​

The stratum corneum is composed of dead, flat cells that lack a blood supply and sensory receptors. Its thickness is related to normal wear of the area it covers. The basal layer is the only layer of the epidermis that is composed of living cells where new cells are formed. As these cells move toward the stratum corneum to replace the cells that have been sloughed off, they die and become filled with a hard protein material called keratin. The relatively waterproof characteristic of keratin prevents body fluids from evaporating and moisture from entering the body. The entire process by which a cell forms in the basal layer, rises to the surface, becomes keratinized, and sloughs off takes about 1 month.

 

In the basal layer, special cells called melanocytes produce a black pigment called melanin. Melanin provides a protective barrier from the damaging effects of the sun’s ultraviolet radiation, which can cause skin cancer. Moderate sun exposure increases the rate of melanin production and results in a suntan. However, overexposure results in sunburn due to melanin’s inability to absorb sufficient ultraviolet rays to prevent the burn.

Differences in skin color are attributed to the amount of melanin in each cell. Dark-skinned people produce large amounts of melanin and are less likely to have wrinkles or skin cancer. Production of melanocytes is genetically regulated and, thus, inherited. Local accumulations of melanin are seen in pigmented moles and freckles. An absence of pigment in the skin, eyes, and hair is most likely due to an inherited inability to produce melanin. An individual who cannot produce melanin, known as an albino, has a marked deficiency of pigment in the eyes, hair, and skin.

​

Dermis

The second layer of the skin, the

(4) dermis, also called corium, lies directly beneath the epidermis. It is composed of living tissue and contains numerous capillaries, lymphatic vessels, an nerve endings. Hair follicles, sebaceous (oil) glands, and sudoriferous (sweat) glands are also located in the dermis.

The (5) subcutaneous layer, also called hypodermis, binds the dermis to underlying structures. It is composed primarily of loose connective tissue and adipose (fat) tissue interlaced with blood vessels. The subcutaneous layer stores fats, insulates and cushions the body, and regulates temperature. The amount of fat in the subcutaneous layer varies with the region of the body and sex, age, and nutritional state.

​

Accessory Organs of the Skin

The accessory organs of the skin consist of integumentary glands, hair, and nails. The glands play an important role in defending the body against disease and maintaining homeostasis, whereas the hair and nails have more limited functional roles.

​

Glands

Two important glands located in the dermis produce secretions:

The (6) sudoriferous (sweat) glands produce sweat and the

(7) sebaceous (oil) glands produce oil. These two glands are exocrine glands because they secrete substances through ducts to an outer surface of the body rather than directly into the bloodstream.

The sudoriferous glands secrete perspiration, or sweat, onto the surface of the skin through pores. Pores are most plentiful on the palms, soles, forehead, and axillae (armpits). The main functions of the sudoriferous glands are to cool the body by evaporation, excrete waste products, and moisten surface cells.

​

The sebaceous glands are filled with cells, the centers of which contain fatty droplets. As these cells disintegrate, they yield an oily secretion called sebum. The acidic nature of sebum helps destroy harmful organisms on the skin, thus preventing infection. When ductules of the sebaceous glands become blocked, acne may result. Congested sebum causes formation of pimples or whiteheads. If the sebum is dark, it forms blackheads. Sex hormones, particularly androgens, regulate production and secretion of sebum. During adolescence, secretions increase; as the person ages, secretions diminish. The loss of sebum, which lubricates the skin, may be one of the reasons for the formation of wrinkles that accompany old age. Sebaceous glands are present over the entire body except on the soles of the feet and the palms of the hands. They are especially prevalent on the scalp and face; around such openings as the nose, mouth, external ear, and anus; and on the upper back.

​

Hair

Hair is found on nearly all parts of the body except for the lips, nipples, palms of the hands, soles of the feet, and parts of the external genitalia. The visible part of the hair is the

(8) hair shaft; the part that is embedded in the dermis is the hair root. The root, together with its coverings, forms the

(9) hair follicle. At the bottom of the follicle is a loop of capillaries enclosed in a covering called the (10) papilla. The cluster of epithelial cells lying over the papilla reproduces and is responsible for the eventual formation of the hair shaft. As long as these cells remain alive, hair will regenerate even if it is cut, plucked, or otherwise removed. Alopecia (baldness) occurs when the hairs of the scalp are not replaced because of death of the papillae (singular, papilla). Like skin color, hair color is related to the amount of pigment produced by epidermal melanocytes. Melanocytes are found at the base of the hair follicle. Melanin ranges in color from yellow to reddish brown to black. Varying amounts of melanin produce hair ranging in color from blond to brunette to black; the more abundant the melanin, the darker the hair. Heredity and aging affect melanin levels. A decrease or an absence of melanin causes loss of hair color.