of frontal bone, located at the midline between the eyebrows greater wings of sphenoid bone lateral projections of the sphenoid bone that form the anterior wall of the middle cranial fossa and an area of the lateral skull hard palate bony structure that forms the roof of the mouth and floor of the nasal cavity, formed by the palatine process of the maxillary bones and the horizontal plate of the palatine bones horizontal plate medial extension from the palatine bone that forms the posterior quarter of the hard palate hypoglossal canal paired openings that pass anteriorly from the anterior-lateral margins of the foramen magnum deep to the occipital condyles hypophyseal (pituitary) fossa shallow depression on top of the sella turcica that houses the pituitary (hypophyseal) gland inferior nasal concha one of the paired bones that project from the lateral walls of the nasal cavity to form the largest and most inferior of the nasal conchae infraorbital foramen opening located on anterior skull, below the orbit infratemporal fossa space on lateral side of skull, below the level of the zygomatic arch and deep (medial) to the ramus of the mandible internal acoustic meatus opening into petrous ridge, located on the lateral wall of the posterior cranial fossa jugular foramen irregularly shaped opening located in the lateral floor of the posterior cranial cavity lacrimal bone paired bones that contribute to the anterior-medial wall of each orbit lacrimal fossa shallow depression in the anterior-medial wall of the orbit, formed by the lacrimal bone that gives rise to the nasolacrimal canal lambdoid suture inverted V-shaped joint that unites the occipital bone to the right and left parietal bones on the posterior skull lateral pterygoid plate paired, flattened bony projections of the sphenoid bone located on the inferior skull, lateral to the medial pterygoid plate lesser wings of the sphenoid bone lateral extensions of the sphenoid bone that form the bony lip separating the anterior and middle cranial fossae lingula small flap of bone located on the inner (medial) surface of mandibular ramus, next to the mandibular foramen mandible unpaired bone that forms the lower jaw bone; the only moveable bone of the skull mandibular foramen opening located on the inner (medial) surface of the mandibular ramus mandibular fossa oval depression located on the inferior surface of the skull mandibular notch large U-shaped notch located between the condylar process and coronoid process of the mandible mastoid process large bony prominence on the inferior, lateral skull, just behind the earlobe maxillary bone (also, maxilla) paired bones that form the upper jaw and anterior portion of the hard palate maxillary sinus air-filled space located with each maxillary bone; largest of the paranasal sinuses medial pterygoid plate paired, flattened bony projections of the sphenoid bone located on the inferior skull medial to the lateral pterygoid plate; form the posterior portion of the nasal cavity lateral wall mental foramen opening located on the anterior-lateral side of the mandibular body mental protuberance inferior margin of anterior mandible that forms the chin middle cranial fossa centrally located cranial fossa that extends from the lesser wings of the sphenoid bone to the petrous ridge middle nasal concha nasal concha formed by the ethmoid bone that is located between the superior and inferior conchae mylohyoid line bony ridge located along the inner (medial) surface of the mandibular body nasal bone paired bones that form the base of the nose nasal cavity opening through skull for passage of air nasal conchae curved bony plates that project from the lateral walls of the nasal cavity; include the superior and middle nasal conchae, which are parts of the ethmoid bone, and the independent inferior nasal conchae bone nasal septum flat, midline structure that divides the nasal cavity into halves, formed by the perpendicular plate of the ethmoid bone, vomer bone, and septal cartilage nasolacrimal canal passage for drainage of tears that extends downward from the medial- anterior orbit to the nasal cavity, terminating behind the inferior nasal conchae occipital bone unpaired bone that forms the posterior portions of the brain case and base of the skull occipital condyle paired, oval-shaped bony knobs located on the inferior skull, to either side of the foramen magnum optic canal opening spanning between middle cranial fossa and posterior orbit orbit bony socket that contains the eyeball and associated muscles palatine bone paired bones that form the posterior quarter of the hard palate and a small area in floor of the orbit palatine process medial projection from the maxilla bone that forms the anterior three quarters of the hard palate paranasal sinuses cavities within the skull that are connected to the conchae that serve to warm and humidify incoming air, produce mucus, and lighten the weight of the skull; consist of frontal, maxillary, sphenoidal, and ethmoidal sinuses parietal bone paired bones that form the upper, lateral sides of the skull perpendicular plate of the ethmoid bone downward, midline extension of the ethmoid bone that forms the superior portion of the nasal septum petrous ridge petrous portion of the temporal bone that forms a large, triangular ridge in the floor of the cranial cavity, separating the middle and posterior cranial fossae; houses the middle and inner ear structures posterior cranial fossa deepest and most posterior cranial fossa; extends from the petrous ridge to the occipital bone pterion H-shaped suture junction region that unites the frontal, parietal, temporal, and sphenoid bones on the lateral side of the skull ramus of the mandible vertical portion of the mandible Sagittal suture joint that unites the right and left parietal bones at the midline along the top of the skull sella turcica elevated area of sphenoid bone located at midline of the middle cranial fossa septal cartilage flat cartilage structure that forms the anterior portion of the nasal septum sphenoid bone unpaired bone that forms the central base of skull sphenoid sinus air-filled space located within the sphenoid bone; most posterior of the paranasal sinuses squamous suture joint that unites the parietal bone to the squamous portion of the temporal bone on the lateral side of the skull styloid process downward projecting, elongated bony process located on the inferior aspect of the skull stylomastoid foramen opening located on inferior skull, between the styloid process and mastoid process superior nasal concha smallest and most superiorly located of the nasal conchae; formed by the ethmoid bone superior nuchal line paired bony lines on the posterior skull that extend laterally from the external occipital protuberance superior orbital fissure irregularly shaped opening between the middle cranial fossa and the posterior orbit supraorbital foramen opening located on anterior skull, at the superior margin of the orbit supraorbital margin superior margin of the orbit suture junction line at which adjacent bones of the skull are united by fibrous connective tissue temporal bone paired bones that form the lateral, inferior portions of the skull, with Squamous, mastoid, and petrous portions temporal fossa shallow space on the lateral side of the skull, above the level of the zygomatic arch temporal process of the zygomatic bone short extension from the zygomatic bone that forms the anterior portion of the zygomatic arch vomer bone unpaired bone that forms the inferior and posterior portions of the nasal septum zygomatic arch elongated, free-standing arch on the lateral skull, formed anteriorly by the temporal process of the zygomatic bone and posteriorly by the zygomatic process of the temporal bone zygomatic bone cheekbone; paired bones that contribute to the lateral orbit and anterior zygomatic arch zygomatic process of the temporal bone extension from the temporal bone that forms the posterior portion of the zygomatic arch The Vertebral Column By the end of this section, you will be able to: Describe each region of the vertebral column and the number of bones in each region Discuss the curves of the vertebral column and how these change after birth Describe a typical vertebra and determine the distinguishing characteristics for vertebrae in each vertebral region and features of the sacrum and the coccyx Define the structure of an intervertebral disc Determine the location of the ligaments that provide support for the vertebral column The vertebral column is also known as the spinal column or spine ([link]). It consists of a sequence of vertebrae (singular = vertebra), each of which is separated and united by an intervertebral disc. Together, the vertebrae and intervertebral discs form the vertebral column. It is a flexible column that supports the head, neck, and body and allows for their movements. It also protects the spinal cord, which passes down the back through openings in the vertebrae. Vertebral Column 7 Cervical vertebrae (C1-C7) form cervical curve 12 Thoracic vertbrae (T1-T12) form thoracic curve 5 Lumbar vertebrae (L1—L5) form lumbar curve Fused vertebrae of sacrum and coccyx form sacrococcygeal curve The adult vertebral column consists of 24 vertebrae, plus the sacrum and coccyx. The vertebrae are divided into three regions: cervical C1—C7 vertebrae, thoracic T1—-T12 vertebrae, and lumbar L1—L5 vertebrae. The vertebral column is curved, with two primary curvatures (thoracic and sacrococcygeal curves) and two secondary curvatures (cervical and lumbar curves). Regions of the Vertebral Column The vertebral column originally develops as a series of 33 vertebrae, but this number is eventually reduced to 24 vertebrae, plus the sacrum and coccyx. The vertebral column is subdivided into five regions, with the vertebrae in each area named for that region and numbered in descending order. In the neck, there are seven cervical vertebrae, each designated with the letter “C” followed by its number. Superiorly, the C1 vertebra articulates (forms a joint) with the occipital condyles of the skull. Inferiorly, C1 articulates with the C2 vertebra, and so on. Below these are the 12 thoracic vertebrae, designated T1—T12. The lower back contains the L1—L5 lumbar vertebrae. The single sacrum, which is also part of the pelvis, is formed by the fusion of five sacral vertebrae. Similarly, the coccyx, or tailbone, results from the fusion of four small coccygeal vertebrae. However, the sacral and coccygeal fusions do not start until age 20 and are not completed until middle age. An interesting anatomical fact is that almost all mammals have seven cervical vertebrae, regardless of body size. This means that there are large variations in the size of cervical vertebrae, ranging from the very small cervical vertebrae of a shrew to the greatly elongated vertebrae in the neck of a giraffe. In a full-grown giraffe, each cervical vertebra is 11 inches tall. Curvatures of the Vertebral Column The adult vertebral column does not form a straight line, but instead has four curvatures along its length (see [link]). These curves increase the vertebral column’s strength, flexibility, and ability to absorb shock. When the load on the spine is increased, by carrying a heavy backpack for example, the curvatures increase in depth (become more curved) to accommodate the extra weight. They then spring back when the weight is removed. The four adult curvatures are classified as either primary or secondary curvatures. Primary curves are retained from the original fetal curvature, while secondary curvatures develop after birth. During fetal development, the body is flexed anteriorly into the fetal position, giving the entire vertebral column a single curvature that is concave anteriorly. In the adult, this fetal curvature is retained in two regions of the vertebral column as the thoracic curve, which involves the thoracic vertebrae, and the sacrococcygeal curve, formed by the sacrum and coccyx. Each of these is thus called a primary curve because they are retained from the original fetal curvature of the vertebral column. A secondary curve develops gradually after birth as the child learns to sit upright, stand, and walk. Secondary curves are concave posteriorly, opposite in direction to the original fetal curvature. The cervical curve of the neck region develops as the infant begins to hold their head upright when sitting. Later, as the child begins to stand and then to walk, the lumbar curve of the lower back develops. In adults, the lumbar curve is generally deeper in females. Disorders associated with the curvature of the spine include kyphosis (an excessive posterior curvature of the thoracic region), lordosis (an excessive anterior curvature of the lumbar region), and scoliosis (an abnormal, lateral curvature, accompanied by twisting of the vertebral column). Note: Disorders of the... Vertebral Column Developmental anomalies, pathological changes, or obesity can enhance the normal vertebral column curves, resulting in the development of abnormal or excessive curvatures ({link]). Kyphosis, also referred to as humpback or hunchback, is an excessive posterior curvature of the thoracic region. This can develop when osteoporosis causes weakening and erosion of the anterior portions of the upper thoracic vertebrae, resulting in their gradual collapse ({link]). Lordosis, or swayback, is an excessive anterior curvature of the lumbar region and is most commonly associated with obesity or late pregnancy. The accumulation of body weight in the abdominal region results an anterior shift in the line of gravity that carries the weight of the body. This causes in an anterior tilt of the pelvis and a pronounced enhancement of the lumbar curve. Scoliosis is an abnormal, lateral curvature, accompanied by twisting of the vertebral column. Compensatory curves may also develop in other areas of the vertebral column to help maintain the head positioned over the feet. Scoliosis is the most common vertebral abnormality among girls. The cause is usually unknown, but it may result from weakness of the back muscles, defects such as differential growth rates in the right and left sides of the vertebral column, or differences in the length of the lower limbs. When present, scoliosis tends to get worse during adolescent growth spurts. Although most individuals do not require treatment, a back brace may be recommended for growing children. In extreme cases, surgery may be required. Excessive vertebral curves can be identified while an individual stands in the anatomical position. Observe the vertebral profile from the side and then from behind to check for kyphosis or lordosis. Then have the person bend forward. If scoliosis is present, an individual will have difficulty in bending directly forward, and the right and left sides of the back will not be level with each other in the bent position. Abnormal Curvatures of the Vertebral Column (a) Scoliosis (b) Kyphosis (c) Lordosis (a) Scoliosis is an abnormal lateral bending of the vertebral column. (b) An excessive curvature of the upper thoracic vertebral column is called kyphosis. (c) Lordosis is an excessive curvature in the lumbar region of the vertebral column. Osteoporosis Normal Bone loss vertebrae amplifies curvature Osteoporosis is an age-related disorder that causes the gradual loss of bone density and strength. When the thoracic vertebrae are affected, there can be a gradual collapse of the vertebrae. This results in kyphosis, an excessive curvature of the thoracic region. Note: — meee OPENStAX COLLEGE Osteoporosis is a common age-related bone disease in which bone density and strength is decreased. Watch this video to get a better understanding of how thoracic vertebrae may become weakened and may fracture due to this disease. How may vertebral osteoporosis contribute to kyphosis? General Structure of a Vertebra Within the different regions of the vertebral column, vertebrae vary in size and shape, but they all follow a similar structural pattern. A typical vertebra will consist of a body, a vertebral arch, and seven processes ({link]). The body is the anterior portion of each vertebra and is the part that supports the body weight. Because of this, the vertebral bodies progressively increase in size and thickness going down the vertebral column. The bodies of adjacent vertebrae are separated and strongly united by an intervertebral disc. The vertebral arch forms the posterior portion of each vertebra. It consists of four parts, the right and left pedicles and the right and left laminae. Each pedicle forms one of the lateral sides of the vertebral arch. The pedicles are anchored to the posterior side of the vertebral body. Each lamina forms part of the posterior roof of the vertebral arch. The large opening between the vertebral arch and body is the vertebral foramen, which contains the spinal cord. In the intact vertebral column, the vertebral foramina of all of the vertebrae align to form the vertebral (spinal) canal, which serves as the bony protection and passageway for the spinal cord down the back. When the vertebrae are aligned together in the vertebral column, notches in the margins of the pedicles of adjacent vertebrae together form an intervertebral foramen, the opening through which a spinal nerve exits from the vertebral column ([link]). Seven processes arise from the vertebral arch. Each paired transverse process projects laterally and arises from the junction point between the pedicle and lamina. The single spinous process (vertebral spine) projects posteriorly at the midline of the back. The vertebral spines can easily be felt as a Series of bumps just under the skin down the middle of the back. The transverse and spinous processes serve as important muscle attachment sites. A superior articular process extends or faces upward, and an inferior articular process faces or projects downward on each side of a vertebrae. The paired superior articular processes of one vertebra join with the corresponding paired inferior articular processes from the next higher vertebra. These junctions form slightly moveable joints between the adjacent vertebrae. The shape and orientation of the articular processes vary in different regions of the vertebral column and play a major role in determining the type and range of motion available in each region. Parts of a Typical Vertebra Anterior Posterior Posterior Spinal cord Spinal cord Spinous process Transverse process Facet of superior articular process Vertebral foramen Intervetebral disc Facet for head Vertebral arch: of rib Lamina Facet of superior articular process Inferior articular process Facet for head of rib Spinal nerve exiting Anterior through intervertebral foramen Spinous process Superior view Left posterolateral view of articulated vertebrae A typical vertebra consists of a body and a vertebral arch. The arch is formed by the paired pedicles and paired laminae. Arising from the vertebral arch are the transverse, spinous, superior articular, and inferior articular processes. The vertebral foramen provides for passage of the spinal cord. Each spinal nerve exits through an intervertebral foramen, located between adjacent vertebrae. Intervertebral discs unite the bodies of adjacent vertebrae. Intervertebral Disc Vertebral body Intervertebral foramen o> Anulus fibrosus ~~ Nucleus pulposus Lateral view Superior view The bodies of adjacent vertebrae are separated and united by an intervertebral disc, which provides padding and allows for movements between adjacent vertebrae. The disc consists of a fibrous outer layer called the anulus fibrosus and a gel-like center called the nucleus pulposus. The intervertebral foramen is the opening formed between adjacent vertebrae for the exit of a spinal nerve. Regional Modifications of Vertebrae In addition to the general characteristics of a typical vertebra described above, vertebrae also display characteristic size and structural features that vary between the different vertebral column regions. Thus, cervical vertebrae are smaller than lumbar vertebrae due to differences in the proportion of body weight that each supports. Thoracic vertebrae have sites for rib attachment, and the vertebrae that give rise to the sacrum and coccyx have fused together into single bones. Cervical Vertebrae Typical cervical vertebrae, such as C4 or C5, have several characteristic features that differentiate them from thoracic or lumbar vertebrae ([link]). Cervical vertebrae have a small body, reflecting the fact that they carry the least amount of body weight. Cervical vertebrae usually have a bifid (Y- shaped) spinous process. The spinous processes of the C3—C6 vertebrae are short, but the spine of C7 is much longer. You can find these vertebrae by running your finger down the midline of the posterior neck until you encounter the prominent C7 spine located at the base of the neck. The transverse processes of the cervical vertebrae are sharply curved (U-shaped) to allow for passage of the cervical spinal nerves. Each transverse process also has an opening called the transverse foramen. An important artery that supplies the brain ascends up the neck by passing through these openings. The superior and inferior articular processes of the cervical vertebrae are flattened and largely face upward or downward, respectively. The first and second cervical vertebrae are further modified, giving each a distinctive appearance. The first cervical (C1) vertebra is also called the atlas, because this is the vertebra that supports the skull on top of the vertebral column (in Greek mythology, Atlas was the god who supported the heavens on his shoulders). The C1 vertebra does not have a body or spinous process. Instead, it is ring-shaped, consisting of an anterior arch and a posterior arch. The transverse processes of the atlas are longer and extend more laterally than do the transverse processes of any other cervical vertebrae. The superior articular processes face upward and are deeply curved for articulation with the occipital condyles on the base of the skull. The inferior articular processes are flat and face downward to join with the superior articular processes of the C2 vertebra. The second cervical (C2) vertebra is called the axis, because it serves as the axis for rotation when turning the head toward the right or left. The axis resembles typical cervical vertebrae in most respects, but is easily distinguished by the dens (odontoid process), a bony projection that extends upward from the vertebral body. The dens joins with the inner aspect of the anterior arch of the atlas, where it is held in place by transverse ligament. Cervical Vertebrae Dens of axis Transverse ligament s Spinous process (bifid) & C4 f SS ers Vertebral foramen C; (atlas) “> ' Cz (axis) —— > Pedicle C3 : Lamina Inferior articular process Superior articular process Bifid spinous process ieee? Transverse process process Transverse C; (vertebra foramen Body Groove prominens) for spinal nerve Structure of a typical cervical vertebra Dens Superior articular Transverse facet fie process Dens - Superior articular Anterior arch Transverse foramen facet Transverse Lamina process Transverse foramen Posterior arch Spinous process Ligament Superior view of atlas Superior view of axis Dens Transverse process Inferior articular Body process Anterior view of axis A typical cervical vertebra has a small body, a bifid spinous process, transverse processes that have a transverse foramen and are curved for spinal nerve passage. The atlas (C1 vertebra) does not have a body or spinous process. It consists of an anterior and a posterior arch and elongated transverse processes. The axis (C2 vertebra) has the upward projecting dens, which articulates with the anterior arch of the atlas. Thoracic Vertebrae The bodies of the thoracic vertebrae are larger than those of cervical vertebrae ({link]). The characteristic feature for a typical midthoracic vertebra is the spinous process, which is long and has a pronounced downward angle that causes it to overlap the next inferior vertebra. The superior articular processes of thoracic vertebrae face anteriorly and the inferior processes face posteriorly. These orientations are important determinants for the type and range of movements available to the thoracic region of the vertebral column. Thoracic vertebrae have several additional articulation sites, each of which is called a facet, where a rib is attached. Most thoracic vertebrae have two facets located on the lateral sides of the body, each of which is called a costal facet (costal = “rib”). These are for articulation with the head (end) of arib. An additional facet is located on the transverse process for articulation with the tubercle of a rib. Thoracic Vertebrae Superior articular process Articular facet for tubercle of rib Transverse process Pedicle Intervertebral Lamina disc Body Spinous rocess ' P Superior costal facet fe vy] // \nferior Inferior costal iff articular facet process A typical thoracic vertebra is distinguished by the spinous process, which is long and projects downward to overlap the next inferior vertebra. It also has articulation sites (facets) on the vertebral body and a transverse process for rib attachment. Rib Articulation in Thoracic Vertebrae Superior L articular facets U a Superior costal facet 7 fi) Crea ZY \ Z - Facet for —_ WAN eS ; \\ YYY tubercle of rib Ay Body of vertebra | we tl Head of rib Zl >: b Ns Intervertebral disc Tranverse r ; processes Fit— Neck of rib él { ! Se Body of vertebra ; 7 = Tubercle of rib BS . . Inferior costal facet Spinous process Thoracic vertebrae have superior and inferior articular facets on the vertebral body for articulation with the head of a rib, and a transverse process facet for articulation with the rib tubercle. Lumbar Vertebrae Lumbar vertebrae carry the greatest amount of body weight and are thus characterized by the large size and thickness of the vertebral body ((link]). They have short transverse processes and a short, blunt spinous process that projects posteriorly. The articular processes are large, with the superior process facing backward and the inferior facing forward. Lumbar Vertebrae Superior articular Transverse process Inferior articular process ; go ST, Spinous ————4—“ : [7 process Intervertebral disc Inferior articular process Lumbar vertebrae are characterized by having a large, thick body and a short, rounded spinous process. Sacrum and Coccyx The sacrum is a triangular-shaped bone that is thick and wide across its superior base where it is weight bearing and then tapers down to an inferior, non-weight bearing apex ([link]). It is formed by the fusion of five sacral vertebrae, a process that does not begin until after the age of 20. On the anterior surface of the older adult sacrum, the lines of vertebral fusion can be seen as four transverse ridges. On the posterior surface, running down the midline, is the median sacral crest, a bumpy ridge that is the remnant of the fused spinous processes (median = “midline”; while medial = “toward, but not necessarily at, the midline”). Similarly, the fused transverse processes of the sacral vertebrae form the lateral sacral crest. The sacral promontory is the anterior lip of the superior base of the sacrum. Lateral to this is the roughened auricular surface, which joins with the ilium portion of the hipbone to form the immobile sacroiliac joints of the pelvis. Passing inferiorly through the sacrum is a bony tunnel