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Chapter 6: Skin and Its Appendages

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INTRODUCTION

Skin (integument) is the body’s largest organ Approximately 1.6 to 1.9 m2 in average-sized

adult Integumentary system describes the skin

and its appendages: hair, nails, and skin glands

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STRUCTURE OF THE SKIN

Skin classified as a cutaneous membrane Two primary layers: epidermis and dermis;

joined by dermoepidermal junction (Figures 6-1 and 6-2)

Hypodermis lies beneath dermis Thin and thick skin (Figure 6-3)

Thin skin covers most of the body’s surface (1 to 3 mm thick)

Thick skin covers soles and palms (4 to 5 mm thick)

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STRUCTURE OF THE SKIN: EPIDERMIS Epidermis

Cell types (Figure 6-4) Keratinocytes: constitute more than 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 and filter ultraviolet (UV) light

Epidermal dendritic cells: branched antigen-presenting cells; 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 spinosum (spiny layer): cells arranged in eight to 10 layers with desmosomes that pull cells into spiny shapes; cells rich in RNA

Stratum germinativum (growth layer): another name for stratum basale or stratum spinosum and stratum basale together

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STRUCTURE OF THE SKIN: EPIDERMIS (cont.)

Stratum granulosum (granular layer): cells arranged in two to four layers and filled with keratohyalin granules; contains 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; thin squamous cells, dead cells filled with keratin (barrier area – to environment and water loss)

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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—approximately 35 days

Epidermal growth factor regulates epidermal growth and repair

Shortened turnover time due to abrasion 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

STRUCTURE OF THE SKIN:

EPIDERMIS (cont.)

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STRUCTURE OF THE SKIN: EPIDERMIS (cont.) Dermoepidermal junction

A basement membrane with unique fibrous elements and a polysaccharide gel “glue” the epidermis to the dermis below

The junction is a partial barrier to the passage of some cells and large molecules

Separation of the epidermis from the DEJ result in blisters

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STRUCTURE OF THE SKIN: DERMIS 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 Contains various structures

Arrector pili muscles and hair follicles (Figure 6-5) Sensory receptors (Figure 6-6) 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 and 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: patterns formed by the collagenous fibers of the reticular layer of the dermis; also called Langer’s lines (Figure 6-7)

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STRUCTURE OF THE SKIN: HYPODERMIS 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 Rich blood supply makes it a perfect site for fast a

relatively pain free absorption of injected material – insulin

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SKIN COLOR

Melanin Basic determinant is quantity, type, and distribution of

melanin Types of melanin

Eumelanin: group of dark brown, almost black, melanins Pheomelanin: group of reddish and orange melanins

Melanin formed from tyrosine by melanocytes (Figure 6-8) Melanocytes release melanin in packets called melanosomes Melanosomes are ingested by surrounding keratinocytes and

form a cap over the nucleus

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SKIN COLOR (cont.)

Albinism: congenital absence of melanin Expression of melanin genes regulated by tyrosinase,

exposure to sunlight (UV radiation), and certain hormones, including melanocortins (adrenocorticotropic hormone, alpha melanocyte stimulating hormone) and endothelin-1 (Figures 6-9 and 6-10)

Cumulative effects of UV ray exposure may produce age spots (Figure 6-11)

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SKIN COLOR: OTHER PIGMENTS Other pigments

Beta-carotene (group of yellowish pigments from food) can also contribute to skin color

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 6-12) Bruising can cause a rainbow of different colors to appear in

the skin (Figure 6-13) Other pigments: from cosmetics, tattoos, and bile pigments

in jaundice (Box 6-4)

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FUNCTIONS OF THE SKIN

Protection (Table 6-2) Physical barrier to microorganisms Barrier to chemical hazards Reduces potential for mechanical trauma Prevents dehydration Protects from excess UV ray exposure (melanin function)

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FUNCTIONS OF THE SKIN (cont.) 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 (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

and uric acid

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FUNCTIONS OF THE SKIN (cont.) Sensation

Skin acts as a sophisticated sense organ Somatic sensory receptors detect stimuli that

detection of pressure, touch, temperature, pain, and other general senses

Flexibility Skin is supple and elastic, thus permitting change

in body contours without injury

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FUNCTIONS OF THE SKIN (cont.) Excretion

Water Urea/ammonia/uric acid

Hormone (vitamin D) production (Figure 6-14) 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 (cont.) 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

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 (cont.)

Homeostasis of body temperature 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 6-15) 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 heat can be lost from the skin

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 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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FUNCTIONS OF THE SKIN (cont.)

Homeostatic regulation of heat loss (Figure 6-16) 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 6-17) Development of hair

Distribution is over entire body except palms of hands and soles of feet and a few other small areas

Fine and soft hair coat present before birth called lanugo Coarse pubic and axillary hair that develops at puberty called

terminal hair 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

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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 6-18) Growth: 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 6-19)

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APPENDAGES OF THE SKIN (cont.) Nails (Figure 6-20)

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 Nail bed: layer of epithelium under nail body; contains abundant

blood vessels Appears pink under translucent nails Nails may have pigmented streaks (Figure 6-21) Separation of a nail from the nail bed is called onycholysis

(Figure 6-22) Growth: nails grow by mitosis of cells in stratum basale beneath

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

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APPENDAGES OF THE SKIN (cont.) Skin glands (Figure 6-23)

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 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 cycle Odor is from bacteria feeding off gland production

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APPENDAGES OF THE SKIN: SKIN GLANDS (cont.)

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 From hair follicles Found in dermis except in palms and soles Secretion increases in adolescence; may lead to formation of

pimples and blackheads Ceruminous glands

Modified apocrine sweat 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

Adults Development and activation of sebaceous and sweat glands Increased sweat production

Body odor Increased sebum production

Acne Old age

Decreased sebaceous and sweat gland activity Wrinkling (Figure 6-24) 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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Severity of Burns First-degree burns

Only epidermis is damaged Skin is red and swollen

Second-degree burns AKA partial thickness burns Epidermis and upper dermis are damaged Skin is red with blisters

Third-degree burns Destroys entire skin layer Burn is gray-white or black

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