Chapter 5 - The Integumentary System PDF

5 The Integumentary System by first asking 5.1 What is the In this chapter, you will learn that The skin and it...

5 The Integumentary System by first asking 5.1 What is the In this chapter, you will learn that The skin and its derivatives serve several (mostly protective) functions then learning about The appendages and next asking 5.8 What are the then asking 5.9 What happens Tissues Ch. 4 using structure of skin? of the skin functions of skin? when things go wrong? looking closer at looking closer at and finally, exploring 5.5 Hair 5.2 Epidermis 5.3 Dermis Developmental Aspects of the Integumentary System and asking 5.6 Nails 5.4 What causes skin color? 5.7 Sweat and sebaceous glands Would you be enticed by an ad for a coat that is water- proof, stretchable, washable, and air-conditioned, that automat- ically repairs small cuts, rips, and burns? How about one that’s CAREER CONNECTION guaranteed to last a lifetime? Sounds too good to be true, but you already have such a coat—your skin. You may not have thought about it, but your skin is an archi- tectural marvel. It covers the entire body, has a surface area of 1.2 to 2.2 square meters, and accounts for about 15% of total body weight in the average adult. The skin serves as much more than a bag for body contents. Pliable yet tough, it takes constant punishment from external agents. Without our skin, we would quickly fall prey to harmful bacteria and perish from water and Play a video to learn how the heat loss. chapter content is used in The skin, its appendages (sweat and oil glands, hairs, and a real healthcare setting nails), and subcutaneous tissue make up a complex set of organs @ Mastering A&P > Study Area. that serves several functions, mostly protective. Together, these organs form the integumentary system (in-teg″u-men′tar-e; integument = “covering”). 150  Chapter 5 The Integumentary System 151 The subcutaneous tissue lies just deep to the skin (Figure 5.1). The skin consists of two layers: 5.1 It is also known as the hypodermis or the superficial fascia the epidermis and dermis (fash′e-ah; “a bond”) because it is superficial to the tough con- nective tissue wrapping (fascia) of the skeletal muscles. The Learning Outcome subcutaneous tissue is not part of the skin, but because it shares N List the two layers of skin and briefly describe some of the skin’s protective functions it is considered part of subcutaneous tissue. the integumentary system. It consists mostly of adipose tissue Varying in thickness from 1.5 to 4.0 millimeters (mm) or more with some areolar connective tissue. in different parts of the body, the skin is composed of two dis- Besides storing fat, the subcutaneous tissue anchors the skin tinct layers (Figure 5.1): to the underlying structures (mostly to muscles), but loosely enough that the skin can slide relatively freely over those struc- The epidermis (ep″ĭ-der′mis), composed of epithelial cells, is tures. Sliding skin protects us by ensuring that everyday bumps the outermost protective shield of the body (epi = upon). often just glance off our bodies. Because of its fatty composi- The underlying dermis, making up the bulk of the skin, is a tion, the subcutaneous tissue also acts as a shock absorber and tough, leathery layer composed mostly of dense connective an insulator that reduces heat loss. tissue. Only the dermis is vascularized. Nutrients reach the epider- Check Your Understanding mis by diffusing through the tissue fluid from blood vessels in 1. Which layer of the skin—dermis or epidermis—is better nourished? the dermis. For answers, see Answers Appendix. 11 5 Hair shaft Dermal papillae Epidermis Papillary layer Sweat pore Appendages of skin Dermis Eccrine sweat gland Reticular layer Arrector pili muscle Sebaceous (oil) gland Hair follicle Hair root Subcutaneous tissue (hypodermis; not part of skin) Nervous structures Sensory nerve fiber with free nerve endings Dermal vascular plexus Lamellar corpuscle Hair follicle receptor (root hair plexus) Adipose tissue Figure 5.1 Skin structure. Three-dimensional view of the skin and underlying subcutaneous tissue. The epidermal and dermal layers have been pulled apart at the upper right corner to reveal the dermal papillae.  152 UNIT 2 Covering, Support, and Movement of the Body Cells of the Epidermis The epidermis is a keratinized 5.2 The cells populating the epidermis include keratinocytes, mel- stratified squamous epithelium anocytes, dendritic cells, and tactile epithelial cells. Learning Outcomes Keratinocytes N Name the tissue type composing the epidermis. The chief role of keratinocytes (kĕ-rat′ĭ-no-sītz″; “keratin N List the major layers of the epidermis and describe the functions of each layer. cells”) is to produce keratin, the fibrous protein that helps give the epidermis its protective properties (Greek kera = horn) The epidermis consists of four distinct cell types and four or (Figure 5.2b, orange cells). Most epidermal cells are five distinct layers (Figure 5.2). 11 5 Keratinocytes Stratum corneum Most superficial layer; 20–30 layers of dead cells, essentially flat membranous sacs filled with keratin. Glycolipids in extracellular space. Stratum granulosum One to five layers of flattened cells, organelles deteriorating; cytoplasm full of lamellar granules (release lipids) and keratohyaline granules. Stratum spinosum Several layers of keratinocytes unified by desmosomes. Cells contain thick bundles of intermediate filaments made of pre-keratin. Stratum basale Deepest epidermal layer; one row of actively mitotic stem cells; some newly formed cells become part of the more superficial layers. See occasional melanocytes and tactile Dermis epithelial cells. (a) Dermis Melanin Sensory Tactile granule nerve epithelial Figure 5.2 Epidermal cells and layers of the epidermis. ending cell (a) Photomicrograph of the four major epidermal layers in thin skin ( 200×). Desmosomes Melanocyte Dendritic cell (b) Diagram showing these four layers and the distribution of different cell types. The stratum lucidum, present only in thick skin, is not illustrated here. (b)  Chapter 5 The Integumentary System 153 keratinocytes. These keratinocytes are tied together by des- Note that the terms “thick skin” and “thin skin” are really mosomes for strength and, in some layers, by tight junctions to misnomers because they refer to the epidermis only. Indeed, if prevent movement of water between cells ( pp. 67–68). you include the dermis, the thickest skin is on the upper back. Keratinocytes arise in the deepest part of the epidermis from a cell layer called the stratum basale. These cells undergo Stratum Basale (Basal Layer) almost continuous mitosis in response to epidermal growth fac- The stratum basale (stra′tum bah-sa′le), the deepest epider- tor, a peptide produced by various cells throughout the body. mal layer, is also called the stratum germinativum ( jer′mĭ- Newly formed keratinocytes are pushed upward by the pro- nă″tiv-um; “germinating layer”). It is attached to the underlying duction of new cells beneath them, all the while making the dermis along a wavy borderline that resembles corrugated keratin that eventually fills them. By the time the keratinocytes cardboard. It consists of a single row of stem cells—a con- approach the skin surface, they are dead, scale-like flat sacs tinually renewing cell population—representing the youngest completely filled with keratin. keratinocytes. The many mitotic nuclei seen in this layer reflect Millions of dead keratinocytes rub off every day, giving us a the rapid division of these cells. Each time one of these basal totally new epidermis every 25 to 45 days. Where the skin expe- cells divides, one daughter cell is pushed into the cell layer just riences friction, both cell production and keratin formation are above to begin its specialization into a mature keratinocyte. The accelerated. Persistent friction (from a poorly fitting shoe, for other daughter cell remains in the basal layer to continue the example) causes a thickening of the epidermis called a callus. process of producing new keratinocytes. Some 10–25% of the cells in the stratum basale are melanocytes, Melanocytes and their branching processes extend among the surrounding cells, Melanocytes (mel′ah-no-sītz), the spider-shaped epithe- reaching well into the more superficial stratum spinosum layer. 11 5 lial cells that synthesize the pigment melanin (mel′ah-nin; melan = black ), are found in the deepest layer of the epider- Stratum Spinosum (Prickly Layer) mis (Figure 5.2b, gray cell). Melanin is made in membrane- The stratum spinosum (spi″no′sum; “prickly”) is several cell bound granules called melanosomes and then transferred layers thick. This layer gets its name from the many spinelike through the cell processes (the “spider legs”) to nearby keratino- extensions of its keratinocytes as seen under a microscope. cytes. As a result, the basal keratinocytes contain more melanin However, these spines do not exist in living cells: They are arti- than do the melanocytes themselves. This melanin clusters on facts created during tissue preparation because the cells shrink the superficial, or “sunny,” side of the keratinocyte nucleus, while holding tight at their many desmosomes. This makes forming a pigment shield that protects the nucleus from the them look like tiny versions of the spiked iron balls used in damaging effects of ultraviolet (UV) radiation in sunlight. medieval warfare. Cells of the stratum spinosum contain thick bundles of intermediate filaments, which consist of a tension- Dendritic Cells resisting protein, pre-keratin. These intermediate filaments The star-shaped dendritic cells arise from bone marrow resist tension in the cell and are anchored to the desmosomes. and migrate to the epidermis. Also called Langerhans cells Scattered among the keratinocytes are dendritic cells, which are (lahng′er-hanz), they ingest foreign substances and are key most abundant in this epidermal layer. activators of our immune system, as described later in this chapter. Their slender processes extend among the surround- Stratum Granulosum (Granular Layer) ing keratinocytes, forming a more or less continuous network The thin stratum granulosum (gran″u-lo′sum) consists of one (Figure 5.2b, purple cell). to five cell layers in which keratinocyte appearance changes drastically, and the process of keratinization (in which the Tactile Epithelial Cells cells fill with keratin) begins. These cells flatten, their nuclei Occasional tactile epithelial cells (or Merkel cells) are present and organelles begin to disintegrate, and they accumulate two at the epidermal-dermal junction. Shaped like a spiky hemi- types of granules. sphere (Figure 5.2b, blue cell), each tactile epithelial cell is The keratohyaline granules (ker″ah-to-hi′ah-lin) help to form intimately associated with a disclike sensory nerve ending. The keratin in the upper layers by providing the “glue” that binds combination functions as a sensory receptor for touch. pre-keratin intermediate filaments together to form keratin. The lamellar granules (lam′el-ar; “a small plate”) contain a Layers of the Epidermis water-resistant glycolipid that is secreted into the extracellu- Variation in epidermal thickness determines if skin is thick or lar space. Together with tight junctions, the glycolipid plays thin. In thick skin, which covers areas subject to abrasion—the a major part in slowing water loss across the epidermis. palms, fingertips, and soles of the feet—the epidermis consists of Proteins within the keratinocytes and lipids deposited outside five layers, or strata (stra′tah; “bed sheets”). From deep to super- them make these cells tough and water resistant. This is why the ficial, these layers are stratum basale, stratum spinosum, stratum outermost layers of skin are strong and watertight. granulosum, stratum lucidum, and stratum corneum. In thin skin, Like all epithelia, the epidermis relies on capillaries in the which covers the rest of the body, the stratum lucidum appears to underlying connective tissue (the dermis in this case) for its be absent and the other strata are thinner (Figure 5.2a, b). nutrients. Above the stratum granulosum, the epidermal cells  154 UNIT 2 Covering, Support, and Movement of the Body are too far from the dermal capillaries to survive. In addition, the glycolipids coating their external surfaces cut them off from nutrients. As a result, they die. This is a normal sequence of events. Stratum Lucidum (Clear Layer) Epidermis The stratum lucidum (loo′sid-um; “light”), found only in thick skin, is visible through a light microscope as a thin translucent band just above the stratum granulosum. It consists of a few rows of flat, dead keratinocytes. Electron microscopy reveals that its cells are identical to those at the bottom of the next layer, the stratum corneum. Stratum Corneum (Horny Layer) Dermis: Papillary dermis An abrupt transition occurs between the nucleated cells of the (areolar connective stratum granulosum and the flattened, anucleate cells of the tissue) stratum corneum (kor′ne-um). This outermost epidermal layer Reticular dermis is a broad zone 20 to 30 cell layers thick that accounts for up (dense irregular to three-quarters of the epidermal thickness (see Figure 5.3). connective tissue) 11 5 Keratin and proteins that accumulate just inside the plasma membrane of cells in this stratum protect the skin against abra- sion and penetration. The glycolipid between its cells helps keep this layer nearly waterproof. It is amazing that even a layer of dead cells can still perform so many functions. The cells of the stratum corneum are shed regularly. These Dermal papillae cells are the dandruff shed from the scalp and the flakes that Figure 5.3 Light micrograph of the dermis. (165×) come off dry skin. In a lifetime, the average person sheds 18 kg (40 lb) of these flakes (which act as fodder for the dust mites that inhabit our homes). The saying “Beauty is only skin deep” is especially interesting in light of the fact that when we look at someone, nearly everything we see is dead! mast cells and white blood cells ( p. 129). Its semifluid matrix, embedded with fibers, binds the entire body together Check Your Understanding like a body stocking. It is your “hide” and corresponds to ani- 2. While walking barefoot, Kwan-Suk stepped on a rusty nail that mal hides used to make leather. penetrated the epidermis. Name the layers the nail pierced The dermis has two layers, the papillary and reticular, which from the superficial skin surface to the junction with the lie next to one another along an indistinct boundary (Figure 5.3). dermis. 3. What is the major function of the stratum basale? Papillary Dermis 4. Why are the desmosomes and tight junctions connecting the The thin, superficial papillary dermis (pap′il-er-e) is areolar keratinocytes so important? connective tissue ( p. 130) in which fine interlacing collagen 5. WHAT IF? Suppose that the dead cells of the stratum corneum and elastic fibers form a loosely woven mat with many small did not shed from your body, but rather stuck to it and blood and lymphatic vessels. The looseness of this connective continued to accumulate. What problems might this cause? tissue allows phagocytes and other defensive cells to wander For answers, see Answers Appendix. freely as they patrol the area for bacteria that have penetrated the skin. Peglike projections from its surface, called dermal papillae (pah-pil′e; papill = nipple ), indent the overlying epidermis (Figures 5.1 and 5.3). Many dermal papillae contain capillary 5.3 The dermis consists of papillary loops. Others house free nerve endings (pain receptors) and dermis and reticular dermis touch receptors called tactile corpuscles or Meissner’s corpus- Learning Outcome cles (mīs′nerz kor′pus-lz). (Note that tactile epithelial cells and tactile corpuscles are different structures.) In thick skin, such as N Name the tissue types composing the dermis. List its the palms of the hands and soles of the feet, these papillae lie major layers and describe the functions of each layer. atop larger mounds called dermal ridges, which in turn cause The dermis (derm = skin) is made up of strong, flexible con- the overlying epidermis to form epidermal ridges (Figure 5.4c). nective tissue. Its cells are typical of those found in any con- Collectively, these skin ridges, referred to as friction ridges, may nective tissue proper: fibroblasts, macrophages, and occasional enhance our ability to grip certain kinds of surfaces. They also  Chapter 5 The Integumentary System 155 contribute to our sense of touch by enhancing vibrations detected skin. These externally invisible lines tend to run longitudinally by the large lamellar corpuscles (receptors) in the dermis. in the skin of the limbs and in circular patterns around the neck Each of us has unique friction ridge patterns that are deter- and trunk (Figure 5.4a). Cleavage lines are important to sur- mined by genetics and the environment inside the womb. geons because when an incision is made parallel to these lines, Because sweat pores open along their crests, our fingertips the skin gapes less and heals more readily. leave identifying films of sweat called fingerprints on almost The collagen fibers of the dermis give skin strength and anything we touch. resiliency that prevent minor jabs and scrapes from penetrating the dermis. Elastic fibers provide the stretch-recoil properties Reticular Dermis of skin. The deeper reticular dermis, accounting for about 80% of the Flexure lines are dermal folds that occur at or near joints, thickness of the dermis, is coarse, dense irregular connective where the dermis is tightly secured to deeper structures. (Notice tissue ( p. 133) (Figure 5.3). The network of blood vessels that the deep creases on your palms.) Since the skin cannot slide eas- nourishes this layer, the dermal vascular plexus, lies between ily to accommodate joint movement in such regions, the dermis this layer and the subcutaneous tissue (see Figure 5.1). The folds and deep skin creases form (Figure 5.4b). Flexure lines are extracellular matrix of the reticular dermis contains thick bun- also visible on the wrists, fingers, soles, and toes. dles of interlacing collagen fibers. The reticular dermis is named for its network of collagen fibers (reticulum = network); the name does not imply any special abundance of reticular fibers. H OMEOSTATIC CLINICAL The collagen fibers run in various planes, but most run par- IMBALANCE 5.1 11 5 allel to the skin surface. Separations, or less dense regions, Extreme stretching of the skin, such as during pregnancy, can between these bundles form cleavage (tension) lines in the tear the dermis, leaving silvery white scars called striae (stri′e; Cleavage lines Represent separations between underyling collagen fiber bundles in the reticular dermis. Run circularly around the trunk and longitudinally in the limbs. Surgical incisions parallel to cleavage lines heal better than those made across them. Flexure lines form where the dermis is Friction ridges Sweat duct openings closely attached to the underlying structures. consist of epidermal along the crests of ridges that lie on top friction ridges help of dermal ridges. make fingerprints. (a) Cleavage lines (b) Flexure lines (c) Friction ridges of fingertip (SEM 12×) Figure 5.4 Dermal modifications result in characteristic skin markings.  156 UNIT 2 Covering, Support, and Movement of the Body many more and darker melanosomes than those of light-skinned individuals, and their keratinocytes retain it longer. Freckles and pigmented nevi (moles) are local accumulations of melanin. When we expose our skin to sunlight, keratinocytes secrete chemicals that stimulate melanocytes. Prolonged sun expo- sure causes a substantial melanin buildup, which helps protect the DNA of skin cells from UV radiation by absorbing the rays and dissipating the energy as heat. Indeed, the initial signal for speeding up melanin synthesis seems to be a faster repair rate of Figure 5.5 Stretch marks (striae). light-damaged DNA. In all but the darkest-skinned people, this defensive response causes skin to darken visibly (tanning occurs). “streaks”), commonly called stretch marks (Figure 5.5). Short- term but acute trauma (as from a burn or from repeated friction) H OMEOSTATIC can cause a blister, a fluid-filled pocket that separates the epi- CLINICAL IMBALANCE 5.2 dermal and dermal layers. Despite melanin’s protective effects, excessive sun exposure eventually damages the skin. It causes elastic fibers to clump, Check Your Understanding which results in leathery skin; temporarily depresses the 6. What type of tissue makes up the papillary and reticular dermis? ­immune system; and can alter the DNA of skin cells, leading 11 5 to skin ­cancer. Although darker-skinned people are less likely 7. APPLY You have just gotten a paper cut. It is very painful, but it doesn’t bleed. Has the cut penetrated into the dermis or just to get melanoma, the deadliest form of skin cancer, they are not the epidermis? ­immune and still need to have suspicious skin changes evaluated. 8. PREDICT Stefan’s front bike tire slipped on a patch of gravel as he Basal cell and squamous cell cancers occur in all skin types. rode to work. He needed five stitches to close the cut above his Many chemicals induce photosensitivity ( photo = light); that left eye. The split skin ran along a cleavage line. Is Stefan likely to is, they increase the skin’s sensitivity to UV radiation and can cause have a major scar or is the wound likely to heal cleanly? Explain. an unsightly skin rash. Such substances include some antibiotic and 9. DRAW Draw a wavy line to represent the junction between the antihistamine drugs, and many chemicals in perfumes and deter- dermis and epidermis. Draw and label the layer of epidermal cells gents. Small, itchy blisters erupt all over the body. next to the dermis, and the layer of dermis that is next to these epidermal cells (use precise names for each layer). Name and label the projections from the dermis that indent the epidermis. Carotene (kar′o-tēn) is a yellow to orange pigment found in For answers, see Answers Appendix. certain plant products such as carrots. It tends to accumulate in the stratum corneum and in the fat of the subcutaneous tissue. In the body, carotene can be converted to vitamin A, a vitamin that is essential for normal vision, as well as for epidermal health. 5.4Melanin, carotene, and The pinkish hue of light skin reflects the crimson color of hemoglobin determine skin color the oxygenated pigment hemoglobin (he′mo-glo″bin) in the red blood cells circulating through the dermal capillaries. Because Learning Outcomes light-skinned people have only small amounts of melanin in N Describe the factors that normally contribute to skin color. their skin, the epidermis is nearly transparent and allows hemo- N Briefly describe how changes in skin color may be used as globin’s color to show through. clinical signs of certain disease states. The pigment melanin is a polymer made of an amino acid H OMEOSTATIC called tyrosine. Its two forms range in color from reddish yellow CLINICAL IMBALANCE 5.3 to brownish black. Melanin synthesis depends on an enzyme in melanocytes called tyrosinase (ti-ro′sĭ-nās). As noted earlier, Health care providers quickly glean important information melanin is transported from melanocytes to the basal keratino- about a person’s health from changes to the color of their skin. cytes. Eventually, lysosomes break down the melanosomes, so Alterations in skin color provide clues about certain disease melanin is found only in the deeper layers of the epidermis. states or even emotional states. Some of these changes are more Human skin comes in different colors. However, distribution obvious in mucous membranes, particularly in people with of those colors is not random—populations of darker-skinned densely pigmented skin. people tend to be found nearer the equator (where greater pro- Blueness, or cyanosis (si″ah-no′sis; (cyan = dark blue): tection from the sun is needed), and those with the lightest skin When hemoglobin is poorly oxygenated, the skin may take are found closer to the poles. Since all humans have the same on a bluish-gray tint. This is especially obvious in the oral relative number of melanocytes, differences in skin coloring mucous membranes and nail beds, particularly in darker- reflect the kind and amount of melanin made and retained. skinned individuals. Cyanosis can be a sign of respiratory or ­Melanocytes of black- and brown-skinned people produce cardiovascular problems.  Chapter 5 The Integumentary System 157 Pallor, or blanching: During emotional stress, blood may be diverted from the skin to internal organs, causing light-skinned people to look “as white as a sheet.” In dark- skinned people, check for pallor in areas with low pigmentation such as palms, nail beds, and inside the lower eyelids. Low blood pressure and anemia (decreased ability of blood to carry oxygen) may also cause pallor. Redness, or erythema (er″ĭ-the′mah): Reddened skin may indicate embarrassment (blushing), fever, inflammation, or allergy. Yellowness: Normally the liver eliminates bilirubin, a waste product from red blood cell breakdown. This yellow pigment accumulates in body tissues when the liver doesn’t function properly. The result is jaundice (jawn′dis), a yellow discoloration most obvi- ous in the sclerae, or whites, of the eyes. Eating large amounts of yellow vegetables, which contain carotene, may cause golden skin, but the sclerae remain white. Red/purple/green/yellow marks: Bruises, also called ecchymoses (ek″ĭ-mo′sēs) or ­hematomas (he″mah-to′mah; “blood swelling”), occur when blood vessels are damaged and leak blood into the surrounding tissue. As the body breaks down the blood, the color of the bruise changes. The size, color, and shape of ecchymoses is important in forensic medicine. Brown or black patches: Hyperpigmented (darker) and thickened areas of skin, often with velvety texture, are usually found where skin folds. They are more common in 11 5 darker-skinned individuals, and may signal an endocrine disorder. Check Your Understanding 10. Melanin and carotene are two pigments that contribute to skin color. What is the third and where is it found? 11. Which alteration in skin color may indicate a liver disorder? For answers, see Answers Appendix. Along with the skin itself, the integumentary system includes a number of associated structures. These skin appendages include hair and hair follicles, nails, sweat glands, and sebaceous (oil) glands. Each plays a unique role in maintaining body homeostasis. Although they all derive from the epithelial cells of the epidermis (i.e., they are epidermal deriva- tives), they all extend into the dermis. We will examine them in the next three modules. 5.5 Hair consists of dead, keratinized cells Learning Outcomes N List the parts of a hair follicle and explain the function of each part. Also describe the functional relationship of arrector pili muscles to the hair follicles. N Name the regions of a hair and explain the basis of hair color. Describe the distribution, growth, replacement, and changing nature of hair during the life span. Millions of hairs are distributed over our entire skin surface except our palms, soles, lips, nipples, and parts of the external genitalia (such as the head of the penis). Although hair helps to keep other mammals warm, our sparse body hair is far less luxuriant and useful. Its main function in humans is to sense insects on the skin before they bite or sting us. Hair on the scalp guards the head against physical trauma, heat loss, and sunlight. Eyelashes shield the eyes, and nose hairs filter large particles like lint and insects from the air we inhale. Together, hairs and their follicles form complex structural units. In these units, the hairs are the long filaments, and the hair follicles are tubular invaginations of the epi- dermis from which the hairs grow. Structure of a Hair Hairs, or pili (pi′li), are flexible strands produced by hair follicles and consist largely of dead, keratinized cells. The hard keratin that dominates hairs and nails has two advan- tages over the soft keratin found in typical epidermal cells: (1) It is tougher and more durable, and (2) its individual cells do not flake off.  158 UNIT 2 Covering, Support, and Movement of the Body Follicle wall Peripheral connective tissue (fibrous) sheath Glassy membrane Epithelial root sheath External root sheath Internal root sheath Hair Cuticle Hair shaft Cortex Medulla (b) Photomicrograph of a cross section of a hair and hair follicle (100×) (a) Diagram of a cross section of a hair within its follicle Arrector 11 5 pili Sebaceous gland Follicle wall Hair root Peripheral connective tissue (fibrous) sheath Hair bulb Glassy membrane Epithelial root sheath External root sheath Internal root sheath Hair root Cuticle Cortex Medulla Hair matrix Hair papilla Melanocyte Subcutaneous adipose tissue (c) Diagram of a longitudinal view of the expanded hair (d) Photomicrograph of longitudinal view bulb of the follicle, which encloses the matrix of the hair bulb in the follicle (150×) Figure 5.6 Structure of a hair and hair follicle. The chief regions of a hair are the root, the part embed- A hair has three concentric layers of keratinized cells: the ded in the skin, and the shaft, the part that projects above the medulla, cortex, and cuticle (Figure 5.6a, b). skin’s surface (Figure 5.6). If the shaft is flat and ribbon­like The medulla (mĕ-dul′ah; “middle”), its central core, consists in cross section, the hair is coily; if it is oval, the hair is silky of large cells and air spaces. The medulla, the only part of the and wavy; if it is round, the hair is straight and tends to be hair that contains soft keratin, is absent in fine hairs. coarse.  Chapter 5 The Integumentary System 159 The cortex, a bulky layer surrounding the medulla, consists Associated with each hair follicle is a bundle of smooth of several layers of flattened cells. muscle cells called an arrector pili (ah-rek′tor pi′li; “raiser The outermost cuticle is formed from a single layer of cells of hair”) muscle. As you can see in Figure 5.1, most hair fol- overlapping one another like shingles on a roof. This arrange- licles approach the skin surface at a slight angle. The arrec- ment helps separate neighboring hairs so the hair does not tor pili muscle is attached in such a way that its contraction mat. (Hair conditioners smooth out the rough surface of the pulls the hair follicle upright and dimples the skin surface to cuticle and make hair look shiny.) The cuticle is the most produce goose bumps in response to cold temperatures or fear. heavily keratinized part of the hair, providing strength and This “hair-raising” response is not very useful to humans, with keeping the inner layers tightly compacted. Because it is sub- our short sparse hairs, but it is an important way for other ani- jected to the most abrasion, the cuticle tends to wear away mals to retain heat and protect themselves. Furry animals stay at the tip of the hair shaft. This allows keratin fibrils in the warmer by trapping a layer of insulating air in their fur; and a cortex and medulla to frizz, creating “split ends.” scared animal with its hair on end looks larger and more formi- dable to its enemy. The more important role of the arrector pili Hair pigment is made by melanocytes at the base of the in humans is that its contractions force sebum out of hair fol- hair follicle and transferred to the cortical cells. Different licles to the skin surface where it acts as a skin lubricant. concentrations of melanin produce hair color from blond to brown to black. Red hair is colored by a related pigment called Types and Growth of Hair pheomelanin. When melanin production decreases and air bub- bles replace melanin in the hair shaft, as commonly occurs with Hairs can be classified as vellus or terminal. The body hair aging, the hair turns gray or white. of children and adult females is pale, fine vellus hair (vel′us; 11 5 vell = wool, fleece). The coarser, longer hair of the eyebrows, Structure of a Hair Follicle eyelashes, and scalp is terminal hair, which may also be darker. At puberty, terminal hairs appear in the axillary and pubic Hair follicles ( folli = bag) fold down from the epidermal sur- regions of males and females and on the face and chest (and face into the dermis (and sometimes into the subcutaneous tissue). typically the arms and legs) of males. These terminal hairs grow The deep end of the follicle, located about 4 mm (1/6 in.) below in response to the stimulating effects of androgens (male sex the skin surface, expands to form a hair bulb (Figure 5.6c, d). hormones, of which testosterone is the most important). A knot of sensory nerve endings called a hair follicle receptor, or root hair plexus, wraps around each hair bulb (see Figure 5.1). Bending the hair stimulates these endings. Consequently, our H OMEOSTATIC CLINICAL hairs act as sensitive touch receptors. IMBALANCE 5.4 A papilla of a hair follicle (or hair papilla) is a dermal When females develop coarse terminal hairs in a masculine dis- papilla that protrudes into the hair bulb. This papilla contains tribution, the condition is called hirsutism (her′soot-izm; hirsut a knot of capillaries that supplies nutrients to the growing hair = hairy). Hirsutism is a cosmetic problem that may signal an un- and signals it to grow. If the hair papilla is destroyed by trauma, derlying medical issue. In polycystic ovary syndrome, the ovaries the follicle permanently stops producing hair. secrete larger amounts of androgens, stimulating hair growth. Less The wall of a hair follicle has a dermal component and an frequently, ovarian or adrenal tumors can secrete androgens. In epidermal component. These components are described from many cases, no clinical problem is found. external to internal (Figure 5.6a–d). Peripheral connective tissue sheath (fibrous sheath). This connective tissue sheath is derived from the dermis. It forms Hairs grow an average of 2 mm per week, although this rate varies widely among body regions as well as with sex and age. the external layer of the follicle wall. Each follicle goes through growth cycles. In each cycle, an Glassy membrane. The glassy membrane is at the junction active growth phase is followed by a resting phase in which the of the fibrous sheath and the epithelial root sheath. It is, in hair matrix is inactive and the follicle shrinks somewhat. At the essence, the basement membrane of the follicle epithelium. start of each active phase, the newly growing hair pushes out Epithelial root sheath. The epithelial root sheath is derived the old hair, which is shed. The life span of hairs varies. In the from the epidermis. It has two components: the external root scalp, the follicles stay active for an average of four years, so sheath, a direct continuation of the epidermis, and the internal individual hairs grow quite long before being shed. The follicles root sheath, which is derived from the matrix cells (see below). in eyebrows, in contrast, are active for only a few months so the Hair grows because cells in the bulb of the follicle divide eyebrows never grow very long. Fortunately, the cycles of adja- rapidly. These dividing cells make up the hair matrix and lie cent hair follicles are not synchronized. For this reason, humans immediately adjacent to the hair papilla. As the matrix produces shed only a small percentage of their hairs at any one time. new hair cells, the older part of the hair is pushed upward, and its fused cells become increasingly keratinized and die. As Hair Thinning and Baldness they divide, hair matrix cells are replenished by stem cells that Given ideal conditions, hair grows fastest from the teen years migrate down to the bulb from a region closer to the skin sur- to the 40s. When hairs are no longer replaced as quickly as face called the hair bulge (not shown in Figure 5.6). they are shed, the hair thins. By age 60 to 65, most people  160 UNIT 2 Covering, Support, and Movement of the Body experience some degree of balding. Coarse terminal hairs are Lunule Lateral nail fold replaced by vellus hairs, and the hair becomes increasingly wispy. True baldness is different. The most common type is male pattern baldness, a genetically determined, male sex hor- mone–influenced condition that changes the response of the hair follicles to androgens. The hair follicles respond to andro- gens with shorter and shorter growth cycles. The cycles become so short that many hairs never even emerge from their follicles before being shed, and those that do are fine vellus hairs that (a) look like peach fuzz in the “bald” area. The drugs used to treat male pattern baldness either inhibit the production of androgens or increase blood flow to the skin and hair follicles. These treat- Free edge Nail Eponychium Nail root ments are only partly successful. of nail plate (cuticle) Proximal Nail nail fold matrix H OMEOSTATIC CLINICAL IMBALANCE 5.5 11 5 Abrupt hair thinning may be distressing, and determining its cause can be challenging. Telogen effluvium (ef-floo′ve-um), or TE, occurs when too many hair follicles enter the resting phase at the same time. TE often occurs after a “shock to the system,” such as surgery, a car crash, hormonal changes with childbirth, (b) crash dieting, or emotional stress. Hair loss or a change in texture may also signal low thyroid Hyponychium Nail bed Phalanx (bone of fingertip) hormone levels. Drugs, including some antidepressants and Figure 5.7 Structure of a nail. (a) Surface view of the distal part cancer chemotherapy drugs, may cause hair thinning. Finally, of a finger. (b) Sagittal section of the fingertip. The nail matrix that severe dietary deficiencies of protein or minerals may result in forms the nail lies beneath the lunule. thin, brittle hair. Check Your Understanding 5.6 Nails are scale-like modifications 12. Why is having your hair cut painless? of the epidermis 13. Use the image below to answer the following questions. What are the names of the parts of the hair labeled a and c? What Learning Outcome are the names of structures b and d and what is the function N Describe the structure of nails. of each of these structures? A nail forms a clear protective covering on the dorsal surface of the distal part of a finger or toe (Figure 5.7). Nails corre- a spond to the hooves or claws of other animals, and are useful as “tools” to help pick up small objects or scratch an itch. In contrast to soft keratin of the epidermis, nails (like hairs) contain hard keratin. Each nail has a proximal root (embedded b in the skin), a nail plate or body (visible attached portion), and a free edge. The nail rests on a bed of epidermis called the nail bed. This bed contains only the deeper layers of the epidermis, because the nail itself corresponds to the superficial keratinized layers. The thickened proximal portion of the nail bed, the nail matrix, is responsible for nail growth. As the nail cells pro- c duced by the matrix become heavily keratinized, the nail body slides distally over the nail bed. d Nails normally appear pink because of the rich bed of capil- laries in the underlying dermis. However, the region that lies over the thick nail matrix appears as a white crescent called the lunule 14. DRAW Draw a cross section of a hair shaft and label its three (lu′nool; “little moon”). The proximal and lateral borders of the concentric regions. nail are overlapped by skin folds, called nail folds. The proximal For answers, see Answers Appendix. nail fold projects onto the nail body as the cuticle, or eponychium  Chapter 5 The Integumentary System 161 (ep″o-nik′e-um; “on the nail”). The thickened region beneath the free edge of the nail where dirt and debris tend to accumulate is 5.7 Sweat glands help control body the hyponychium (“below nail”). It secures the free edge of the nail plate at the tip of the finger or toe. temperature, and sebaceous glands secrete sebum Learning Outcomes H OMEOSTATIC IMBALANCE 5.6 CLINICAL N Compare the structure and locations of sweat and oil glands. Also compare the composition and functions of Changes in nail appearance can their secretions. help diagnose certain conditions. N Compare and contrast eccrine and apocrine glands. For example, yellow-tinged nails may indicate a respira- Sweat glands, also called sudoriferous glands (su″do-rif′er-us; tory or thyroid gland disorder. sudor = sweat ), are distributed over the entire skin surface (Thickened yellow nails are usu- except the nipples and parts of the external genitalia. Their ally due to a fungus infecting the number is staggering—up to 3 million per person. nail.) An outward concavity of We have two types of sweat glands: eccrine and apocrine. In the nail (koilonychia, or “spoon both types, the secretory cells are associated with myoepithelial nail,” Figure 5.8) may signal an Figure 5.8 Koilonychia. cells, specialized cells that contract when stimulated by the nerv- iron deficiency. Horizontal lines ous system. Their contraction forces the sweat into and through the gland’s duct system to the skin surface. Sweating is regulated 11 5 (Beau’s lines) across the nails can be a sign of severe illness that affects the whole body such as uncontrolled diabetes, a heart by a branch of the autonomic (involuntary) nervous system. ­attack, or cancer chemotherapy. Eccrine Sweat Glands Eccrine sweat glands (ek′rin; “secreting”), also called mero- crine sweat glands, are far more numerous than apocrine sweat Check Your Understanding glands and are particularly abundant on the palms, soles of the 15. Why is the lunule of a nail white instead of pink like the rest of feet, and forehead. Each is a simple, coiled, tubular gland. The the nail? secretory part lies coiled in the dermis, and the duct extends to 16. Why are nails so hard? open in a funnel-shaped pore ( por = channel) at the skin sur- For answers, see Answers Appendix. face (Figure 5.9b). (These sweat pores are different from the Sweat pore Sebaceous Dermal gland connective tissue Eccrine Sebaceous gland gland duct Duct Hair in Dermal connective hair follicle tissue Secretory cells (a) Sebaceous gland (90×) (b) Eccrine gland (140×) Figure 5.9 Cutaneous glands.  162 UNIT 2 Covering, Support, and Movement of the Body Table 5.1 Summary of Cutaneous Glands ECCRINE SWEAT GLANDS APOCRINE SWEAT GLANDS SEBACEOUS GLANDS 11 5 Functions Temperature control May act as sexual scent glands Lubricate skin and hair Some antibacterial properties Help prevent water loss Antibacterial properties Type of Hypotonic filtrate of blood Filtrate of blood with added proteins Sebum (an oily secretion) Secretion and fatty substances Method of Merocrine (exocytosis) Merocrine (exocytosis) Holocrine Secretion Secretion Skin surface Usually upper part of hair follicle; Usually upper part of hair follicle; Exits Duct At rarely, skin surface sometimes, skin surface Body Location Everywhere, but especially palms, Mostly axillary and anogenital regions Everywhere except palms and soles soles, forehead so-called pores of a person’s complexion, which are openings eccrine glands, they lie deeper in the dermis or even in the sub- of hair follicles.) cutaneous tissue, and their ducts empty into hair follicles. Eccrine gland secretion, commonly called sweat, is a hypo- Apocrine secretion contains the same basic components as tonic filtrate of the blood that passes through the secretory cells true sweat, plus fatty substances and proteins. Consequently, it of the sweat glands and is released by exocytosis. It is 99% is viscous and sometimes has a milky or yellowish color. The water, with some salts (mostly sodium chloride), traces of meta- secretion is odorless, but when bacteria on the skin decom- bolic wastes (urea, uric acid, and ammonia), and a microbe- pose its organic molecules, it takes on a musky and generally killing peptide called dermcidin. Normally, sweat is acidic with unpleasant odor, the basis of body odor. a pH between 4 and 6. Apocrine glands begin functioning at puberty under the influ- Sweating’s major role is to prevent the body from overheat- ence of the male sex hormones (androgens) and play little role in ing. Heat-induced sweating begins on the forehead and spreads maintaining a constant body temperature. Their precise function inferiorly over the remainder of the body. Emotionally induced is not yet known. Three lines of evidence suggest that they may sweating—the “cold sweat” brought on by fright or nervous- be the human equivalent of other animals’ sexual scent glands: ness—begins on the palms, soles, and axillae (armpits) and then (1) Sexual foreplay increases their activity; (2) they enlarge spreads to other body areas. and recede with the phases of a female’s menstrual cycle; and (3) behavioral studies show that their secretions may act as pher- Apocrine Sweat Glands omones (chemical messengers released by one individual that The approximately 2000 apocrine sweat glands (ap′o-krin) are trigger a response in other members of the same species). largely confined to the axillary and anogenital areas. In spite Two important types of modified apocrine glands are: of their name, they are merocrine glands, which release their Ceruminous glands. Ceruminous glands (sĕ-roo′mĭ-nus; product by exocytosis like the eccrine sweat glands. Larger than cera = wax ) are modified apocrine glands found in the  Chapter 5 The Integumentary System 163 lining of the external ear canal. Their secretion mixes with Check Your Understanding sebum produced by nearby sebaceous glands to form a sticky, 17. Which cutaneous glands are associated with hair follicles? bitter substance called cerumen, or earwax, that is thought to 18. When Carmela returned home from a run in 85°F weather, her deter insects and block entry of foreign material. face was dripping with sweat. Why? Mammary glands. Mammary glands, another type of spe- 19. What is the difference between heat-induced sweating and a cialized sweat gland, secrete milk. Although they are properly “cold sweat,” and which variety of sweat gland is involved? part of the integumentary system, we will consider the mam- 20. APPLY Sebaceous glands are not found in thick skin. Why is mary glands in Chapter 27 with female reproductive organs. their absence in those body regions desirable? For answers, see Answers Appendix. Sebaceous Glands The sebaceous glands (se-ba′shus; “greasy”), or oil glands (Figure 5.9a), are simple branched alveolar glands that are 5.8 First and foremost, the skin is a found all over the body except in the thick skin of the palms and soles. They are small on the body trunk and limbs, but barrier quite large on the face, neck, and upper chest. These glands Learning Outcome secrete an oily substance called sebum (se′bum). The central N Describe how the skin accomplishes at least five different cells of the alveoli accumulate oily lipids until they become so functions. engorged that they burst, so functionally these glands are holo- crine glands ( p. 126). The accumulated lipids and cell frag- Like the skin of a grape, our skin keeps its contents juicy and 11 5 ments constitute sebum. whole. The skin and its appendages perform a variety of func- Most, but not all, sebaceous glands develop as outgrowths tions, including protection, body temperature regulation, cutane- of hair follicles and secrete sebum into a hair follicle, or occa- ous sensation, metabolic functions, blood reservoir, and excretion. sionally to a pore on the skin surface. Arrector pili contractions force sebum out of the hair follicles to the skin surface. Protection Sebum softens and lubricates the hair and skin, prevents hair Given its superficial location, the skin is our most vulnerable from becoming brittle, and slows water loss from the skin. Per- organ system, exposed to microorganisms, abrasion, tempera- haps even more important is its bactericidal (bacterium-killing) ture extremes, harmful chemicals, and UV radiation. The skin action. Sebaceous glands increase their activity during puberty constitutes at least three types of barriers: chemical, physical, under the influence of male sex hormones. and biological. H OMEOSTATIC Chemical Barriers CLINICAL IMBALANCE 5.7 Chemical barriers include skin secretions and melanin. Acne is an active inflammation of the sebaceous glands ac- Although the skin’s surface teems with bacteria, the low pH of companied by “pimples” (pustules or cysts), whiteheads, or skin secretions—the acid mantle—retards their multiplication. blackheads on the skin. If In addition, dermcidin in sweat and bactericidal substances accumulated sebum blocks a in sebum kill many bacteria outright. Skin cells also secrete sebaceous gland duct, it can natural antibiotics called defensins that literally punch holes in cause a whitehead (closed bacteria, making them leak like sieves. Wounded skin releases comedo, plural: comedones) large quantities of protective peptides called cathelicidins that or a blackhead (open comedo). are particularly effective in preventing infection by group A Acne is associated with infec- streptococcus bacteria. tion by Propionibacterium As discussed earlier, melanin provides a chemical pigment ­acnes. It can range from mild shield to prevent UV damage to skin cells. to so severe that it leads to Physical Barriers permanent scarring. O ve r a c t ive s e b a c e o u s The continuity of skin and the hardness of its keratinized cells glands can cause seborrhea provide physical barriers. As a physical barrier, the skin is a (seb″o-re′ah; “fast-f lowing Figure 5.10 Cradle cap remarkable compromise. A thicker epidermis would be more sebum”), known as “cradle (seborrhea) in a newborn. impenetrable, but we would pay the price in loss of suppleness cap” in infants (Figure 5.10). and agility. Seborrhea begins on the scalp as pink, raised lesions that The outstanding barrier capacity of the skin arises from the gradually become yellow to brown and begin to slough off structure of its stratum corneum, which has been compared oily scales. to bricks and mortar. Multiple layers of dead flat cells are the bricks and the glycolipids surrounding them are the mortar. Epi- dermal continuity works hand in hand with the acid mantle and Table 5.1 summarizes the three types of cutaneous glands. chemicals in skin secretions to ward off bacterial invasion.  164 UNIT 2 Covering, Support, and Movement of the Body The water-resistant glycolipids of the epidermis block most blood vessels to dilate and the sweat glands into vigorous secre- diffusion of water and water-soluble substances between cells, tory activity. On a hot day, sweat becomes noticeable and can preventing both their loss from and entry into the body through account for the loss of up to 12 L (about 3 gallons) of body the skin. However, there is a continual small loss of water water in one day. This visible output of sweat is called sensi- through the epidermis. ble perspiration. Evaporation of sweat from the skin surface Substances that do penetrate the skin in limited amounts dissipates body heat and efficiently cools the body, preventing include: overheating. Lipid-soluble substances, such as oxygen, carbon dioxide, fat- When the external environment is cold, dermal blood vessels soluble vitamins (A, D, E, and K), and steroids (estrogens) constrict. Their constriction causes the warm blood to bypass the skin temporarily and allows skin temperature to drop to that Oleoresins (o″le-o-rez′inz) of certain plants, such as poison of the external environment. This slows passive heat loss from ivy and poison oak the body, conserving body heat. We will discuss body tempera- Organic solvents, such as acetone, dry-cleaning fluid, and ture regulation in more detail in Chapter 24. paint thinner, which dissolve the cell lipids Salts of heavy metals, such as lead and mercury Cutaneous Sensation Certain drugs (nitroglycerin, seasickness medications) The skin is richly supplied with cutaneous sensory receptors, Pharmaceuticals called penetration enhancers that help ferry which are actually part of the nervous system. The cutaneous other drugs into the body receptors are classified as exteroceptors (ek″ster-o-sep′torz) 11 5 because they respond to stimuli arising outside the body. For example: H OMEOSTATIC CLINICAL Tactile (Meissner’s) corpuscles (in the dermal papillae) and IMBALANCE 5.8 Organic solvents and heavy metals are devastating to the body tactile epithelial cells with their associated sensory nerve end- and can be lethal. Passage of organic solvents through the skin ings allow us to become aware of a caress or the feel of our into the blood can shut down the kidneys and also cause brain clothing against our skin. damage. Absorption of lead results in anemia and neurological Lamellar (also called Pacinian) corpuscles (in the deeper defects. These substances should never be handled with bare dermis or subcutaneous tissue) alert us to bumps or contacts hands. involving deep pressure. Hair follicle receptors report on wind blowing through our hair and a playful tug on a ponytail. Biological Barriers Free nerve endings, present throughout the skin, sense pain- Biological barriers include the dendritic cells of the epidermis ful stimuli (irritating chemicals, extreme heat or cold, and and macrophages in the dermis: others). Dendritic cells ( p. 153) are part of the immune system. Figure 5.1 illustrates all the cutaneous receptors mentioned They patrol beneath the skin’s surface, engulfing any for- above with two exceptions: tactile corpuscles (found only in eign invaders that have penetrated the epidermis. Once they skin that lacks hairs) and tactile epithelial cells (shown in Fig- have captured their prey, they leave the skin and migrate to ure 5.2b). We will discuss these cutaneous receptors in more the nearest lymph node. There they display their prey to other detail in Chapter 13. immune cells. This triggers an immune response. Dermal macrophages ( p. 129) constitute a second line of Metabolic Functions defense. They dispose of viruses and bacteria that manage to penetrate the epidermis. Like dendritic cells, macrophages can The skin is a chemical factory, fueled in part by the sun’s rays. initiate an immune response. When sunlight bombards the skin, modified cholesterol mol- ecules are converted to a vitamin D precursor. This precursor Body Temperature Regulation is transported via the blood to other body areas to be converted to vitamin D, which plays various roles in calcium metabolism. The body works best when its temperature remains within For example, calcium cannot be absorbed from the digestive homeostatic limits. Like car engines, we need to get rid of the tract without vitamin D. heat generated by our internal reactions. As long as the external Among its other metabolic functions, the epidermis makes temperature is lower than body temperature, the skin surface chemical conversions that supplement those of the liver. For loses heat to the air and to cooler objects in its environment, example, keratinocyte enzymes can: just as a car radiator loses heat to the air and other nearby parts. Under normal resting conditions, and as long as the environ- “Disarm” many cancer-causing chemicals that penetrate the mental temperature is below 31–32°C (88–90°F), sweat glands epidermis secrete about 500 ml (0.5 L) of sweat per day. This routine and Activate some steroid hormones—for instance, they can unnoticeable sweating is called insensible perspiration. When transform cortisone applied to irritated skin into hydrocorti- body temperature rises, the nervous system stimulates dermal sone, a potent anti-inflammatory drug  Chapter 5 The Integumentary System 165 Skin cells also make several biologically important proteins, Loss of homeostasis in body cells and organs reveals itself on including collagenase, an enzyme that aids the natural turnover the skin, sometimes in startling ways. The skin can develop of collagen (and deters wrinkles). more than 1000 different conditions and ailments. The most common skin disorders are bacterial, viral, or yeast infections Blood Reservoir (see Related Clinical Terms on pp. 169–170). Less common, The dermal vascular supply is extensive and can hold about 5% but far more damaging to body well-being, are skin cancer and of the body’s entire blood volume. When other body organs, burns, considered next. such as vigorously working muscles, need a greater blood sup- ply, the nervous system constricts the dermal blood vessels. Skin Cancer This constriction shunts more blood into the general circula- Skin cancer is the most common type of cancer—one in five tion, making it available to the muscles and other body organs. of us will develop skin cancer at some point. The single most important risk factor for skin cancer is exposure to the UV radi- Excretion ation in sunlight and tanning beds, which damages DNA bases The body eliminates limited amounts of nitrogen-containing ( p. 54). Adjacent pyrimidine bases often respond by fusing, wastes (ammonia, urea, and uric acid) in sweat, although most forming lesions called dimers. UV radiation also appears to dis- such wastes are excreted in urine. Profuse sweating is an impor- able a tumor suppressor gene ( p. 144). In limited numbers of tant avenue for water and salt (sodium chloride) loss. cases, frequent irritation of the skin by infections, chemicals, or physical trauma seems to be a predisposing factor. The three major forms of skin cancer are basal cell carci- 11 5 Check Your Understanding noma, squamous cell carcinoma, and melanoma. 21. What chemicals produced in the skin help provide barriers to bacteria? List at least three and explain how the chemicals are protective. Basal Cell Carcinoma 22. Which epidermal cells play a role in body immunity? Basal cell carcinoma (kar″sĭ-no′mah), the least malignant and 23. How is sunlight important to bone health? most common, accounts for nearly 80% of cases. Stratum basale 24. MAKE CONNECTIONS When blood vessels in the dermis cells proliferate, invading the dermis and subcutaneous tissue. constrict or dilate to help maintain body temperature, which The cancer lesions occur most often on sun-exposed areas of the type of muscle tissue that you learned about (in Chapter 4) face and appear as shiny, dome-shaped nodules that later develop acts as the effector that causes blood vessel dilation or a central ulcer with a pearly, beaded edge (Figure 5.11a). Basal constriction? cell carcinoma is relatively slow-growing, and metastasis sel- For answers, see Answers Appendix. dom occurs. Full cure by surgical excision is the rule in 99% of cases. 5.9 Skin cancer and CLINICAL Squamous Cell Carcinoma Squamous cell carcinoma, the second most common skin can- burns are major challenges to the body cer, arises from the keratinocytes of the stratum spinosum. The Learning Outcomes lesion appears as a scaly reddened papule (small, rounded ele- N Summarize the characteristics of the three major types of vation) that arises most often on the head (scalp, ears, and lower skin cancers. lip) and hands (Figure 5.11b). It tends to grow rapidly and N Explain why serious burns are life threatening. Describe metastasize if not removed. If it is caught early and removed how to determine the extent of a burn and whether it is surgically or by radiation therapy, the chance of complete cure a first-, second-, or third-degree burn. is good. (a) Basal cell carcinoma (b) Squamous cell carcinoma (c) Melanoma Figure 5.11 Photographs of skin cancers.  166 UNIT 2 Covering, Support, and Movement of the Body Melanoma Melanoma (mel″ah-no′mah), cancer of melanocytes, is the most dangerous skin cancer because it is highly metastatic and resistant to chemotherapy. It accounts for only about 1% of skin 1st-degree burn cancers, but most skin cancer deaths. Its incidence is increasing rapidly. Melanoma can begin wherever there is pigment. Most such cancers appear spontaneously, and about one-third develop from preexisting moles. It usually appears as a spreading brown to black patch (Figure 5.11c) that metastasizes rapidly to sur- rounding lymphatic and blood vessels. The key to surviving melanoma is early detection. Survival rates decline with increasing thickness of the melanoma, degree 2nd-degree burn of involvement of nearby lymph nodes, and extent of metasta- sis. In advanced cases, surgical treatment is followed by immu- notherapy, radiation therapy, or, more recently, targeted gene therapy that has had promising results by shrinking tumors and/ (a) Skin bearing partial-thickness or prolonging life. burn (1st- and 2nd-degree burns) The American Cancer Society suggests that we regularly 11 5 examine our skin for new moles or pigmented spots. Apply the ABCD rule for recognizing melanoma: Asymmetry: The two sides of the pigmented spot or mole do not match. 3rd-degree burn Border irregularity: The borders of the lesion exhibit ­indentations. Color: The pigmented spot contains several colors (blacks, browns, tans, and sometimes blues and reds). Diameter: The spot is larger than 6 mm in diameter (the size of a pencil eraser). Some experts add an E, for evolution or evolving (changes with time). (b) Skin bearing full-thickness Burns burn (3rd-degree burn) Burns are a devastating threat to the body primarily because of Figure 5.12 Partial-thickness and full-thickness burns. their effects on the skin. A burn is tissue damage inflicted by intense heat, electricity, radiation, or certain chemicals, all of which kill cells in the affected areas. painful, but skin regeneration occurs with little or no scarring The immediate threat to life resulting from severe burns is a within three to four weeks if care is taken to prevent infection. catastrophic loss of body fluids containing proteins and elec- First- and second-degree burns are considered partial-thickness trolytes. This leads to dehydration and electrolyte imbalance, burns (Figure 5.12a). and then circulatory shock (inadequate blood circulation due to Third-degree burns are full-thickness burns, involving the reduced blood volume) and renal failure (kidney shutdown). To entire thickness of the skin (Figure 5.12b). The burned area save the patient, the lost fluids must be replaced immediately appears gray-white, cherry red, or blackened, and initially by administration of intravenous (IV) fluids. there is little or no edema. Since the nerve endings have been destroyed, the burned area is not painful. Although skin might Evaluating Burns eventually regenerate, it is usually impossible to wait that long Burns are classified according to their severity (depth) as first-, because of fluid loss and infection. Skin grafting is advised. second-, or third-degree burns. In first-degree burns, only the In general, burns are considered critical if any of the follow- epidermis is damaged. Symptoms include localized redness, ing conditions exists: swelling, and pain. First-degree burns tend to heal in two to Over 25% of the body has second-degree burns three days without special attention. Sunburn is usually a first- degree burn. Over 10% of the body has third-degree burns Second-degree burns injure the epidermis and the upper There are third-degree burns of the face, hands, or feet region of the dermis. Symptoms mimic those of first-degree (facial burns introduce the possibility of burned respiratory burns, but blisters also appear. The burned area is red and passageways, which can swell and cause suffocation)  Chapter 5 The Integumentary System 167 Totals other pathogens easily invade areas where the skin barrier is destroyed, and they multiply rapidly in the nutrient-rich envi- 41⁄2% ronment of dead tissues. As you might expect, antibiotics play Anterior and posterior an important role in burn treatment. head and neck, 9% Longer-term treatment of full-thickness burns usually involves a skin graft. To prepare a burned area for a skin graft, the eschar (es′kar), or burned skin, must first be debrided Anterior and posterior (removed). Then healthy skin is transplanted to the burned site. upper limbs, 18% If possible, the skin is the patient’s own (an autograft). Other- wise, there is a good chance that the patient’s immune system will reject the foreign skin. Even if the graft “takes,” extensive scar tissue often forms—a major problem because it can limit 41⁄2% Anterior 41⁄2% Anterior and posterior trunk, 18% trunk, 36% mobility at joints and other critical areas. Alternatively, synthetic skin—a silicone “epidermis” bound to a spongy “dermal” layer composed of collagen and ground cartilage—can be applied to the debrided area. In time, the patient’s own dermal tissue absorbs and replaces the artificial one. Then the silicone sheet is peeled off and replaced with a network of epidermal cells cultured from the patient’s own skin. 11 5 Check Your Understanding 25. Which type of skin cancer develops from the youngest 9% 9% epidermal cells? Perineum, 1% 26