Melanocyte
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Melanocytes are cells located in the bottom layer of the skin's epidermis. With a process called melanogenesis, they produce melanin, a pigment in the skin, eyes, and hair. In Caucasians, melanocytes are only triggered by ultraviolet rays, thus requiring sun exposure in order to tan. In other people, the melanocytes constantly produce melanin and this can also be enhanced by sun exposure[1] (http://www.yourperfectan.com/htm/yourtan/skintype.htm)[2] (http://www.perfectimage85.com/TanGuide/TGuidemain.htm).
The typical density of melanocytes is between 1000 and 2000 cells per square millimeter of skin, comprising between 5-10% of cells in the skin. The difference between fair people and dark people is not the number of melanocytes, but how active the melanocytes are.
Albinos lack an enzyme, tyrosinase, that is required for melanocytes to produce melanin.
Embryologically, melanocytes come from the neural crest. This is completely different to the surrounding skin cells (keratinocytes). This fact also means that a cancer of a melanocyte, a melanoma, will spread (metastasize) very easily. For this reason melanomas are often fatal, and when being removed a lot of surrounding tissue needs to be taken as well.
See also
- Chromatophore (the cells that allow octopuses and cuttlefish to change their color rapidly)