- A pigment-producing cell in the skin, hair and eye that determines their color. The pigment that melanocytes make is called melanin. The major determinant of color is not the number but rather the activity of the melanocytes. Melanin production takes place in unique organelles (tiny structures within the cell) known as melanosomes. Darkly pigmented skin, hair and eyes have melanosomes that contain more melananin. The melanosome: Melanosomes rearrange themselves within the cell in response to external cues such as ultraviolet rays or mutations in their transport system. Melanosomes usually cluster together near the center of the cell but can rapidly redistribute themselves to the ends of dendritic (tooth-like) processes projecting from the cell. Melanosome redistribution involves long-range transport from the center along microtubules and, along the periphery, short-range capture and transport. Each melanocyte supplies melanin to approximately 30 nearby keratinocytes via its dendrites. Embyrology: Melanocytes originate in the neural crest and migrate to the basal layer of the epidermis and the hair matrices. These neural crest-derived cells are not confined to the skin and hair. They also migrate to the inner ear, uveal tract in the eye and the leptomeninges in the brain. The failure of melanocytes to migrate to these locations explains the association of congenital white spotting of the skin (piebaldism) with heterochromia (the juxtaposition of different colors) in the iris as well as congenital deafness in Waardenburg syndrome. Forms of melanin: There are two major forms of melanin — eumelanin and pheomelanin. Eumelanin is brown to black while pheomelanin is yellow to red. The regulation of the production of eumelanin versus pheomelanin involves the interaction of the melanocortin 1 receptor (MC1R) on the surface of the melanocyte with melanocyte stimulating hormone (MSH) or with the agouti signaling protein. The binding of MSH to MC1R results in the formation of eumelanin while the binding of the agouti protein to MC1R leads to the production of pheomelanin. Tanning of the skin due to UV exposure represents an increase in the content of eumelanin within the epidermis and its major purpose is increased photoprotection. Disorders of pigmentation: These can result from a change in the number of melanocytes or a decrease or increase in their activity. Leukoderma in association with inflammatory disorders of the skin such as atopic dermatitis and vitiligo are two of the more common disorders of hypopigmentation. One of the most common disorders of hyperpigmentation is melasma (also known as mask of pregnancy) which is seen primarily, but not exclusively, in women and may or may not have its onset during pregnancy. Exposure to the sun plays a very important role in the induction and maintenance of these areas of hyperpigmentation on the face.
* * *A pigment-producing cell located in the basal layer of the epidermis with branching processes by means of which melanosomes are transferred to epidermal cells, resulting in pigmentation of the epidermis. SYN: melanodendrocyte, pigment cell of skin. [melano- + G. kytos, cell]
* * *me·la·no·cyte mə-'lan-ə-.sīt, 'mel-ə-nō- n an epidermal cell that produces melanin compare MELANOBLAST
* * *n.any one of the cells, concentrated within the epidermis of skin, that produces the dark brown pigment melanin.
* * *mel·a·no·cyte (melґə-no-sīt) (mə-lanґo-sīt) [melano- + -cyte] a type of dendritic clear cell found in the stratum basale of the epidermis; it synthesizes tyrosinase and contains melanosomes that produce melanin and can be transferred from melanocytes to nearby keratinocytes.
Medical dictionary. 2011.