Introduction

Various shades and colors of human skin are created by the brown pigment, melanin. Without melanin, the skin would be pale white with varying shades of pink caused by the blood flowing through it. Fair-skinned people produce very little melanin; darker-skinned people produce moderate amounts; and very dark skinned people produce a great deal. People with albinism have no melanin.

Melanin is produced by special cells (melanocytes) that are interspersed among the other cells in the top layer of the skin, the epidermis. After melanin is produced, it spreads into other nearby skin cells.

When exposed to sunlight, melanocytes produce increased amounts of melanin, causing the skin to darken, or tan. In some fair-skinned people, certain melanocytes produce more melanin than others in response to sunlight. This uneven melanin production results in spots of pigmentation known as freckles. A tendency to freckles runs in families. Increased amounts of melanin can also occur in response to hormonal changes, such as those that may take place in Addison's disease, in pregnancy, or with oral contraceptive use. Some cases of skin darkening, however, are not related to increased melanin at all, but rather to abnormal pigments that make their way into the skin. Diseases such as hemochromatosis or hemosiderosis or some drugs that are applied to the skin, swallowed, or injected can cause skin darkening. A buildup of bilirubin (the main pigment in bile) causes the skin to turn yellow (jaundice).

An abnormally low amount of melanin (hypopigmentation) may affect large areas of the body or small patches. Decreased melanin usually results from a previous injury to the skin such as a blister, ulcer, burn, or skin infection. Sometimes pigment loss results from an inflammatory condition of the skin or, in rare instances, is hereditary. A common skin infection, tinea versicolor (see Section 18, Chapter 212), can also cause pigment loss in patches of skin.