by Alicia M Prater PhD

Created on: January 19, 2012   Last Updated: April 27, 2012

Melanocytes are the pigment cells of the human body. These specialized cells determine what color the hair and skin are. They 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, providing pigmentation to the iris.

Cell characteristics

Melanocytes originate in the neural crest during embryonic development. They migrate to the skin, inner ear, eye, and medulla oblongata (the lower part of the brainstem). The skin contains about 10 times as many keratinocytes (i.e. keratin protein-producing skin cells), but each melanocyte functions enough to pigment 30 keratinocytes. Melanocytes contain

melanosomes, a unique organelle found only in this cell lineage (i.e. skin melanocytes, choroidal melanocytes, and retinal pigment epithelial cells).

Melanin production

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.

Skin melanocytes

One such stimulus is exposure to sunlight, which induces the production of melanocyte stimulating hormone (MSH), which 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. Pheomelanin is induced by a different protein, agouti signal protein, binding to the melanocortin 1 receptor. This protein also inhibits eumelanin binding to the melanocortin receptor.

Choroidal melanocytes

The irises of the eye are also pigmented. The variations in 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 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.

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