Chapter 7: Skin and Its Appendages

1. Chapter 7: Skin and Its Appendages Anatomy & Physiology

2. Introduction • Skin (integument) is body’s largest organ • Skin measures approximately 1.6 to 1.8 m2 in average-sized adult • Integumentary system describes the skin and its appendages—the hair, nails, and skin glands 2

3. Structure of the Skin • Skin classified as cutaneous membrane • Two primary layers—epidermis and dermis; joined by dermoepidermal junction (Figures 7-1 and 7-2) • Hypodermis lies beneath dermis • Thin and thick skin (Figure 7-3) • “Thin skin” —covers most of body surface (1 to 3 mm thick); has hair and smooth surface • “Thick skin”—soles and palms (4 to 5 mm thick); ridged surface with no hair 3

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7. Structure of the Skin • Epidermis • Cell types (Figure 7-4) • Keratinocytes—constitute over 90% of cells present; principal structural element of the outer skin; sometimes called corneocytes after they are fully keratinized • Melanocytes—pigment-producing cells (5% of the total); contribute to skin color; filter ultraviolet light • Epidermal dendritic cells—branched antigen-presenting cells (APCs); they play a role in immune response; also called Langerhans cells • Tactile epithelial cells (Merkel cells)—attach to sensory nerve endings to form “light touch” receptors 7

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9. Structure of the Skin • Epidermis (cont) • Cell layers • Stratum basale (base layer)—single layer of columnar cells; only these cells undergo mitosis and then migrate through the other layers until they are shed; stratumgerminativum (growth layer) is another name for stratum basale (or stratum spinosum and stratum basale together) • Stratum spinosum (spiny layer)—cells arranged in 8 to 10 layers with desmosomes that pull cells into spiny shapes; cells rich in RNA 9

10. Structure of the Skin • Cell layers • Stratum granulosum (granular layer)—cells arranged in two to four layers and filled with keratohyalin granules; contain high levels of lysosomal enzymes • Stratum lucidum (clear layer)—cells filled with keratin precursor called eleidin; absent in thin skin • Stratum corneum (horny layer)—most superficial layer; dead cells filled with keratin (barrier area) 10

11. Structure of the Skin • Epidermis (cont) • Epidermal growth and repair • Turnover or regeneration time refers to time required for epidermal cells to form in the stratum basale and migrate to the skin surface—about 35 days • Several hormones support normal growth and repair of the epidermis: epidermal growth factor (EGF), insulin-like growth factor 1 (EGF-1), and growth hormone (GH) 11

12. Structure of the Skin • Epidermal growth and repair (cont) • Shortened turnover time will increase the thickness of the stratum corneum and result in callus formation • Normally 10% to 12% of all cells in stratum basale enter mitosis daily • Each group of 8 to 10 basal cells in mitosis with their vertical columns of migrating keratinocytes is called an epidermal proliferating unit, or EPU 12

13. Structure of the Skin • Dermopidermal junction (DEJ) • A basement membrane, with unique fibrous elements, and a polysaccharide gel serve to “glue” the epidermis to the dermis below • The junction serves as a partial barrier to the passage of some cells and large molecules 13

14. Structure of the Skin • Dermis • Sometimes called “true skin”—much thicker than the epidermis and lies beneath it • Gives strength to the skin • Serves as a reservoir storage area for water and electrolytes 14

15. Structure of the Skin • Dermis (cont) • Contains various structures • Arrector pili muscles and hair follicles (Figure 7-5) • Sensory receptors (Figure 7-6) • Sweat and sebaceous glands • Blood vessels • Rich vascular supply plays a critical role in temperature regulation 15

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18. Structure of the Skin • Dermis (cont) • Layers of dermis • Papillary layer—composed of dermal papillae that project into the epidermis; contains fine collagenous and elastic fibers; contains the dermoepidermal junction; forms a unique pattern that gives individual fingerprints • Reticular layer—contains dense, interlacing white collagenous fibers and elastic fibers to make the skin tough yet stretchable; when processed from animal skin, produces leather 18

19. Structure of the Skin • Dermis (cont) • Dermal growth and repair • The dermis does not continually shed and regenerate itself as does the epidermis • During wound healing, fibroblasts begin forming an unusually dense mass of new connective fibers; if not replaced by normal tissue, this mass remains a scar • Cleavage lines (Figure 7-7)—patterns formed by the collagenous fibers of the reticular layer of the dermis; also called Langer’s lines 19

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21. Structure of the Skin • Hypodermis • Also called the subcutaneous layer or superficial fascia • Located deep to the dermis; forms connection between skin and other structures • Not part of the skin 21

22. Skin Color • Melanin • Basic determinant is quantity, type, distribution of melanin • Types of melanin • Eumelanin—group of dark brown (almost black) melanins • Pheomelanin—group of reddish and orange melanins 22

23. Skin Color • Melanin (cont) • Melanin formed from tyrosine by melanocytes (Figure 7-8) • Melanocytes release melanin in packets called melanosomes • Melanosomes are ingested by surrounding keratinocytes and form a cap over the nucleus • Albinism—congenital absence of melanin 23

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25. Skin Color • Melanin (cont) • Process regulated by tyrosinase, exposure to sunlight (UV radiation), and certain hormones, including melanocortins (ACTH, a-MSH) and ET-1 (Figures 7-9 and 7-10) • Cumulative effects of UV exposure may produce age spots (Figure 7-11) 25

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29. Skin Color • Other Pigments • Beta-carotene (group of yellowish pigments from food) can also contribute to skin color • Lipofuscin—accumulates in cells that have ceased mitosis in aging skin, producing brown-yellow age spots 29

30. Skin Color • Other Pigments (cont) • Hemoglobin—color changes also occur as a result of changes in blood flow • Redder skin color when blood flow to skin increases • Cyanosis—bluish color caused by darkening of hemoglobin when it loses oxygen and gains carbon dioxide (Figure 7-12) • Bruising can cause a rainbow of different colors to appear in the skin (Figure 7-13) • Other pigments—from cosmetics, tattoos, bile pigments in jaundice (Box 7-4) 30

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33. Functions of the Skin (Table 7-2) • Protection • Physical barrier to microorganisms • Barrier to chemical hazards • Reduces potential for mechanical trauma • Prevents dehydration • Protects against excess UV exposure (melanin function) 33

34. Functions of the Skin • Surface film • Emulsified protective barrier formed by mixing of residue and secretions of sweat and sebaceous glands with sloughed epithelial cells from skin surface; shedding of epithelial elements is called desquamation • Functions • Antibacterial, antifungal activity • Lubrication • Hydration of skin surface • Buffer of caustic irritants • Blockade of toxic agents 34

35. Functions of the Skin • Surface film (cont) • Chemical composition • From epithelial elements—amino acids, sterols, and complex phospholipids • From sebum—fatty acids, triglycerides, and waxes • From sweat—water, ammonia, urea, and lactic acid and uric acid 35

36. Functions of the Skin • Sensation • Skin acts as a sophisticated sense organ • Somatic sensory receptors detect stimuli that permit us to detect pressure, touch, temperature, pain, and other general senses • Flexibility • Skin is supple and elastic, thus permitting change in body contours without injury • Excretion • Water • Urea/ammonia/uric acid 36

37. Functions of the Skin • Hormone (vitamin D) production (Figure 7-14) • Exposure of skin to UV light converts 7-dehydrocholesterol to cholecalciferol—a precursor to vitamin D • Blood transports precursor to liver and kidneys where vitamin D is produced • Process and end result fulfill the necessary steps required for vitamin D to be classified as a hormone 37

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39. Functions of the Skin • Immunity • Phagocytic cells destroy bacteria • Epidermal dendritic cells trigger helpful immune reaction working with “helper T cells” • Homeostasis of body temperature • To maintain homeostasis of body temperature, heat production must equal heat loss; skin plays a critical role in this process 39

40. Functions of the Skin • Homeostasis of body temperature (cont) • Heat production • By metabolism of foods in skeletal muscles and liver • Chief determinant of heat production is the amount of muscular work being performed 40

41. Functions of the Skin • Homeostasis of body temperature (cont) • Heat loss—approximately 80% of heat loss occurs through the skin; remaining 20% occurs through the mucosa of the respiratory, digestive, and urinary tracts (Figure 7-15) • Evaporation—to evaporate any fluid, heat energy must be expended; this method of heat loss is especially important at high environmental temperatures when it is the only method by which heat can be lost from the skin 41

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43. Functions of the Skin • Homeostasis of body temperature (cont) • Heat loss (cont) • Radiation—transfer of heat from one object to another without actual contact; important method of heat loss in cool environmental temperatures • Conduction—transfer of heat to any substance actually in contact with the body; accounts for relatively small amounts of heat loss • Convection—transfer of heat away from a surface by movement of air; usually accounts for a small amount of heat loss 43

44. Functions of the Skin • Homeostasis of body temperature (cont) • Homeostatic regulation of heat loss (Figure 7-16) • Heat loss by the skin is controlled by a negative feedback loop • Receptors in the hypothalamus monitor the body’s internal temperature • If body temperature is increased, the hypothalamus sends a nervous signal to the sweat glands and blood vessels of the skin • The hypothalamus continues to act until the body’s temperature returns to normal 44

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46. Appendages of the Skin • Hair (Figure 7-17) • Development of hair • Distribution—over entire body except palms of hands and soles of feet and a few other small areas • Fine and soft hair coat existing before birth called lanugo • Coarse pubic and axillary hair that develops at puberty called terminal hair • Hair follicles and hair develop from epidermis; mitosis of cells of germinal matrix forms hairs 46

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48. Appendages of the Skin • Hair (cont) • Development of hair (cont) • Papilla—cluster of capillaries under germinal matrix • Root—part of hair embedded in follicle in dermis • Shaft—visible part of hair • Medulla—inner core of hair; cortex—outer portion 48

49. Appendages of the Skin • Hair (cont) • Appearance of hair • Color—result of different amounts, distribution, types of melanin in cortex of hair (Figure 7-18) • Growth—hair growth and rest periods alternate; hair on head averages 5 inches of growth per year • Sebaceous glands—attach to and secrete sebum (skin oil) into follicle • Male pattern baldness (androgenic alopecia) results from combination of genetic tendency and male sex hormones (Figure 7-19) 49

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