1 mosby items and derived items © 2010, 2007, 2003 by mosby, inc., an affiliate of elsevier inc....

1Mosby items and derived items © 2010, 2007, 2003 by Mosby, Inc., an affiliate of Elsevier Inc.

Chapter 6: Skin and Its Appendages


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


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)


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


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



• 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; dead cells filled with keratin (barrier area)



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


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


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

Dermal growth and repair• The dermis does not continually shed and

regenerate itself as does the epidermis


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


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


SKIN COLOR (cont.)

Albinism: congenital absence of melanin Process 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)


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)


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)


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


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• Hair follicles and hair develop from epidermis; mitosis of

cells of germinal matrix forms hairs• Root: part of hair embedded in follicle in dermis• Shaft: visible part of hair• Medulla: inner core of hair• Cortex: outer portion


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)


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


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


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• Simple, branched glands• 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• 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


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

1 mosby items and derived items © 2010, 2007, 2003 by mosby, inc., an affiliate of elsevier inc....

Documents