Melanocyte
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Melanocytes are cells located in the bottom layer of the skin's epidermis and in the middle layer of the eye, the uvea. Through a process called melanogenesis, these cells produce melanin, a pigment in the skin, eyes, and hair. There are both basal and activated levels of melanogenesis; lighter-skinned people generally have low basal levels of melanogenesis, and exposure to UV radiation generally causes increased melanogenesis.
There are typically between 1000 and 2000 melanocytes per square millimeter of skin. They comprise from 5% to 10% of the cells in the basal layer of epidermis. Although their size can vary, melanocytes are typically 7 micrometers in length. The difference in skin color between fair people and dark people is due not to the number of melanocytes in their skin, but to the melanocytes' level of activity.
Albinos lack an enzyme, tyrosinase, that is required for melanocytes to produce melanin from the amino acid tyrosine.
Embryologically, melanocytes come from the neural crest, a completely different source than that of the surrounding skin cells (keratinocytes). All melanocytes have the capacity to migrate widely in the embryo, and therefore a cancer of a melanocyte, a melanoma, will spread (metastasize) very easily. For this reason, melanomas are often fatal, and when surgically removed, a lot of surrounding tissue must be taken as well.
The production of melanin is initiated by either MSH (melanocyte-stimulating hormone), ACTH (another hormone) or exposure to ultraviolet light. Once made, melanin is stored in “warehouses”, arm-like structures called dendrites. From there the melanin is shipped to the keratinocytes.
When ultraviolet rays penetrate the skin and damage DNA; thymidine dinucleotide (pTpT) fragments from damaged DNA will trigger release of the hormone alpha-MSH, which can then bind to melanocytes to cause them to produce melanin. Melanin vesicles from the melanocytes are then transferred to surrounding keratinocytes. The skin will then become darker as the keratinocytes that have been filled with melanin move toward the skin surface.The melanin is in organelles called "melanosomes" that are organized as a cap protecting the nucleus of the keratinocyte.
[edit] See also
- Chromatophore (the pigment cell type found in cold blooded animals)
- Eye color
