6. Integumentary System

INTRODUCTION TO THE INTEGUMENTARY SYSTEM

The skin and the glands, hair, nails, and other structures that are derived from it make up the integumentary (in-teg- yoo-MEN-tar-ee) system. Because it is on the outside of the body, this organ system is our contact with the external environment and is subjected to continual abuse from the environment. However, the skin is resilient and versatile. Generally, it quickly repairs itself and continues to perform its many functions year after year.

STRUCTURE OF THE SKIN

The skin (sometimes called the cutaneous [kyoo-TAY-nee- us] membrane) consists of two distinct layers of tissues. The outer layer is the epidermis, and the inner layer is the dermis. These are anchored to underlying structures by a third layer, the hypodermis or subcutaneous tissue.

The structure of the skin is illustrated in Figure 6-1.

Epidermis

The outer layer of the skin is the epidermis. This layer con­sists of stratified squamous epithelium (see Figure 6-1). There are no blood vessels present in the epidermis, and the cells receive their nutrients by diffusion from vessels in the underlying tissue. The bottom row of cells in the epidermis is called the stratum basale. It consists of actively dividing (mitotic) columnar cells and melanocytes. This is the layer next to the basement membrane and closest to the blood supply. As older cells are pushed upward toward the surface by the growing cells next to the basement membrane, they receive fewer nutrients. They also undergo a process called keratinization (ker-ah-tin-ih-ZAY-shun). During keratini- zation, a protein called keratin is deposited in the cell. This causes the chemical composition of the cell to change, and the cell changes shape. By the time the cells reach the surface, they are flat or squamous. They are also dead from lack of nutrients and are sloughed off. They are replaced by other cells that are pushed upward from the stratum basale. About one fourth of the cells in the stratum basale are melanocytes (meh-LAN-oh-sytes). Melanocytes are special­ized epithelial cells that produce a dark pigment called melanin (MEL-ah-nin), which is primarily responsible for skin color.

The outermost or surface region of the epidermis is the stratum corneum (KOR-nee-um). It makes up about three fourths of the epidermal thickness and consists of 20 to 30 layers of flattened, dead, completely keratinized cells. The cells in the stratum corneum are continually shed and replaced. About 5 weeks after a cell has been produced in the stratum basale, it is sloughed off the surface of the stratum corneum. The keratin that is present is a tough, water-repellent protein, and its inclusion in the stratum corneum provides protection against water loss from the body.

Dermis

The dermis, or stratum corium (KOR-ee-um), is dense con­nective tissue that is deeper and usually thicker than the epidermis (see Figure 6-1). Hair, nails, and certain glands (although derived from the stratum basale of the epidermis) are embedded in the dermis. The dermis contains both col­lagenous and elastic fibers to give it strength and elasticity. If the skin is overstretched, the dermis may be damaged, leaving white scars called striae (STRY-ee), commonly called “stretch marks.” Fibers also form a framework for the numerous blood vessels and nerves that are present in the dermis but generally absent in the epidermis. Many of the nerves in the dermis have specialized endings called sensory receptors that detect changes in the environment, such as heat, cold, pain, pressure, and touch. Because there are no nerves in the epidermis, these receptors are the body’s contact with the environment.

The upper region of the dermis has numerous papillae, or projections, that extend into the epidermis. Blood vessels, nerve endings, and sensory receptors extend

Highlight on the Integumentary System

Skin: For an “average” person, the skin weighs about 5 kg (11 lb), has a surface area of approximately 2 m[1] (21 ft2), and varies in thickness from 0.05 to 0.4 cm (0.02 to 0.16 inch).

Neoplasms: Cancerous neoplasms composed of melanocytes, called malignant melanomas, account for 3% of all cancers, and the incidence is rising at a rate of 4.5% annually. Exposure to sunlight is the major risk factor for the development of malignant melanoma, and individuals with fair skin and light hair are at greatest risk. Melanomas often metastasize to the lung, liver, and brain.

Dermis: The dermis is the portion of an animal’s skin that is used to make leather because the collagen in the dermis becomes tough when treated with tannic acid.

Blister: A blister is a fluid-filled pocket between the dermis and the epidermis. When the skin is burned or irritated, some plasma escapes from the blood vessels in the dermis and accumulates between the two layers, where it forms the blister.

Dermal blood vessels: In people with light skin, when dermal blood vessels dilate and blood flow increases (e.g., during blushing and increased temperature), the skin may be quite red. If the vessels constrict and blood flow decreases, the individual is pale or “white as a sheet.”

Hair type: The shape of the hair shaft determines whether hair is straight or curly. If the shaft is round, the hair is straight. If it is oval, the hair is wavy. If it is flat, the hair is curly or kinky. To make their hair curly, individuals often get a “permanent,” which flattens the hair.

Acne: Acne is a problem that plagues many teenagers. Increased hormone activity at puberty causes an increase in sebaceous gland activity. Sebum and dead cells may block the hair follicle and form blackheads. Bacteria infect the blocked follicle, and the sebum-dead cell mixture accumulates until the follicle ruptures. This initiates an inflammatory response that soon appears on the surface as a pus-filled pimple.

Adipose tissue: People who lose weight rapidly may feel cold because they have reduced their adipose insulation. ■

into the papillae to bring them into closer proximity to the epidermis and the surface. On the palms, the fingertips, and the soles of the feet, the papillae form distinct patterns or ridges that provide friction for grasping objects. The pat­terns are genetically determined and are unique for each individual. These are the basis of fingerprints and footprints.

Subcutaneous Layer

The subcutaneous layer (see Figure 6-1) is not actually a part of the skin, but it loosely anchors the skin to underly­ing organs. Because it is beneath the dermis, it is sometimes called the hypodermis. It is also referred to as superficial fascia. The subcutaneous layer consists largely of loose con­nective tissue and adipose tissue. The fibers in the loose connective tissue are continuous with those in the dermis, and as a result there is no distinct boundary between the dermis and the subcutaneous tissue.

The adipose tissue in the subcutaneous layer cushions the underlying organs from mechanical shock and acts as a heat insulator in temperature regulation. Fat in the adipose tissue can be mobilized and used for energy when necessary. The distribution of subcutaneous adipose tissue is largely responsible for the differences in body contours between men and women.

SKIN COLOR the melanocytes, however, is genetically controlled. Although many genes are responsible for skin color, a single mutation can result in an inability to produce melanin. This results in a condition called albinism (AL-bih-nizm) in which individuals have light skin, white hair, and unpig­mented irises in the eyes.

Some people have the yellowish pigment carotene (KAIR- oh-teen) in addition to melanin. This gives a yellow tint to the skin. A pinkish tint in the skin is attributable to the blood vessels in the dermis. Ultraviolet light increases mela­nocyte activity so that more melanin is produced and the skin becomes darker or tanned.

EPIDERMAL DERIVATIVES

Accessory structures of the skin include hair, nails, sweat glands, and sebaceous glands. They are derived from the stratum basale of the epidermis and are embedded in the dermis. Figure 6-1 illustrates some of the accessory struc­tures associated with the skin.

Hair and Hair Follicles

Hair is found on nearly all body surfaces, but it is absent on the palms of the hands and the soles of the feet. All hair has essentially the same structure. It consists of a shaft and a root that are composed of dead, keratinized epithelial cells. The root is enclosed in a hair follicle that extends through the epidermis and is embedded in the dermis.

The shaft of a hair is that portion that extends beyond the surface of the epidermis. It is the part that you can see. Because it contains no nerves, it can be cut with no sensa­tion of pain. The root is the portion of the hair that is below the surface of the skin. It is surrounded by a hair follicle.

The shaft and root are continuous and together make up the hair, which is produced by the hair follicle. The outer­most covering on a hair is a single layer of overlapping, keratinized cells called the cuticle. On the shaft of the hair, the cuticle is exposed to the environment and subjected to abrasion. It tends to wear away at the tip of the shaft. When this happens, the inner portion projects from the tip of the shaft, resulting in “split ends.”

The root of a hair is enclosed in a tubular hair follicle that is embedded in the dermis. Blood vessels in the dermis provide the blood supply for the epithelial cells of the hair follicle. Stratum basale cells, like those in the skin, provide the mitotic cells that divide and undergo keratinization to produce the hair.

Hair color is determined by the type of melanin pro­duced by the melanocytes in the stratum basale. Yellow, brown, and black pigments are present in varying propor­tions to produce different hair colors. With age, the mela­nocytes become less active. Hair in which melanin is replaced with air bubbles is white.

A bundle of smooth muscle cells, called the arrector pili muscle, is associated with each hair follicle. Most hair fol­licles are at a slight angle to the surface of the skin. The arrector pili muscles are attached to the hair follicles in such a way that contraction pulls the hair follicles into an upright position or causes the hair to “stand on end.” Contraction of the arrector pili muscles also causes raised areas on the skin, or “goose bumps.” Action of the arrector pili muscles is controlled by the nervous system in response to cold and fright.

Nails

Nails are thin plates of dead stratum corneum that contain a very hard type of keratin and cover the dorsal surfaces of the distal ends of the fingers and toes. Each nail has a free edge; a nail body, which is the visible portion; and a nail root, which is covered with skin. The eponychium (eh-poh- NICK-ee-um) or cuticle is a fold of stratum corneum that grows onto the proximal portion of the nail body. Stratum basale from the epidermis grows under the nail body and is responsible for nail growth. The portion of the body over the growth area appears as a whitish, crescent-shaped area called the lunula (LOO-nyoo-lah). Nails appear pink because of the rich supply of blood vessels in the underlying dermis.

Glands

The two major glands associated with the skin are the sebaceous glands and the sweat glands. A third type, the ceruminous glands, are modified sweat glands.

Sebaceous Glands

Generally, sebaceous glands are associated with hair follicles and are found in all areas of the body that have hair (see Figure 6-1). Those not associated with hair follicles open directly onto the surface of the skin. The oily secretion, called sebum, is transported by a duct into a hair follicle, and from there it reaches the surface of the skin. Sebum functions to keep hair and skin soft and pliable. It also inhibits growth of bacteria on the skin and helps to prevent water loss. Secretory activity of the sebaceous glands is stimulated by sex hormones; consequently, the glands are relatively inactive in childhood, become highly active during puberty, and decrease in activity during old age. Decreased sebum, in part, accounts for the dry skin and brittle hair that are common in older people.

Sweat (Sudoriferous) Glands

Sweat glands (also called sudoriferous glands) are widely distributed over the body. They are most numerous in the palms and soles. The glandular portion of a sweat gland is a coiled tube that is embedded in the dermis of the skin, and the duct opens onto the surface of the skin through a sweat pore (see Figure 6-1). The secretion of these glands is primarily water with a few salts. When the body’s tempera­ture increases, the glands are stimulated to produce sweat, which evaporates and has a cooling effect. Sweat, or perspi­ration, is also produced in response to nerve stimulation as a result of emotional stress.

Ceruminous Glands

Ceruminous glands are modified sweat glands that are found in the external auditory (ear) canal. They secrete an oily, sticky substance called cerumen (see-ROOM-men), or earwax, that is thought to repel insects and trap foreign material.

FUNCTIONS OF THE SKIN

Protection

The skin forms a protective covering over the entire body. The keratin in the cells waterproofs the cells and helps prevent fluid loss from the body. This waterproofing also prevents too much water from entering the body during swimming and bathing. Unbroken skin forms the first line of defense against bacteria and other invading organisms. The oily secretions of the sebaceous glands are acidic and inhibit bacterial growth on the skin. Melanin pigment absorbs light and helps protect underlying tissues from the damaging effects of ultraviolet light. Skin also protects underlying tissues from mechanical, chemical, and thermal injury.

Sensory Reception

The dermis contains numerous sensory receptors for heat, cold, pain, touch, and pressure. Even though hair itself has no sensory receptors, the movement of hair can be detected by receptors clustered around a hair follicle. The sensory receptors in the dermis relay information about the environ­ment to the brain so that changes can be made to prevent or minimize injury. The sensory receptors are also a means of communication between individuals.

Regulation of Body Temperature

Normally, body temperature is maintained at 37° C (98.6° F). It is important that body temperature be regulated because changes in temperature alter the speed of chemical reactions in the body. The skin helps to regulate body tem­perature in two ways: by dilation and constriction of blood vessels, and by activity or inactivity of the sweat glands. Both of these mechanisms are examples of negative feed­back in maintaining homeostasis. Blood vessels dilate and sweat glands become active in response to an increase in body temperature. Both mechanisms tend to remove heat from the body. In response to cold, blood vessels constrict and sweat glands are inactive to conserve body heat. The adipose tissue in the subcutaneous layer also helps by acting as an insulator.

Synthesis of Vitamin D

Vitamin D is required for calcium and phosphorus absorp­tion in the small intestine. The calcium and phosphorus are essential for normal bone metabolism and muscle function. Skin cells contain a precursor molecule that is converted to vitamin D when the precursor is exposed to ultraviolet rays in sunlight. It takes only a small amount of ultraviolet light to stimulate vitamin D production, so this should not be used as an excuse to expose the skin to sun unnecessarily and to risk the damage that may result.

AGING OF THE INTEGUMENTARY SYSTEM

As the skin ages, the number of elastic fibers decreases and adipose tissue is lost from the dermis and subcutaneous layer. This causes the skin to wrinkle and sag. Loss of collagen fibers in the dermis makes the skin more fragile and makes it heal more slowly. Mitotic activity in the stratum basale slows so that the skin becomes thinner and appears more transparent. Reduced sebaceous gland activity causes dry, itchy skin. Loss of adipose tissue in the subcu­taneous layer and reduced sweat gland activity lead to an intolerance to cold and susceptibility to heat. The ability of the skin to regulate temperature is reduced. There is a general reduction in melanocyte activity, which decreases protection from ultraviolet light, resulting in increased sus­ceptibility to sunburn and skin cancer. Some melanocytes, however, may increase melanin production, resulting in “age spots.”

Despite all the creams and “miracle” lotions, there is no known way to prevent skin from aging. Good nutrition and cleanliness may slow the aging process. Because skin that is exposed to sunlight ages more rapidly than unexposed skin, one of the best ways to slow the aging process is to avoid exposure by wearing protective clothing and by using sun­block whenever possible.

Highlight on Conditions Affecting the Integumentary System

Alopecia (al-oh-PEE-shee-ah) Absence of hair from skin areas where it normally grows; baldness; may be hereditary or caused by disease, injury, or chemotherapy or may occur as part of aging

Basal cell carcinoma (BAY-sal SELL kar-sih-NOH-mah) Malig­nant tumor of the basal cell layer of the epidermis; most common form of skin cancer and usually grows slowly Cellulitis (sell-yoo-LYE-tis) Infection of connective tissue with severe inflammation of the dermis and subcutaneous layers of the skin

Dermatitis (der-mah-TYE-tis) Inflammation of the skin Eczema (ECK-zeh-mah) An inflammatory skin disease with red, itching, vesicular lesions that may crust over; common allergic reaction, but may occur without any obvious cause Eschar (ESS-kar) A slough produced by a burn or gangrene Impetigo (im-peh-TYE-go) Superficial skin infection caused by staphylococcal or streptococcal bacteria and characterized by vesicles, pustules, and crusted-over lesions; most common in children

Malignant melanoma (mah-LIG-nant mel-ah-NOH-mah) Cancer­ous growth composed of melanocytes; often arises in a preex­isting mole; an alarming increase in the prevalence of malignant melanoma is attributed to excessive exposure to sunlight

Nevus (NEE-vus) An elevated, pigmented lesion on the skin; commonly called a mole; a dysplastic nevus is a mole that does not form properly and may progress to a type of skin cancer; plural, nevi

Pruritus (proo-RYE-tus) Severe itching; one of the most common problems in dermatology; arises as a result of stimulation of nerves in the skin by enzymes released in allergic reactions or by other irritating substances

Urticaria (ur-tih-KAIR-ee-ah) Allergic transient skin eruptions characterized by elevated lesions, called wheals, and often accompanied by severe itching and burning; also called hives

Wart (WORT) Epidermal growth on the skin caused by a virus; plantar warts occur on the soles of the feet, juvenile warts occur on the hands and face of children, and venereal warts occur in the genital area

Xeroderma pigmentosum (zee-roh-DER-mah pig-men-TOH- sum) A pigmentary and atrophic inherited disease of the skin and eyes that is characterized by vascular lesions, excessive freckling, keratinous growths, carcinoma, photophobia, ocular opacities, and tumors; involves defect in the enzymes active in the repair of DNA damaged by ultraviolet light ■

Medical TermWord PartsDefinition
Arrector pilipil/o: hairMuscle associated with hair follicles.
Ceruminous glandcerumin/o: cerumenA gland in the ear canal that produces cerumen or ear wax.
Dermisderm/o: skinInner layer of the skin that contains the blood vessels, nerves, glands, and hair follicles.
Epidermisepi-: above, upon -derma: skinOutermost layer of the skin.
Keratinizationkerat/o: hard, horny tissue -ation: process, conditionProcess by which the cells of the epidermis become filled with keratin and move to the surface where they are sloughed off.
Melaninmelan/o: blackA dark brown or black pigment found in parts of the body, especially skin and hair.
Sebaceous glandseb/o: sebum -ous: pertaining toAn oil gland of the skin that produces sebum or body oil.
Subcutaneous layersub-: under, below cutane/o: skin -ous: pertaining toBelow the skin; a sheet of areolar connective tissue and adipose tissue beneath the dermis of the skin; also called hypodermis or superficial fascia.
Sudoriferous glandsud-: sweat -ous: pertaining toA gland in the skin that produces perspiration; also called sweat gland.

Skin color is a result of many factors: some genetic, some physiologic, and some environmental. Basic skin color is caused by the dark pigment melanin produced by the mela­nocytes in the stratum basale of the epidermis. Everyone has about the same number of melanocytes. The activity of