Browsing by Subject "Melanocytes"

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    Hypomelanosis in chickens
    (1994) Marco, Heather Gaile; Kidson, Susan H
    Hypomelanosis, a severe reduction in pigmentation, is a widespread phenomenon which affects many different vertebrate species, including humans and chickens. The cause(s) of various forms of hypomelanosis is (are) not known. The aim of this study was to determine the cause of hypomelanosis in a breed of white chickens (White Plymouth Rock x Pile Game). It was hoped that this hypomelanotic breed may provide insight into the etiopathogenesis of certain human hypomelanotic disorders, such as vitiligo and albinism. To determine whether melanocytes are present in the hypomelanotic skin, two melanocyte-specific assays were carried out, in situ DOPA histochemistry and a sensitive radiometric assay for tyrosinase. The results show that active tyrosinase was present in 8, 9 and 10 day skins. However, unlike normal black skin, the level of tyrosinase did not increase with age, suggesting that the melanocytes either die or that they do not continue to synthesise tyrosinase. Ultrastructurally, these melanocytes appeared to be morphologically normal and they did not show signs of premature degeneration. Unlike black chick melanocytes, however, they contained very few premelanosomes and fully melanised melanosomes were never observed, suggesting that hypomelanosis results from the arrested development (melanisation) of melanosomes in vivo. Two different experiments were carried out to determine whether this blockage in melanogenesis is intrinsic in the melanocyte or whether it is caused by extrinsic environmental factors. The outcome of these studies were conflicting: 1) In culture, white chick neural crest cells produced pigment, suggesting that the melanocyte is not defective. However, ultrastructural examination of these cultured melanocytes showed that they contained a large proportion of partially melanised melanosomes. 2) Black chick neural crest cells migrated into white skin explants and contributed towards pigment in the developing feathers, suggesting that the white chick tissue environment is also not defective. The results hint that hypomelanosis in the white chicks is caused by the interaction of at least two genetic defects: an intrinsic mutation of the melanocyte, as well as an extrinsic mutation in the melanocyte environment that, in combination, exert an inhibitory influence on melanin synthesis. This study shows that, in situ, white chick melanocytes share some features with ty-pos albino melanocytes and may be representative of this pigmentary disorder. White Plymouth Rock x Pile Game chicks may also be useful as a model for the multi-faceted disorder, vitiligo.
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    Synthesis and activity of tyrosinase in mouse skin melanocytes
    (1990) Nkabinde, Nkosana Cyril; Kidson, Susan H
    Tyrosinase (E.C. 1.14.18.1) is a key enzyme in the biosynthesis of melanin. The control of melanin sythesis was explored in skin melanocytes of the following strains; wild type (C57BL/6J-C/C) (which maximally synthesize melanin at normal mammalian body temperature, Himalayan (C57BL/6J-cʰ/cʰ) (which maximally synthesize melanin at temperatures below 37°C) and albino (Balb c-c/c) (a mutant which does not synthesize melanin) The effect of a-MSH on tyrosinase activity was initially investigated. A skin culture tyrosinase assay that made it possible to measure the effect of α-MSH on the activity of this enzyme in vitro was first developed. It was found that α-MSH activated the wild type and Himalayan tyrosinase in a dose-dependent manner and that this activation did not require the de novo synthesis of new enzyme. The role of glycosylation on the wild type and particularly the Himalayan tyrosinase activity was next investigated. The results do not support, but are not in conflict with the theory that the Himalayan tyrosinase is inherently underglycosylated. Translation and transcription as additional control mechanisms of tyrosinase activity was finally investigated. The correlation between the levels of tyrosinase activity, abundance of the enzyme and the synthesis of tyrosinase mRNA in wild type, Himalayan and albino mice was determined. It was shown that the levels of newly synthesized tyrosinase and tyrosinase mRNA transcripts were higher in the wild type than in the Himalayan skin. This could account for the reduced tyrosinase activity in the Himalayan mutant at normal body temperature. Low levels of tyrosinase mRNA were found in the albino skin though there was no immunodetectable enzyme in this tissue.