Seeley Anatomy & Physiology 13e (2023) Integumentary System PDF

5 C H A P T E R Integumentary System The skin is composed of the epidermis...

5 C H A P T E R Integumentary System The skin is composed of the epidermis and the dermis. Accessory structures of the skin include nails, hair, and glands. Photo: Gulosus chirographi imputat plane fragilis syrtes.Bellus oratori fermetet uin que nnali. rures lucide circumg gulosus The integumentary system includes the skin and accessory structures zothecas incredibilitier such as hair, glands, qunquen. and©Credit nails. ISTUDY van03883_ch05_158-181.indd 158 16/11/21 12:07 PM  I t is the morning of “the big day.” You look in the mirror and, much to your dismay, there is a big, red bump on your chin. Just when you needed to look your best, this Understand Learn to Predict had to happen! For most people, blemish-free skin is highly desirable, and any sign of acne is cause for embarrassment. Hair loss and crows’ feet also cause consternation It was a dream job—summer days spent in some people. It goes without saying that much time, effort, and money are spent on poolside, soaking up the sun. Following her changing the appearance of the integumentary system. Think about the amount of coun- first year at college, Laura worked as a life- ter space dedicated to skin care products, hair care products, and cosmetics in a typical guard at the country club in her hometown. discount store. People apply lotion to their skin, color their hair, and trim their nails. By the end of summer, she had a golden They try to prevent sweating by using antiperspirants and reduce or even mask body tan and was eager to show off her new look odor by washing and using deodorants and perfumes. on campus. However, after returning to The integumentary (in-teg-you-MEN-tah-ree) system consists of the skin and school in the fall, she was disappointed to accessory structures such as hair, glands, and nails. Although people are concerned see that her skin kept getting lighter, and in about the appearance of their integumentary system for vanity’s sake, its appearance only a few weeks it had returned to its nor- can also indicate physiological imbalances. Some disorders, such as acne or warts, mal paler tone. affect only the integumentary system. Other disorders affect different body parts but By combining your understanding of epi- are reflected in the integumentary system, which provides useful signs for diagnosis. thelial tissue from chapter 4 with further For example, reduced blood flow through the skin during a heart attack can cause information about skin structure and pig- a person to look pale, whereas increased blood flow as a result of fever can cause a mentation in this chapter, explain how and flushed appearance. Some diseases cause skin rashes such as those characteristic of why Laura’s tan faded in the fall. measles, chickenpox, and allergic reactions. Answers to this question and the chapter’s odd-numbered Predict questions can be found in Appendix E. 5.1 Functions of the Integumentary System LEARNING OUTCOME After reading this section, you should be able to A. Describe the general functions of the integumentary system. Although we are often concerned with how the integumentary system looks, it has many important functions that go beyond ­ appearance. The integumentary system includes the skin and accessory structures such as hair, nails, and glands. It forms the boundary ­between the body and the external environment, thereby separating us from the external environment while allowing us to interact with it. The major functions of the integumentary system are: 1. Protection. The skin is the covering of the body. Though exposed to the external environment, the skin’s structure reduces the negative and harmful effects of ultraviolet light. Acting as a barrier, the skin also keeps microorganisms from entering the body and prevents dehydration by reducing water loss. 2. Sensation. The integumentary system has sensory receptors that can detect heat, cold, touch, pressure, and pain. 3. Temperature regulation. The skin plays a major role in regulating body tempera- ture through the modulation of blood flow through the skin and the activity of sweat glands. 4. Vitamin D production. When exposed to ultraviolet light, the skin produces a molecule that can be transformed into the hormonal form of vitamin D, an important regulator of calcium homeostasis. 5. Excretion. Small amounts of waste products are excreted through the skin and glands. ASSESS YOUR PROGRESS Answers to these questions are found in the section you have just completed. Re-read the section if you need help in answering these questions. Module 4 1. Provide an example for each function of the integumentary system. Integumentary System 159 ISTUDY van03883_ch05_158-181.indd 159 16/11/21 12:07 PM  160 PART 2 Support and Movement 5.2 Skin ∙∙ Merkel cells are specialized ­epidermal cells associated with the nerve endings responsible for detecting light touch and superficial pressure (see figure 5.1 and chapter 14). LEARNING OUTCOMES The keratinocytes of the epidermis are constantly lost at its After reading this section, you should be able to surface but are also constantly replaced by new keratinocytes A. Describe the structure of the epidermis. moving from deeper layers. These new keratinocytes are produced B. Describe the function of the epidermis. when keratinocyte stem cells undergo mitosis in the deepest layer of the epidermis. As new cells form, they push older cells to the C. Discuss the epidermal strata and relate them to the process of keratinization. surface, where they slough off. The outermost cells in this strati- fied arrangement protect the cells underneath, and the deeper, D. Differentiate between thick and thin skin as to the replicating cells replace cells lost from the surface. (Note: This layers present and their locations. information will be useful when answering the Learn to Predict E. Explain the major factors affecting skin color. question at the beginning of the chapter.) F. Describe the structure of the dermis. As keratinocytes move from the deeper epidermal layers to the G. Describe the functions of the dermis. ­surface, the cells change shape and chemical composition, through the process called keratinization (KER-ah-tin-i-ZAY-shun), The skin is made up of two major tissue layers, the epidermis where the cells accumulate keratin. During keratinization, the cells and the dermis (figure 5.1). The epidermis (ep-ih-DER-miss; eventually die and produce an outer layer of dead, hard cells that upon the dermis) is the superficial layer of the skin, consist- resists abrasion and forms a permeability barrier. The study of ing of stratified squamous epithelial tissue. The multiple cell keratinization is important because many skin diseases result from layers of the epidermis protect against the potential damage malfunctions in this process. For example, large scales of epider- from abrasion on the skin’s surface. In addition, the epidermis mal tissue are sloughed off in psoriasis (soh-RYE-ah-sis; reduces water loss through the skin. The epidermis rests on table 5.2). By comparing normal and abnormal keratinization, the dermis (DER-miss; skin), a layer of connective tissue. The scientists may be able to develop effective therapies for psoriasis. structure of the dermis is responsible for most of the strength of Although keratinization is a continual process, distinct tran- the skin. Leather is a durable material used to make items such sitional stages can be recognized as the cells change. On the as furniture coverings. Leather is produced from the skin of an basis of these stages, the many layers of cells in the epidermis animal by removing the epidermis and preserving the dermis in are divided into regions, or strata (sing. stratum; figure 5.2). a process called tanning. From the deepest to the most superficial, the five strata are the The skin rests on the subcutaneous tissue, or hypodermis (1) stratum basale, (2) stratum spinosum, (3) stratum granulosum, ­(high-poh-DER-miss), a layer of loose connective tissue (figure (4) stratum lucidum, and (5) stratum corneum. The number of cell 5.1). The subcutaneous tissue is not part of the skin or the integu- layers in each stratum and even the number of strata in the skin mentary system, but it does connect the skin to underlying muscle vary, depending on the location in the body. or bone. Table 5.1 summarizes the structures and functions of the skin and subcutaneous tissue. Stratum Basale The stratum basale (ba-SAL-eh), or stratum germinativum Epidermis (JER-mi-nah-TYVE-um; see figure 5.2) is the deepest portion of The epidermis is stratified squamous epithelium and as such has the epidermis. It is a single layer of cuboidal or columnar cells. many of the characteristics described in chapter 4. It is separated At the stratum basale, the epidermis is anchored to the basement from the underlying dermis by a basement membrane. The epider- membrane by hemidesmosomes. In addition, desmosomes hold mis contains no blood vessels. The living cells of the epidermis the keratinocytes together (see ­ chapter 4). The connections receive nutrients and excrete waste products by the diffusion of formed by the hemidesmosomes and desmosomes provide struc- substances between the epidermis and the capillaries of the dermis tural strength to the epidermis. Keratinocytes are strengthened (figure 5.1a). Though the epidermis is stratified, it is not as thick internally by keratin fibers (intermediate filaments) that insert as the dermis. into the desmosomes. The epidermis is composed of several types of cells. Most cells Keratinocyte stem cells of the stratum basale undergo mitotic of the epidermis are called keratinocytes (ke-RAT-i-noh-sytes) divisions approximately every 19 days. One daughter cell remains because they produce a protein mixture called keratin (KER-ah-tin), a stem cell in the stratum basale and divides again, but the other which makes the cells more durable. It is the durability of the kera- daughter cell is pushed toward the surface and becomes keratin- tinocytes that gives the epidermis its ability to resist abrasion and ized. It takes approximately 40–56 days for the cell to reach the reduce water loss. Other cells of the epidermis include ­melanocytes, epidermal surface and slough off. Langerhans cells, and Merkel cells. ∙∙ Melanocytes (MEL-an-oh-sytes) produce the pigment melanin, Stratum Spinosum which contributes to skin color. Superficial to the stratum basale is the stratum spinosum ∙∙ Langerhans cells are part of the immune system (see chapter 22). ­(spye-NOH-sum), consisting of 8–10 layers of many-sided cells ISTUDY van03883_ch05_158-181.indd 160 16/11/21 12:07 PM  FUNDAMENTAL Figure Hairs Free nerve endings Epidermis Sebaceous gland Skin Arrector pili (smooth muscle) Dermis Hair follicle Pacinian corpuscle Nerve Vein Artery Subcutaneous tissue (hypodermis) Sweat gland (sudoriferous) Adipose tissue (a) Epidermis FIGURE 5.1 Skin and Subcutaneous Tissue (a) The skin, consisting of the epidermis and the dermis, is connected Dermis by the subcutaneous tissue to underlying structures. Note the acces- sory structures (hairs, glands, sensory receptor, and arrector pili), some of which project into the subcutaneous tissue, as well as the large amount of adipose tissue in the subcutaneous tissue. (b) Photo- micrograph of the skin showing the dermis covered by the epidermis. LM 40x (b) Victor P. Eroschenko (b) 161 ISTUDY van03883_ch05_158-181.indd 161 16/11/21 12:07 PM  FUNDAMENTAL Figure Superficial Intercellular lipids 5 Keratin Stratum Direction corneum of cell Lamellar body movement releases lipids. 4 Protein Stratum envelope lucidum Keratohyalin 3 granules Stratum granulosum Lipid-filled lamellar body 2 Keratin fiber Stratum spinosum Desmosome 1 Nucleus Stratum basale Hemidesmosome Dermal papilla Basement extending into LM 500x membrane Deep the epidermis (a) (b) FIGURE 5.2 Epidermal Layers and Keratinization (a) As keratinocytes progress from one stratum to the next, they undergo a series of changes called keratinization. (b) Photomicrograph of the epidermis resting on the papillary layer of the dermis (b) Alvin Telser/McGraw Hill Education. TABLE 5.1 Comparison of the Skin (Epidermis and Dermis) and Subcutaneous Tissue Part Structure Function Epidermis Superficial part of skin; stratified squamous Prevents water loss and the entry of chemicals and microorganisms; epithelium; composed of four or five strata protects against abrasion and harmful effects of ultraviolet light; pro- duces vitamin D; gives rise to hair, nails, and glands Stratum corneum Most superficial stratum of the epidermis; 25 or Provides structural strength due to keratin within cells; prevents water more layers of dead squamous cells loss due to lipids surrounding cells; sloughing off of most superficial cells resists abrasion Stratum lucidum Three to five layers of dead cells; appears transparent; Disperses keratohyalin around keratin fibers present in thick skin, absent in most thin skin Stratum granulosum Two to five layers of flattened, diamond-shaped Produces keratohyalin granules; lamellar bodies release lipids from cells cells; cells die Stratum spinosum A total of 8–10 layers of many-sided cells Produces keratin fibers; lamellar bodies form inside keratinocytes Stratum basale Deepest stratum of the epidermis; single layer of Produces cells of the most superficial strata; melanocytes produce cuboidal or columnar cells; basement membrane and contribute melanin, which protects against ultraviolet light of the epidermis attaches to the dermis Dermis Deep part of skin; connective tissue composed of Is responsible for the structural strength and flexibility of the skin; the two layers epidermis exchanges gases, nutrients, and waste products with blood vessels in the dermis Papillary layer Papillae project toward the epidermis; loose Brings blood vessels close to the epidermis; dermal papillae form connective tissue fingerprints and footprints Reticular layer Mat of collagen and elastic fibers; dense irregular Is the main fibrous layer of the dermis; strong in many directions; connective tissue forms cleavage lines Subcutaneous Not part of the skin; loose connective tissue with Attaches the dermis to underlying structures; adipose tissue provides tissue abundant deposits of adipose tissue energy storage, insulation, and padding; blood vessels and nerves from the subcutaneous tissue supply the dermis 162 ISTUDY van03883_ch05_158-181.indd 162 16/11/21 12:07 PM  CHAPTER 5 Integumentary System 163 (see figure 5.2). As the cells in this stratum are pushed to the Lipids are released from lamellar bodies surrounding the skin surface, they flatten; desmosomes break apart, and new ­ cells. The lipids are responsible for many of the skin’s permeabil- ­desmosomes form. During preparation for microscopic observa- ity characteristics. tion, the cells usually shrink from one another, except where they are ­attached by desmosomes, causing the cells to appear spiny— hence the name stratum spinosum. As keratinocytes progress through this stratum, additional keratin fibers and lipid-filled, Understand Predict 1 ­membrane-bound organelles called lamellar (LAM-eh-lar, lah- Some drugs are administered by applying them to the skin (e.g., a nicotine skin MEL-ar) b ­ odies form inside the cells. patch to help a person stop smoking). The drug diffuses through the epidermis to blood vessels in the dermis. What kinds of substances can pass easily through Stratum Granulosum the skin by diffusion? What kinds of substances have difficulty diffusing through The stratum granulosum (gran-yoo-LOH-sum) consists of two to the skin? five layers of somewhat flattened, diamond-shaped cells. The long axes of these cells are oriented parallel to the surface of the skin ­(see figure 5.2). This stratum derives its name from the presence of protein granules of keratohyalin (KER-ah-toh-HIGH-ah-lin), which accumulate in the cytoplasm of the keratinocytes. The Thick and Thin Skin lamellar bodies, formed as the cells pass through the stratum When we say a person has thick or thin skin, we are usually spinosum, move to the plasma membrane and release their lipid referring metaphorically to the person’s ability to take criticism. contents into the extracellular space. Inside the keratinocyte, a However, in a literal sense all of us have both thick and thin skin. protein envelope forms beneath the plasma membrane. In the most Skin is classified as thick or thin based on the structure of the superficial layers of the stratum granulosum, the nucleus and other epidermis. Thick skin has all five epithelial strata, and the stratum organelles degenerate, and the keratinocyte dies. Unlike the other corneum has many layers of cells. Thick skin is found in areas organelles and the nucleus, however, the keratin fibers and kerato- subject to pressure or friction, such as the palms of the hands, the hyalin granules within the cytoplasm do not degenerate. soles of the feet, and the fingertips. Thin skin covers the rest of the body and is more flexible than thick skin. The stratum lucidum is generally absent in thin Stratum Lucidum skin. Additionally, each stratum in thin skin consists of fewer lay- The stratum lucidum (LOO-see-dum) is a thin, clear zone above ers of cells than are found in thick skin. For example, the stratum the stratum granulosum (see figure 5.2). It consists of several lay- granulosum frequently has only one or two layers of cells, as ers of dead keratinocytes with indistinct boundaries. Keratin opposed to the three to five layers of cells in thick skin. Another ­fibers are present, but the keratohyalin, which was evident as important difference between thick and thin skin is that hair is granules in the stratum granulosum, has dispersed around the found only in thin skin. Recall the first key concept of anatomy keratin fibers, and the cells appear somewhat transparent. The and physiology is the relationship between structure and func- stratum lucidum is present in only a few areas of the body (see the tion. This variation in the structure of skin relative to degree section “Thick and Thin Skin”). of exposure to pressure and friction is a good example of this relationship. Stratum Corneum The entire skin, including both the epidermis and the dermis, The last, and most superficial, stratum of the epidermis is the varies in thickness from 0.5 mm in the eyelids to 5.0 mm on the stratum corneum (COR-nee-um; see figure 5.2). This stratum back and shoulders. The terms thin and thick refer only to the is composed of 25 or more layers of dead, overlapping squa- ­epidermis and should not be used when total skin thickness is mous cells joined by desmosomes. Eventually, the desmosomes considered. It is actually variation in the thickness of the dermis break apart, and the cells are shed from the surface of the skin. that accounts for most of the difference in total skin thickness. For Excessive shedding of the stratum corneum of the scalp results example, the skin of the back is thin skin, whereas that of the palm in dandruff. Less noticeably, skin cells are continually shed of the hand is thick skin; however, because the dermis of the skin from other areas as clothes rub against the body or as the skin is of the back is thicker, the total skin thickness of the back is greater washed. than that of the palm. The stratum corneum consists of cornified cells, which are In skin subjected to friction or pressure, an area called a dead keratinocytes, with a hard protein envelope, filled with the callus (KAL-us) often forms when the stratum corneum of the protein keratin. Keratin is a mixture of keratin fibers and kerato- epidermis increases in thickness. The increased number of cell hyalin. The envelope and the keratin are responsible for the struc- layers provides added protection in the area that experiences more tural strength of the stratum corneum. The type of keratin found mechanical stress. The skin over bony prominences develop a in the skin is soft keratin. Another type of keratin, hard keratin, cone-shaped structure called a corn. The base of the cone is at the is found in nails and the external parts of hair. Cells containing surface, but the apex extends deep into the epidermis, and pressure hard keratin are more durable than cells with soft keratin, and on the corn may be quite painful. Calluses and corns can develop they are not shed. in both thin and thick skin. ISTUDY van03883_ch05_158-181.indd 163 16/11/21 12:07 PM  164 PART 2 Support and Movement Skin Color be brown, black, yellow, or red. The types of melanin produced The factors that determine skin color include (1) pigments in the are determined by genetic factors. The degree to which melanin skin, (2) blood circulating through the skin, and (3) the thickness is produced is influenced by exposure to light and hormones. of the stratum corneum. The number of melanocytes in the skin is relatively the same for Melanin (MEL-ah-nin) is the group of pigments primarily all humans, regardless of race. Racial variations in skin color are responsible for skin, hair, and eye color. Melanin also provides determined by several major factors: protection against ultraviolet light from the sun. Large amounts of ∙∙ types of melanin produced melanin are found in certain regions of the skin, such as freckles, ∙∙ amount of melanin produced moles, the nipples, the areolae of the breasts, the axillae, and the ∙∙ size of melanosomes (melanin-filled vesicles in cells) genitalia. Other areas of the body, such as the lips, palms of the ∙∙ number of melanosomes hands, and soles of the feet, contain less melanin. ∙∙ distribution of melanosomes Melanin is produced by melanocytes (MEL-an-oh-sytes), irregularly shaped cells with many long processes that extend Although many genes are responsible for skin color, a single between the keratinocytes of the stratum basale and the stratum spi- mutation (see chapter 29) can prevent the manufacture of mela- nosum. ­Figure 5.3 illustrates how melanin, produced by melano- nin. ­Albinism (AL-bi-niz-em) is usually a recessive genetic trait cytes, is transferred to keratinocytes, resulting in a relatively that results from an inability to produce tyrosinase. The result is consistent skin tone across the body. a deficiency or an absence of pigment in the skin, the hair, and the irises of the eyes. For those individuals who are able to pro- 1 Melanocytes produce and package melanin into vesicles called duce melanin, exposure to ultraviolet light darkens the melanin melanosomes (MEL-ah-no-sohms). already present in the skin and stimulates melanin production, 2 These melanosomes move into cell processes of the melanocytes. resulting in tanning. (Note: This information will be helpful 3 Keratinocytes phagocytize (see c­ hapter 3) the tips of the melanocyte when answering the Learn to Predict question at the beginning cell processes, thereby a ­ cquiring melanosomes. of the chapter.) Melanin production involves the enzymatic modification of Certain conditions may cause increases in melanin production the amino acid tyrosine to an intermediate. This intermediate and therefore changes in pigmentation in areas of the body. During may be modified in different ways producing pigments that can pregnancy, certain hormones, such as estrogen and melanocyte- stimulating hormone, cause the mother’s body to increase melanin production, which causes darkening of the nipples, areolae, and genitalia. The cheekbones, forehead, and chest also may darken, resulting in the “mask of pregnancy.” In addition, a dark line of PROCESS Figure pigmentation may appear on the midline of the abdomen. Diseases that cause increased secretion of adrenocorticotropic hormone and melanocyte-stimulating hormone, such as Addison disease, also cause increased pigmentation. Skin color can also show variation due to changes in blood flow. These changes in skin color are not as long lasting and often Keratinocyte dissipate in a few minutes or hours. Erythema and cyanosis are phagocytizes melanocyte examples of these types of skin color changes. Erythema (er- Keratinocyte 3 process ih-THEE-mah) is a condition in which the skin turns a reddish hue Melanocyte when the amount of blood flowing through the skin increases. An 2 Melanosomes Melanosomes move into process inflammatory response (see section 4.8, “Tissue Damage and Inflammation”) stimulated by infection, sunburn, allergic reactions, Nucleus insect bites, or other causes can produce erythema. Exposure to the Golgi apparatus 1 Melanocyte produces melanin cold and blushing or flushing when angry or hot can also produce Basement erythema. A decrease in blood flow, as occurs in shock, can make membrane the skin appear pale, and a decrease in the blood O2 content pro- duces cyanosis (sigh-ah-NOH-sis), a bluish skin color (see Clinical Impact 5.2). PROCESS Figure 5.3 Carotene (KAIR-oh-teen) is a yellow pigment found in plants, such as carrots and corn. Humans normally ingest carotene Melanin Transfer to Keratinocytes and use it as a source of vitamin A. Carotene is lipid-soluble and, Melanocytes make melanin, which is packaged into melanosomes and transferred to many keratinocytes. Melanin protects a cell’s DNA from UV when large amounts of carotene are consumed, the excess accu- damage. mulates in the stratum corneum and in adipocytes of the dermis and subcutaneous tissue, causing the skin to develop a yellowish Based on this information, around which cellular tint. The yellowish tint slowly disappears once carotene intake is Apply structures would you expect the melanosomes to congregate? reduced. ISTUDY van03883_ch05_158-181.indd 164 16/11/21 12:07 PM  CHAPTER 5 Integumentary System 165 Clinical Skin Cancer IMPACT 5.1 S kin cancer is the most common type of carcinoma, and (3) melanoma (figure 5.4). Early detection and treatment of mela- cancer. Most skin cancers result from Basal cell carcinoma, the most common type, noma before it metastasizes can prevent death. damage caused by the ultraviolet (UV) affects cells in the stratum basale. Basal cell Melanoma can be detected by routine examina- ­radiation in sunlight. Some skin cancers are carcinomas have a varied appearance. Some tion of the skin and application of the ABCDE induced by chemicals, x-rays, depression of are open sores that bleed, ooze, or crust for rule, which states the signs of melanoma: the immune system, or inflammation, whereas several weeks. Others are reddish patches; A stands for asymmetry (one side of the others are inherited. shiny, pearly, or translucent bumps; or scarlike lesion does not match the other side), UV radiation damages the genes (DNA) areas of shiny, taut skin. Removal or destruc- B is for border irregularity (the edges are in epidermal cells, producing mutations. If tion of the tumor cures most cases. ragged, notched, or blurred), a mutation is not repaired, the mutation is Squamous cell carcinoma is the second C is for color (pigmentation is not uniform), passed to one of the two daughter cells when a most common type of skin cancer. Squamous D is for diameter (greater than 6 mm), and cell divides by mitosis. If the mutations affect cell carcinoma affects cells in the stratum E is for evolving (lesion changes over genes that regulate the cell cycle, uncontrolled spinosum and can appear as a wartlike growth; time). Evolving lesions change size, shape, cell division and skin cancer can result. a persistent, scaly red patch; an open sore; or elevation, or color; they may bleed, crust, The amount of protective melanin in the an elevated growth with a central depression. itch, or become tender. skin affects the likelihood of developing skin These lesions may bleed. ­Removal or destruc- cancer. Fair-skinned individuals, who have less tion of the tumor cures most cases. Limiting exposure to the sun and using melanin, are at an increased risk of developing Melanoma (MEL-ah-NOH-mah) is the sunscreens can reduce everyone’s likelihood of skin cancer compared with dark-skinned indi- least common, but most deadly, type of skin developing skin cancer, especially those who viduals, who have more melanin. Long-term or cancer, accounting for over 77% of the skin have a genetic susceptibility. Two types of UV intense exposure to UV radiation also increases cancer deaths in the United States. Because radiation play a role. Ultraviolet-B (UVB; 290– the risk. Thus, individuals who are older than they arise from melanocytes, most melano- 320 nm) radiation is the most potent for causing 50, who have engaged in repeated recreational mas are black or brown, but occasionally sunburn; it is also the main cause of basal and or occupational exposure to the sun, or who a melanoma stops producing melanin and squamous cell carcinomas and a significant have experienced sunburn are at increased risk. appears skin-colored, pink, red, or purple. cause of melanoma. Ultraviolet-A (UVA; 320– Most skin cancers develop on the parts of the About 40% of melanomas develop in preex- 400 nm) also contributes to skin cancer devel- body that are frequently exposed to sunlight, isting moles. Treatment of melanomas when opment, especially melanoma. It, too, penetrates such as the face, neck, ears, and dorsum of the they are confined to the epidermis is almost the dermis, causing wrinkling and leathering of forearm and hand. A physician should be con- always successful. However, if a melanoma the skin. The Skin Cancer Foundation recom- sulted if skin cancer is suspected. invades the dermis and metastasizes (spreads) mends using a broad-spectrum sunscreen that There are three types of skin cancer: to other parts of the body, it is difficult to protects against both UVB and UVA, with a sun (1) basal cell carcinoma, (2) squamous cell treat and can be deadly. protection factor (SPF) of at least 15. (a) Basal cell carcinoma (b) Squamous cell carcinoma (c) Melanoma FIGURE 5.4 Cancer of the Skin There are three types of skin cancer: (a) basal cell carcinoma, (b) squamous cell carcinoma, (c) melanoma. (a) Dr. P. Marazzi/Science Photo Library/Getty Images; (b) Dr. P. Marazzi/Science Source; (c) National Cancer Institute (NCI) The location of pigments and other substances in the skin dust particles in the air. The deeper within the dermis or subcuta- affects the color produced. For example, light reflected off dark neous tissue any dark pigment is located, the bluer the pigment pigment in the dermis or subcutaneous tissue can be scattered by appears because of the light-scattering effect of the overlying tis- collagen fibers of the dermis to produce a blue color. The same sue. This effect causes the blue color of tattoos, bruises, and some effect produces the blue color of the sky as light is reflected from superficial blood vessels. ISTUDY van03883_ch05_158-181.indd 165 16/11/21 12:08 PM  166 PART 2 Support and Movement ASSESS YOUR PROGRESS varied in structure and function. They include (1) free nerve end- ings for pain, itch, tickle, and temperature sensations; (2) hair fol- 2. From deepest to most superficial, name and describe the five strata of the epidermis. In which stratum are new cells licle receptors for light touch; (3) Pacinian corpuscles for deep formed by mitosis? Which strata have live cells, and which pressure; (4) Meissner corpuscles for detecting ­ simultaneous strata have dead cells? ­stimulation at two points on the skin; and (5) Ruffini end organs for sensing continuous touch or pressure (see figure 5.5). 3. Describe the structural features resulting from keratinization that make the epidermis structurally strong and resistant to The dermis is composed of two tissue layers (see ­figure 5.5): water loss. the superficial papillary (PAP-i-lar-ee) layer and the deeper ­reticular (re-TIK-yoo-lar) layer. The papillary layer derives its 4. Compare the structure and location of thick and thin skin. name from projections, called dermal papillae (pah-PIL-ee), that Is hair in thick or thin skin? extend toward the epidermis. The papillary layer is loose connec- 5. Which cells of the epidermis produce melanin? What tive ­tissue with thin fibers that are somewhat loosely arranged. happens to the melanin once it is produced? The papillary layer also contains blood vessels that supply the 6. How do genetic factors, exposure to sunlight, and overlying epidermis with O2 and nutrients, remove waste prod- hormones determine the amount of melanin in the skin? ucts, and aid in regulating body temperature. 7. How do carotene, blood flow, blood O2 content, and The dermal papillae under the thick skin of the palms of the collagen affect skin color? hands and soles of the feet lie in parallel, curving ridges. These ridges shape the overlying epidermis into patterns called friction ridges. The impressions left on surfaces by these friction ridges Dermis form fingerprints and footprints. The friction ridges improve the The dermis is connective tissue. Recall from chapter 4 that con- grip of the hands and feet. Everyone has unique friction ridge pat- nective tissue consists of cells distributed widely in an extensive terns, even identical twins. extracellular matrix, which includes protein fibers. The cells of the The reticular layer, which is composed of dense irregular dermis include fibroblasts, a few adipo­cytes, and macrophages. connective tissue, is the main layer of the dermis. It is continu- Collagen is the main type of protein fiber of the extracellular ous with the subcutaneous tissue and forms a mat of irregularly matrix, but elastic and reticular fibers are also present. The dermis arranged fibers that are resistant to stretching in many directions. contains blood vessels that allow for nutrient and waste exchange The elastic and collagen fibers are oriented more in some direc- to the cells of the dermis and the living cells of the epidermis. The tions than in others and produce cleavage lines, or tension lines, dermis also contains nerve endings, hair follicles, smooth muscles, in the skin (figure 5.6). It is important for health professionals glands, and lymphatic vessels (figure 5.5). The nerve endings are to understand cleavage line ­directions because an incision made Hair Meissner corpuscles (touch) Epidermis Free nerve endings (respond to pain, tickle, temperature, and itch sensations) Papillary layer Dermis Hair follicle receptor (detects light touch and Reticular slight bending of the hair) layer Pacinian corpuscle Ruffini end organ (detects deep pressure) (detects continuous touch or pressure) FIGURE 5.5 Dermis The dermis is the deep layer of the skin, composed of two layers: the papillary layer and the reticular layer. Sensory receptors of the dermis include free nerve endings, hair follicle receptors, Pacinian corpuscles, Meissner corpuscles, and Ruffini end organs. ISTUDY van03883_ch05_158-181.indd 166 16/11/21 12:08 PM  CHAPTER 5 Integumentary System 167 pregnancy or on the skin of athletes who have quickly increased muscle size by intense weight training. ASSESS YOUR PROGRESS 8. Name and compare the two layers of the dermis. Which layer is responsible for most of the structural strength of the skin? 9. What are formed by the dermal papillae in thick skin? (b) What roles do they have? 10. What are cleavage lines, and how are they related to the healing of a cut? 5.3 Subcutaneous Tissue LEARNING OUTCOMES After reading this section, you should be able to A. Describe the structure of the subcutaneous tissue. B. Describe the functions of the subcutaneous tissue. Deep to the skin is the subcutaneous tissue. The subcutaneous tis- sue attaches the skin to underlying bone and muscle and supplies the skin with blood vessels and nerves (see figure 5.1). The sub- (c) cutaneous tissue consists of loose connective tissue with collagen and elastic fibers. The main types of cells within the subcutaneous tissue are fibroblasts, adipocytes, and macrophages. The subcuta- (a) neous tissue, which is not part of the skin, is sometimes called the hypodermis. FIGURE 5.6 Cleavage Lines Approximately half the body’s stored lipids are in the subcuta- (a) The orientation of collagen fibers produces cleavage lines, or tension neous tissue, where they function in insulation and padding and as lines, in the skin. The left half of the image illustrates cleavage lines of the posterior surface of the body. The right half of the image illustrates a source of energy. The subcutaneous tissue can be used to estimate cleavage lines of the anterior surface of the body. (b) An incision made total body fat by pinching the skin at selected locations and mea- across cleavage lines can gap, increasing the time needed for healing and suring the thickness of the skin fold and underlying subcutaneous resulting in increased scar tissue formation. (c) An incision made parallel to tissue. The thicker the fold, the greater the amount of total body fat. cleavage lines results in less gapping, faster healing, and less scar tissue. The amount of adipose tissue in the subcutaneous tissue varies with age, sex, and diet. Most infants have a chubby appearance parallel to the cleavage lines is less likely to gap than an incision ­because they have proportionately more adipose tissue than adults. made across them. The development of infections and the forma- Females have proportionately more adipose tissue than males, tion of scar tissue are reduced in wounds where the edges are especially over the thighs, buttocks, and breasts. This accounts for closer together. some of the differences in body shape between the sexes and is also If the skin is overstretched, the dermis may rupture and leave responsible for some of the differences in body shape between lines that are visible through the epidermis. These lines of scar tis- individuals of the same sex. sue, called stretch marks, can develop in the skin of individuals Injections are used to introduce certain substances, such who have experienced rapid growth. For example, stretch marks as medication and vaccines, into the body by puncturing the commonly form on the abdomen and breasts of a woman during skin. There are three types of injections (figure 5.7). (1) An FIGURE 5.7 Injection (a) An intradermal injection introduces sub- stances to the dermis. (b) A subcutaneous Epidermis injection introduces substances into the subcutaneous tissue. (c) An intramuscular Dermis injection introduces substances into a muscle Subcutaneous deep to the subcutaneous tissue. layer Muscle (a) Intradermal (b) Subcutaneous (c) Intramuscular ISTUDY van03883_ch05_158-181.indd 167 16/11/21 12:08 PM  168 PART 2 Support and Movement intradermal injection (figure 5.7a), as is used for the tubercu- Hair lin skin test, goes into the dermis. It is administered by drawing The presence of hair is one of the characteristics of all mammals; the skin taut and inserting a small needle at a shallow angle if the hair is dense and covers most of the body surface, it is called into the skin. (2) A subcutaneous injection (figure 5.7b) extends fur. In humans, hair is found everywhere on the skin except the into the subcutaneous tissue; an insulin injection is one example. palms, the soles, the lips, the nipples, parts of the external genitalia, A subcutaneous injection is achieved by pinching the skin to form and the distal segments of the fingers and toes. a “tent,” into which a short needle is inserted. (3) An intramus- Hair structure and coloration change as a person ages cular injection (figure 5.7c) reaches a muscle deep to the subcu- (figure 5.8). By the fifth or sixth month of fetal development, taneous tissue. It is accomplished by inserting a long needle at a delicate, unpigmented hair called lanugo (la-NYU-go) has 90-degree angle to the skin. Intramuscular injections are used for developed and covered the fetus. Near the time of birth, long, injecting most vaccines and certain antibiotics. coarse, and pigmented hairs called terminal hairs replace the lanugo of the scalp, eyelids, and eyebrows. Vellus (VEL-us) ASSESS YOUR PROGRESS hairs, which are short, fine, and usually unpigmented, replace 11. Name the types of tissue forming the subcutaneous tissue the lanugo on the rest of the body. At puberty, terminal hair, layer. especially in the pubic and axillary regions, replaces much of 12. How is the subcutaneous tissue related to the skin? the vellus hair. The hair of the chest, legs, and arms is approxi- mately 90% terminal hair in males and approximately 35% in 13. List the functions of the adipose tissue within the subcutaneous tissue. females. In males, terminal hairs replace the vellus hairs of the face to form the beard. These changes in hair type at puberty are universal and have biological significance. The beard, pubic, 5.4 Accessory Skin Structures and axillary hair are visible signs of sexual maturity. In addi- tion, pubic and axillary hair may function as wicks for dispers- ing odors produced by secretions from specialized glands in the LEARNING OUTCOMES pubic and axillary regions acting as olfactory signals to others. It also has been suggested that pubic hair protects against abra- After reading this section, you should be able to sion during intercourse and axillary hair reduces friction when A. Describe the structure of a hair. the arms move against the body. B. Discuss the phases of hair growth. Hair Structure C. Explain the function of the arrector pili muscle. A hair is divided into the shaft, which protrudes above the sur- D. List the glands of the skin and describe the secretions face of the skin, and the root, located below the surface (figure they produce. 5.9a). The base of the root is expanded to form the hair bulb. E. Describe the parts of a nail. Most of the root and the shaft are composed of columns of dead, F. Explain how nails grow. keratinized epithelial cells arranged in three concentric layers: (1) the medulla, (2) the cortex, and (3) the cuticle (figure 5.9c, d). The skin includes several types of structures that are embedded The medulla (meh-DULL-ah) is the central axis of the hair, and in or otherwise associated with the epidermis and dermis and are it consists of two or three layers of cells containing soft keratin. therefore referred to as accessory structures. The accessory skin The cortex surrounds the medulla and forms the bulk of the structures include the hair, glands, and nails. hair. The cells of the cortex contain hard keratin. The cortex is (a) Lanugo (b) Vellus hair (c) Terminal hair FIGURE 5.8 Types of Hair (a) An infant with lanugo (b) Vellus hair (c) Terminal hair of the scalp, eyebrows, eyelashes, and beard (a) Ivan Lonan/Shutterstock; (b) Bele Olmez/Getty Images; (c) Diego Cervo/Shutterstock ISTUDY van03883_ch05_158-181.indd 168 16/11/21 12:09 PM  CHAPTER 5 Integumentary System 169 Hair shaft (above skin surface) Medulla Cortex Hair Hair root (below skin Cuticle surface) Arrector pili (smooth muscle) Sebaceous gland Hair root Dermal root sheath External epithelial Hair root sheath follicle Hair bulb Internal epithelial (base of LM 200x root sheath hair root) Artery Matrix Vein Hair papilla Adipose (a) tissue (b) Medulla Cortex Cuticle Dermal root sheath External epithelial root sheath Matrix Internal epithelial (growth zone) root sheath Melanocyte Stratum basale Hair papilla Basement membrane (c) (d) Hair Hair follicle FIGURE 5.9 Hair Follicle (a) The hair follicle contains the hair and consists of a dermal and an epithelial root sheath. (b) Photomicrograph of hair follicle with associated sebaceous gland. (c) Enlargement of the hair follicle wall and hair bulb. (d) Cross section of a hair within a hair follicle. (b) Al Telser/McGraw Hill covered by the cuticle (KEW-tih-cul), a single layer of cells also internal parts (figure 5.9c, d). At the opening of the follicle, the containing hard keratin. The edges of the cuticle cells overlap external epithelial root sheath has all the strata found in thin like shingles on a roof. skin. Deeper in the hair follicle, the number of cells decreases The hair follicle is a tubelike invagination of the epidermis until, at the hair bulb, only the stratum basale is present. This that extends into the dermis. A hair develops and grows within arrangement of epidermal cells has important consequences for each hair follicle. A hair follicle consists of a dermal root skin repair. If the epidermis and the superficial part of the dermis sheath and an epithelial root sheath. The dermal root sheath is are damaged, the keratinocyte stem cells located in the stratum the portion of the dermis that surrounds the epithelial root basale of the undamaged part of the hair follicle can be a source sheath. The epithelial root sheath is divided into external and of new epithelium. ISTUDY van03883_ch05_158-181.indd 169 16/11/21 12:09 PM  170 PART 2 Support and Movement hair root. Eventually, hair growth stops; the hair follicle shortens Clinical and holds the hair in place. A resting period follows, after which a IMPACT 5.2 new cycle begins. With this new cycle, a new hair forms to replace the old hair, which falls out of the hair follicle. Thus, losing a hair normally means that the hair is being replaced. The length of each The Integumentary System stage depends on the hair—eyelashes grow for approximately 30 as a Diagnostic Aid days and rest for 105 days, whereas scalp hairs grow for 3 years T he integumentary system can be used in diagnosis because and rest for 1–2 years. At any given time, an estimated 90% of the it is easily observed and often reflects events occurring in scalp hairs are in the growing stage. It is normal for a person to other parts of the body. For example, cyanosis, a bluish lose approximately 100 scalp hairs per day. color to the skin that results from decreased blood O2 content, is Hair loss can be more extensive than the normal 100 hairs an indication of impaired cardiovascular or respiratory function. per day. In some cases, extensive hair loss may occur, but hair When red blood cells wear out, they are broken down, and the liver growth may return, such as occurs in patients receiving certain excretes part of their contents as bile pigments into the small intes- chemotherapies. Alopecia areata, often called spot baldness, tine. Jaundice (JAWN-dis), a yellowish skin color, occurs when involves varying degrees of hair loss and can affect all areas of the excess bile pigments accumulate in the blood. If a disease, such as body. The condition is most likely due to an autoimmune response, viral hepatitis, damages the liver, bile pigments are not excreted and where the defense cells of the body destroy normal, healthy cells. accumulate in the blood. Regrowth of hair occurs in many individuals, but some experience Rashes and lesions in the skin can be symptomatic of problems elsewhere in the body (see table 5.2). For example, scarlet fever permanent hair loss. results from a bacterial infection in the throat. The bacteria release The most common kind of permanent hair loss is “pattern bald- a toxin into the blood that causes the pink-red rash for which this ness.” Hair follicles shrink and revert to producing vellus hair, which disease was named. In allergic reactions (see chapter 22), histamine is very short, transparent, and for practical purposes invisible. Even- released into the tissues produces swelling and reddening. Develop- tually, hair production in these smaller follicles may completely ment of the skin rash called hives can indicate an allergy to foods or cease. Although baldness is more common and more pronounced drugs, such as penicillin. in certain males, it can also occur in females. Genetic factors and The condition of the skin, hair, and nails is affected by nutri- the hormone testosterone are involved in causing pattern baldness. tional status. Vitamin A deficiency causes the skin to produce excess The average rate of hair growth is approximately 0.3 mm per day, keratin and assume a characteristic sandpaper texture, whereas iron- although hairs grow at different rates, even in the same approximate deficiency anemia causes the nails to lose their normal contour and location. Cutting, shaving, or plucking hair does not alter the growth become flat or concave (spoon-shaped). Hair concentrates many substances, which can be detected by rate or the character of the hair, but hair can feel coarse and bristly laboratory analysis, and comparing a patient’s hair with “normal” shortly after shaving because the short hairs are less flexible. hair can be useful in certain diagnoses. For example, lead poisoning Maximum hair length is determined by the rate of hair growth and results in high levels of lead in the hair. Forensic analyses can also the length of the growing phase. For example, scalp hair can be conducted on hair samples to test for drug use and poisoning. become very long, but eyelashes stay short. However, hair analyses are not reliable for determining a person’s general health or nutritional status. Hair Color Hair color is the result of melanin production by melanocytes and the distribution of melanin to the cells of the hair. Similar to how mela- nocytes of the epidermis produce melanin and pass it to the keratino- The internal epithelial root sheath of the hair follicle has raised cytes of the skin, melanocytes within the hair bulb matrix (see edges that mesh closely with the raised edges of the hair cuticle, figure 5.9c) produce melanin and pass it to keratinocytes in the hair and this arrangement holds the hair in place. When a hair is pulled cortex and medulla. As with the skin, varying amounts and types of out, the internal epithelial root sheath usually comes out as well melanin cause different shades of hair color. Blonde hair has little and is plainly visible as whitish tissue around the root of the hair. black-brown melanin, whereas jet black hair has the most. Intermedi- The hair bulb is an expanded knob at the base of the hair root ate amounts of melanin account for different shades of brown. Red (figure 5.9a, c). Inside the hair bulb is a mass of undifferentiated hair is caused by varying amounts of a red type of melanin. Hair epithelial cells called the matrix. The matrix produces both the hair sometimes contains both black-brown and red melanin. Hair color is and the internal epithelial root sheath. The dermis of the skin pro­ controlled by several genes, and dark hair color is not necessarily jects into the hair bulb as a hair papilla. Within the hair papilla are dominant over light. With age, the amount of melanin in hair can blood vessels that provide nourishment to the cells of the matrix. decrease, causing hair color to fade or become white (i.e., no mela- nin). Gray hair is usually a mixture of unfaded, faded, and white hairs. Hair Growth Hair is produced in cycles that involve a growth stage and a rest- Muscles ing stage. During the growth stage, a hair increases in length as Associated with each hair follicle are smooth muscle cells called new matrix cells are produced, differentiate, become keratinized, the arrector pili (ah-REK-tor PIE-lee). The arrector pili extend and die. The hair grows longer as cells are added at the base of the from the dermal root sheath of the hair follicle to the papillary ISTUDY van03883_ch05_158-181.indd 170 16/11/21 12:09 PM  CHAPTER 5 Integumentary System 171 layer of the dermis (see figure 5.9a). Normally, the hair follicle and the hair inside it are at an oblique angle to the surface of the skin. When the arrector pili muscles contract, however, they pull the follicle into a more perpendicular position, causing the hair to “stand on end.” Movement of the hair follicles produces raised Sweat pores areas called “goose bumps.” This is a common response to cold Duct of eccrine temperatures as well as strong emotional responses. In other mam- sweat gland mals, arrector pili muscle contractions are important for warmth Duct of Arrector pili as well as communication of aggression or fear. apocrine (smooth muscle) sweat gland ASSESS YOUR PROGRESS Hair follicle Sebaceous gland 14. When and where are lanugo, vellus, and terminal hairs found in the skin? Eccrine 15. What are the regions of a hair? What type of cells make Hair bulb sweat (sudoriferous) up most of a hair? gland 16. Describe the three layers of a hair seen in cross section. 17. Describe the parts of a hair follicle. How is the epithelial Apocrine root sheath important in skin repair? sweat (sudoriferous) 18. In what part of a hair does growth take place? What are the gland stages of hair growth? Do all hairs grow at the same rate? 19. What determines the different shades of hair color? FIGURE 5.10 Glands of the Skin 20. Explain the location and action of arrector pili muscles. Sebaceous glands and apocrine sweat glands empty into a hair follicle. Eccrine sweat glands empty onto the surface of the skin. Glands The skin has several types of glands that secrete materials onto are normally only functional in lactating women. The major its surface; however, not all are functional in all individuals. glands of the skin are the sebaceous glands and the sweat glands For example, mammary glands are a type of skin gland that (figure 5.10). MICROBES Using Bacteria to Fight Bacteria In Your Body 5.1 A cne (acne vulgaris) is the most com- have identified three unique strains of C. acnes. “good” C. acnes or similar bacteria to their skin mon skin condition in the United Of the three strains, one is more dominant in to prevent the “bad” C. acnes from taking over; States. Though 80% of all American people with acne-free skin. Research has shown so-called skin probiotics. adolescents develop acne, adults can also be that this strain of C. acnes does not adversely affected by it. When considering all age groups, affect the host. However, the other two strains approximately 40 to 50 million ­ Americans of C. acnes are pathogenic to humans. So Apply Predict 2 suffer from acne. Unfortunately, there is not how does this information help scientists learn a tried-and-true cure for acne; however, new how to prevent acne? It seems that the “good” You just learned that acne-causing bacteria tend research examining the skin microbiome may C. acnes prevents invasion of the skin by certain to live in areas of the skin with many sebaceous have found a natural and effective treatment to bacteria through a natural metabolic process. glands. However, in section 5.4, it states that