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17

Disorders of the Skin, Hair, and Nails

The skin is the largest organ of the body. It contains millions of tiny nerve endings (called receptors) that respond to pressure and changes in temperature to provide you with information about your surroundings. Your skin also contains many tiny glands. Sebaceous glands produce an oily substance called sebum that helps keep the skin’s surface supple and prevent it from drying and cracking. When you are hot or have a fever, sweat glands produce a watery liquid that evaporates on the skin to cool your body, and small blood vessels in your skin dilate (widen) to allow heat to escape. When you are cold, your skin conserves heat by constricting (narrowing) the blood vessels.

Hair and nails are extensions of the skin and are composed mostly of keratin (the main constituent of the outermost layer of the skin). There are thousands of hair follicles in the skin. Follicles are pits of actively dividing cells that continuously make hairs. Relatively larger, thicker hairs grow on the scalp and pubic area; smaller, finer hairs (some that can barely be seen) grow elsewhere on the body. Fingernails and toenails, like hair, are continuously produced by actively dividing cells under the fold of skin at the base and sides of each nail.

Cross section of the skin

The skin consists mainly of two layers—the epidermis (the thin, protective outer layer made up mostly of dead cells) and the dermis (the thicker layer containing blood vessels, nerves, oil glands, and hair follicles). The subcutaneous tissue is a layer of fat under the skin.

Skin Disorders

Skin is composed of two layers. The surface layer that you see is a thin covering called the epidermis. Below the epidermis is a thicker layer called the dermis. The dermis contains most of the living elements of the skin, such as hair follicles and sweat glands. Below the dermis is a layer of fat called subcutaneous tissue.

The epidermis is a very active layer of cells. Cells at its base are continuously dividing to produce new cells. These cells gradually die as they fill up with a hard substance called keratin. As the cells die, they move up to the skin surface, where they are shed or rubbed away by movement (such as friction from your clothes or washing). In this turnover of cells, it takes an average of 1 month for a single epidermal cell to move from the base of the epidermis to its surface.

On parts of the body where pressure and friction are greatest, the epidermis is thicker, and it takes longer for cells to travel from the base to the surface. The normal turnover of cells can be disrupted by some skin disorders. For example, in psoriasis (see page 1064), new cells are produced at a faster rate than old cells are shed.

Birthmarks

Birthmarks are persistent (but not always permanent) areas of discolored skin that are present from birth. They may be pink, red, or purple marks caused by concentrations of blood vessels in the skin or tan or brownish black areas of intense pigmentation. Some birthmarks fade with time or disappear altogether. Birthmarks that bleed or that do not fade as a child gets older can be removed with laser surgery (see page 1088). Birthmarks can also be removed when a child is only a few weeks old.

Capillary mark

A capillary mark is a pink or pinkish brown spot that is present at birth but gradually fades and usually disappears before a baby is 18 months old.

Strawberry mark

A strawberry mark or hemangioma (see page 117) is a bright red, raised area up to 4 inches across that can occur anywhere on the body. It may be barely noticeable at birth, but it will grow rapidly for a few weeks and then increase in size proportionately with the child. Large strawberry marks may bleed easily if they are bumped or scratched. When the child is about 6 to 10 months old, small gray-white areas in the mark will spread and gradually replace the red tissue, and the area will flatten. Strawberry marks usually disappear by the time a child is 5 years old, leaving