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Figure

Being bald can be beautiful, but many people with hair loss seek ways to grow hair.

hair within the follicle stands on end when the muscle contracts. If a person is upset or very cold, nerve impulses may stimulate the arrector pili muscles to contract, raising gooseflesh, or goose bumps. Each hair follicle also has associated with it one or more sebaceous (oil-producing) glands, discussed later in the chapter.

Nails

Nails are protective coverings on the ends of the fingers and toes. Each nail consists of a nail plate that overlies a surface of skin called the nail bed. Specialized epithelial cells that are continuous with the epithelium of the skin produce the nail bed. The whitish, thickened, half-moon-shaped region (lunula) at the base of a nail plate is the most active growing region. The epithelial cells here divide, and the newly formed cells are kera-tinized. This gives rise to tiny, keratinized scales that become part of the nail plate, pushing it forward over the nail bed. In time, the plate extends beyond the end of the nail bed and with normal use gradually wears away (fig. 6.8).

Some interesting hair characteristics are inherited. The direction of a cowlick is inherited, with a clockwise whorl being more common than a counterclockwise whorl. A white forelock, and hairy ears, elbows, nose tip, or palms are also inherited.

Shier-Butler-Lewis: II. Support and Movement 6. Skin and the © The McGraw-Hill

Human Anatomy and Integumentary System Companies, 2001

Physiology, Ninth Edition

Nail plate Lunula. Nail bed s

Figure

Nails grow from epithelial cells that divide and become keratinized in the lunula.

Nail appearance mirrors health. Bluish nail beds may reflect a circulatory problem. A white nail bed or oval depressions in a nail can indicate anemia. A pigmented spot under a nail that isn't caused by an injury may be a melanoma. Horizontal furrows may result from a period of serious illness or indicate malnutrition. Certain disorders of the lungs, heart, or liver may cause extreme curvature of the nails. Red streaks in noninjured nails may be traced to rheumatoid arthritis, ulcers, or hypertension.

Skin Glands

Sebaceous glands (se-ba shus glandz) (see fig. 6.2) contain groups of specialized epithelial cells and are usually associated with hair follicles. They are holocrine glands (see chapter 5, page 148), and their cells produce globules of a fatty material that accumulate, swelling and bursting the cells. The resulting mixture of fatty material and cellular debris is called sebum.

Sebum is secreted into hair follicles through short ducts and helps keep the hairs and the skin soft, pliable, and waterproof (fig. 6.9). Acne results from excess sebum secretion (Clinical Application 6.3).

Sebaceous glands are scattered throughout the skin but are not on the palms and soles. In some regions, such as the lips, the corners of the mouth, and parts of the external reproductive organs, sebaceous glands open directly to the surface of the skin rather than being connected to hair follicles.

Sweat glands (swet glandz) (sudoriferous glands) are widespread in the skin. Each gland consists of a tiny tube that originates as a ball-shaped coil in the deeper dermis or superficial subcutaneous layer. The coiled por-

Sebaceous

Sebaceous

Figure

A sebaceous gland secretes sebum into a hair follicle (shown here in cross section: 175x micrograph enlarged to 300x).

Figure

A sebaceous gland secretes sebum into a hair follicle (shown here in cross section: 175x micrograph enlarged to 300x).

tion of the gland is closed at its deep end and is lined with sweat-secreting epithelial cells. The most numerous sweat glands, called eccrine glands (ek rin glandz), respond throughout life to body temperature elevated by environmental heat or physical exercise (fig. 6.10). These glands are common on the forehead, neck, and back, where they produce profuse sweat on hot days or during intense physical activity. They also cause the moisture that appears on the palms and soles when a person is emotionally stressed.

The fluid the eccrine sweat glands secrete is carried by a tube (duct) that opens at the surface as a pore (fig. 6.11). Sweat is mostly water, but it also contains small quantities of salts and wastes, such as urea and uric acid. Thus, sweating is also an excretory function.

The secretions of certain sweat glands, called apo-crine glands (ap oi-krin glandz), develop a scent as they are metabolized by skin bacteria (see fig. 6.10). (Although they are currently called apocrine, these glands secrete by the same mechanism as eccrine glands—see merocrine glands described in chapter 5, page 148.) Apocrine sweat glands become active at puberty and can wet certain areas of the skin when a person is emotionally upset, frightened, or in pain. Apocrine sweat glands are also active during sexual arousal. In adults, the apocrine glands are most numerous in axillary regions, the groin, and the area around the nipples. Ducts of these glands open into hair follicles.

Other sweat glands are structurally and functionally modified to secrete specific fluids, such as the cerumi-nous glands of the external ear canal that secrete ear wax

Nail plate Lunula. Nail bed s

Acne

Many young people are all too familiar with acne vulgaris, a disorder of the sebaceous glands. Excess sebum and squa-mous epithelial cells clog the glands, producing blackheads and whiteheads (comedones). The blackness is not dirt but results from the accumulated cells blocking light. In addition, the clogged sebaceous gland provides an attractive environment for anaerobic bacteria that signals the immune system to trigger inflammation. The inflamed, raised area is a pimple (pustule).

A Hormonal Problem

Acne is the most common skin disease, affecting 80% of people at some time between the ages of eleven and thirty. It is largely hor-monally induced. Just before puberty, the adrenal glands increase production of androgens, which stimulate increased secretion of sebum. At puberty, sebum production surges again. Acne usually develops because the sebaceous glands are extra responsive to an-drogens, but in some cases, andro-gens may be produced in excess.

Acne can cause skin blemishes far more serious than the perfect models in acne medication ads depict (fig. 6D). Scarring from acne can lead to emotional problems. Fortunately, several highly effective treatments are available.

What to Do-And

Not Do

Acne is not caused by uncleanliness or eating too much chocolate or greasy food. Although cleansing products containing soaps, deter-

gents, or astringents can remove surface sebum, they do not stop the flow of oil that contributes to acne. Abrasive products are actually harmful because they irritate the skin and increase inflammation.

Several acne treatments are available, but most take weeks to months to work. Women with acne are sometimes prescribed birth control pills because the estrogens counter androgen excess. Other drugs with estrogenic effects are available in Europe but not in the United States or Canada. Isotretinoin is a derivative of vitamin A that helps nearly all people achieve relief or even permanent cures, but it has several side effects and causes birth defects. Systemic antibiotics can treat acne by clearing bacteria from sebaceous glands. Topical treatments include tretinoin (another vitamin A derivative), salicylic acid (an aspirin solution), and benzoyl peroxide.

Treatment for severe acne requires a doctor's care. Drug combinations are tailored to the severity of the condition (table 6A). ■

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