chapter 7: skin and its appendages

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Chapter 7: Skin and Its Appendages

Anatomy & Physiology


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

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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

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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

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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; stratum germinativum (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

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Structure of the Skin

Cell layersStratum 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)

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Structure of the Skin Epidermis (cont)

Epidermal growth and repairTurnover 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)

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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)

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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

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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)

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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)

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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 hairMedulla—inner core of hair; cortex—outer portion

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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)

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Nails (Figure 7-20) Consist of epidermal cells converted

to hard keratin Nail body—visible part of each nail Root—part of nail in groove hidden by

fold of skin, the cuticle Lunula—moon-shaped white area

nearest root

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Nails (cont) Nail bed—layer of epithelium under nail body;

contains abundant blood vessels Appears pink under translucent nails Nails may have pigmented streaks (Figure 7-21) Separation of a nail from the nail bed is called

onycholysis (Figure 7-22) Growth—nails grow by mitosis of cells in statum

basale beneath the lunula; average growth about 0.5 mm per week, or slightly over 1 inch per year

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Appendages of the Skin Skin glands (Figure 7-23)

Two types of sweat glands Eccrine glands

Most numerous sweat glands; quite small

Distributed over total body surface with exception of a few small areas

Simple, coiled, tubular glands Function throughout life Secrete perspiration, or sweat; eliminate

wastes and help maintain a constant core temperature

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Two types of sweat glands (cont) Apocrine glands

Located deep in subcutaneous layer Limited distribution—axilla, areola of

breast, and around anus Large (often more than 5 mm in

diameter) Simple, branched, tubular glands Begin to function at puberty Secretion shows cyclic changes in

female with menstrual cycle59



Skin glands (cont) Sebaceous glands

Secrete sebum—oily substance that keeps hair and skin soft and pliant; prevents excessive water loss from skin

Lipid components have antifungal activity Simple, branched glands Found in dermis except in palms and

soles Secretion increases in adolescence; may

lead to formation of pimples and blackheads

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Appendages of the Skin Skin glands (cont)

Ceruminous glands Modified apocrine sweat glands Simple, coiled, tubular glands Empty contents into external ear canal

alone or with sebaceous glands Mixed secretions of sebaceous and

ceruminous glands called cerumen (wax) Function of cerumen to protect area from

dehydration; excess secretion can cause blockage of ear canal and loss of hearing

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Cycle of Life: Skin

Children Skin is smooth, unwrinkled, and

characterized by elasticity and flexibility

Few sweat glands Rapid healing

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Cycle of Life: Skin

Adults Development and activation of

sebaceous and sweat glands Increased sweat production; can

result in body odor Increased sebum production; can

result in acne

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Cycle of Life: Skin

Old age Decreased sebaceous and sweat

gland activity Wrinkling (Figure 7-24) Decrease in body's ability to

cool itself

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The Big Picture: Skin and the Whole Body

Skin is a major component of the body’s structural framework

Skin defines the internal environment of the body

Primary functions are support and protection

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chapter 7: skin and its appendages

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