melanin

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melanin (mĕl´ənĬn), water-insoluble polymer of various compounds derived from the amino acid tyrosine. It is one of two pigments found in human skin and hair and adds brown to skin color; the other pigment is carotene, which contributes yellow coloring. The synthesis of melanin reactions is catalyzed by the enzyme tyrosinase; an inherited lack of tyrosinase activity results in one of the forms of albinism. Tyrosinase is found in only one specialized type of cell, the melanocyte, and in this cell melanin is found in membrane-bound bodies called melanosomes. Melanosomes can be transferred from their site of synthesis in the melanocytes to other cell types. The various hues and degrees of pigmentation found in the skin of human beings are directly related to the number, size, and distribution of melanosomes within the melanocytes and other cells. Besides it role in pigmentation, melanin, which absorbs ultraviolet light, plays a protective role when skin is exposed to the damaging rays of the sun (see sunburn; skin cancer).

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melanin a brown pigment in the skin and elsewhere. It is made in melanocytes, which are cells in the deepest layer of the epidermis, and these distribute granules of pigment to the other skin cells. Synthesis is stimulated by sunlight, and also by a hormone from the anterior pituitary gland. This is the pigment in simple moles and freckles, and in the areolar of the nipples; melanocytes are the cells which become cancerous in a malignant melanoma. Other sites are in hair, behind the retina of the eye, and in part of the adrenal gland.

S. J.


See pigmentation; skin.
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melanin Any of a group of polymers, derived from the amino acid tyrosine, that cause pigmentation of eyes, skin, and hair in vertebrates. Melanins are produced by specialized epidermal cells called melanophores (or melanocytes); their dispersion in these cells is controlled by melanocyte-stimulating hormone and melatonin. Certain invertebrates, fungi, and microorganisms also produce melanin pigments. The `ink' of the octopus and squid is a notable example. Hereditary albinism is caused by the absence of the enzyme tyrosinase, which is necessary for melanin production.

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melanin One of a group of dark pigments found in the skin and formed in melanoblast cells through the oxidation of tyrosine, phenylalanine, and other aromatic compounds. There are two chemically differing forms: the black-brown eumelanin and the red-yellow phaeomelanin. In many mammals, these alternate during the growth of a hair, resulting in a banded (‘agouti’) hair.

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melanin Dark pigment found in the skin, hair, and parts of the eye. It helps protect skin from ultraviolet radiation from the Sun. The amount of melanin determines skin colour. Absence of melanin results in an albino.

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mel·a·nin / ˈmelənin/ • n. a dark brown to black pigment occurring in the hair, skin, and iris of the eye in people and animals. It is responsible for tanning of skin exposed to sunlight.

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melanin Brown pigment formed when phenolic compounds in cut fruit and vegetables are exposed to air and oxidized; also the brown and black pigments of skin and hair.

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melanin (mel-ăn-in) n. a dark-brown to black pigment occurring in the hair, the skin, and in the iris and choroid layer of the eyes.