Melanocytes are the pigment cells of the human body, determining the color of the hair and skin. These specialized cells are found in the basal (i.e. bottom) layer of the epidermis, where the hair roots start. Related cells are also found in the eye, producing pigment for the iris.
Description of cell characteristics
Melanocytes originate in the neural crest during embryonic development. They migrate to the skin, inner ear, eye, and the lower part of the brainstem, called the medulla oblongata. The skin contains about 10 times more keratinocytes (i.e. cells that produce keratin protein), but each melanocyte functions enough to pigment 30 keratinocytes. Inside melanocytes are a unique organelle called the melanosome. The cell structure is found only in the skin melanocytes, choroidal melanocytes, and retinal pigment epithelial cells.
Melanosomes resemble granules and are the location for the synthesis and storage of melanin, the human pigment protein. Two types of melanin are produced based on the stimulus – eumelanin (brown to black) and pheomelanin (yellow to red). Melanosome activity is regulated both genetically and environmentally.
Function of specific cell types
Skin melanocytes: One stimulus of melanin production is exposure to sunlight, which induces the production of melanocyte stimulating hormone (MSH). MSH binds to the melanocortin 1 receptor, increasing the production of eumelanin. The pigment is transported to surrounding keratinocytes, offering the skin protection from the ultraviolet rays of the sun. Pheomelanin is induced by a different protein, agouti signal protein, binding to the melanocortin 1 receptor. This protein also inhibits eumelanin from binding to the melanocortin receptor. The melanin from these cells also becomes part of each hair strand as it grows from a follicle in the skin, coloring the naturally white hair to a shade of eumelanin or pheomelanin.
Choroidal melanocytes: Variations in eye color are due to how light reflects off the surface of the iris, which is actually a ring of muscle fibers and connective tissue that controls pupil size. The wavelengths that are reflected are determined by the pigment that is present in the iris, which itself is decided by complex genetics. The darker pigment (eumelanin) absorbs light, so the distribution of pigment types in various parts of the iris epithelial layer determines the type and extent of eye coloration.
Retinal pigment epithelium: The third type of melanocyte, the retinal pigment epithelium, forms the blood-brain barrier of the retina. On top of the retinal epithelium is the neural retina. As such, these cells maintain the visual receptors and are necessary for vision.
To summarize, the melanocytes are specialized cells that contain melanosomes. This organelle produces a pigment that protects the skin from light damage and is responsible for the color of a person’s skin, hair, and eyes. How dark a person’s complexion is depends on the activity of their melanocytes and the type of melanin they produce. For a primer on the genetics underlying skin color diversity, see this 2009 review from Human Molecular Genetics.