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Concept
Amelanism
Résumé
Amelanism (also known as amelanosis) is a pigmentation abnormality characterized by the lack of pigments called melanins, commonly associated with a genetic loss of tyrosinase function. Amelanism can affect fish, amphibians, reptiles, birds, and mammals including humans. The appearance of an amelanistic animal depends on the remaining non-melanin pigments. The opposite of amelanism is melanism, a higher percentage of melanin.
A similar condition, albinism, is a hereditary condition characterised in animals by the absence of pigment in the eyes, skin, hair, scales, feathers or cuticle. This results in an all white animal, usually with pink or red eyes.
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. Hair acquires pigment from melanocytes in the root bulb, which deposit melanosomes into the growing hair structure. 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.
Source officielle
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