Melanocytes are cells that produce melanin, a dark pigment responsible for the coloration of hair and skin. In addition to being found in the skin, melanocytes are also present in the brain, inner ear, heart, and eye, among other locations in the body. They usually are buried below the surface. The cells produce melanin in response to environmental cues, including exposure to ultraviolet radiation and certain chemicals. The melanin travels out of the melanocytes and up to the surface of the tissue where the cells are found. Over time, it breaks down and needs to be replaced with fresh supplies of melanin produced by the layer of underlying melanocytes.

Pigmentation disorders, including congenital conditions, as well as acquired ones, can impact the way melanocytes work. Some suppress activity on the part of these cells, causing pale or white patches to appear on the body in locations where no melanin is being made. Others do the opposite, stimulating overproduction of melanin and causing darkening of the skin. Certain medications have the same effect, explaining why people on some prescriptions develop darkened skin.

In the skin, melanin has a protective quality. This pigment absorbs ultraviolet radiation, preventing it from traveling to the surrounding tissues of the body. As a result, people from regions where sun exposure is frequent and intense often have more active melanocytes. The cells work to ensure that their bodies have a healthy layer of melanin so they are less likely to burn and suffer UV damage to the sun.

Melanin also plays some other roles in the body, including the brain, where it appears to be a source for the basic ingredients for some neurotransmitters. The brain needs to synthesize chemicals as it runs out of them and in people with limited melanin production in the brain, reductions in some neurotransmitters have been observed. People with degenerative brain diseases may also experience destruction of melanocytes and subsequent declines in melanin function, which lead to decreased brain function. The area of the brain known as the substantia nigra is named for the high levels of melanin it contains.