Amelanism - Melanins and Melanin Production

Melanins and Melanin Production

Melanin is a compound found in plants, animals, and protists, and is derived from the amino acid tyrosine. Melanin is a photoprotectant, absorbing the DNA-damaging ultraviolet radiation of the sun. Vertebrates have melanin in their skin and hair, feathers, or scales. They also have two layers of pigmented tissue in the eye: the stroma, at the front of the iris, and the iris pigment epithelium, a thin but critical layer of pigmented cells at the back of the iris. Melanin is also present in the inner ear, and is important for the early development of the auditory system. Melanin is also found in parts of the brain and adrenal gland.

Melanins are produced in organelles called melanosomes. The production of melanins is called melanogenesis. Melanosomes are found in specialized pigment cells called melanocytes, but may also be engulfed by other cells, which are then called melanophages. A critical step in the production of melanins is the catalysis of tyrosine by an enzyme called tyrosinase, producing dopaquinone. Dopaquinone may become eumelanin, or phaeomelanin. Eumelanin, meaning true black, is a dense compound that absorbs most wavelengths of light, and appears black or brown as a result. Phaeomelanin, meaning rufous-black, is characterized by the presence of sulfur-containing cysteine, and it appears reddish to yellowish as a result. Melanosomes containing eumelanin are eumelanosomes, while those containing phaeomelanin are phaeomelanosomes. A hormone called Melanocortin 1 receptor (MC1R) commits melanocytes to the production of eumelanin. Another chemical, Agouti signalling peptide, can attach itself to MC1R and interfere with this signalling. In the absence of MC1R, melanocytes produce phaeomelanin. Hair acquires pigment from melanocytes in the root bulb, which deposit melanosomes into the growing hair structure.

Melanocytes, and the parallel melanophores found in fishes, amphibians, and reptiles, are derived from a strip of tissue in the embryo called the neural crest. Stem cells in the neural crest give rise to the cells of the autonomic nervous system, supportive elements of the skeleton such as chondrocytes, cells of the endocrine system, and melanocytes. This strip of tissue is found along the dorsal midline of the embryo, and multipotent cells migrate down along the sides of the embryo, or through germ layers, to their ultimate destinations. Melanocyte stem cells are called melanoblasts. Conditions associated with abnormalities in the migration of melanoblasts are known collectively as piebaldism. Pigment cells of the iris pigment epithelium have a separate embryological origin. Piebaldism and amelanism are distinct conditions.