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Amelanism (also known as amelanosis) is a pigmentation abnormality characterized by the lack of pigments called , commonly associated with a Genetics loss of tyrosinase function. Amelanism can affect fish, , , , and 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.
Melanins are produced in called . The production of melanins is called melanogenesis. Melanosomes are found in specialized pigment cells called , 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. 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. Melanocyte-stimulating hormone (MSH) binds to the Melanocortin 1 receptor (MC1R) and commits melanocytes to the production of eumelanin. In the absence of this signal, melanocytes produce phaeomelanin. Another chemical, Agouti signalling peptide (ASP), can attach itself to MC1R and interfere with MSH/MC1R signalling. In many mammals, variation in the level of ASP switches melanocytes between eumelanin and phaeomelanin production, resulting in coloured patterns.
Melanocytes, and the parallel Chromatophore found in fishes, amphibians, and reptiles, are derived from a strip of tissue in the embryo called the neural crest. in the neural crest give rise to the cells of the autonomic nervous system, supportive elements of the skeleton such as , 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 , 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.Robins, Ashley H. (1991) pg. 75 Piebaldism and amelanism are distinct conditions.
Loss of melanogenesis function is linked to the gene that encodes tyrosinase. Certain of this gene, TYR, at the Color locus, cause oculocutaneous albinism type 1 in humans and the familiar red-eyed albino conditions in mice and other mammals.
/ref> Aeumelanic hair coats, associated with mutations of the MC1R gene, have also been identified in mice, cattle, dogs, and horses. These coat colors are called "yellow" in mice and dogs, "red" in cattle and chestnut in horses. The loss of eumelanin in the coat is, in these species, harmless. The distinction between aeumelanism and hyperphaeomelanism – over abundance of phaeomelanin – is semantic.
/ref> and horses. The appearance of mammals with recessive agouti mutations is typically dense black. As with aeumelanism, the difference between lack of phaeomelanin and abundance of eumelanin is one of words. Some agouti alleles in mice are associated with health defects, but this is not the case in dogs, cats, or horses.
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