Abstract

PIP: This paper reviews the action of hormones on the epidermal melanin unit. Slight attention is paid to the hormonal regulation of the melanophore. Melanin serves 2 main functions in the epidermis: 1) The pigment protects underlying structures from the harmful effects of sunlight; and 2) It serves to influence the color of the epidermis. 1 mechanism by which hormones can influence the amount of melanin involves changes in the activity to the enzyme tryosinase in the melanoblasts which synthesize the pigment. A second mechanism may involve changes in the activity of keratinocytes which engulf the melanin discharged from melanoblasts. Hormones can also influence the dispersion of melanin. Interstitial cell-stimulating hormone, estrogens, melanocyte-stimulating hormone and adrenocorticotrophin appear to increase epidermal melanin by enhancing the activity of tyrosinase. The action of interstitialcell-stimulating hormone (ICSH) upon melanogenesis has been studied in weaver birds. It has been shown that estrogens are capable of accelerating the synthesis of melanin, and that the action is a direct effect of the hormone itself, because the response occurs locally when the hormone is applied directly to the skin. It has been observed that skin color varies with the menstrual cycle. Such variations may result from the synergistic action of estrogens and progesterone. A similar mechanism accounts for the pigmentation of pregnancy. Studies have shown that estrogens also influence the color of feathers in certain birds. Melanocyte-stimulating hormone (MSH) has been shown to increase the melanin content of the epidermis of the guinea pig, and promotes the dispersion of melanin into the dendritic processes of the melanocyte and into adjacent keratinocytes. Adrenocorticotropic hormone (ACTH) prossesses some MSH activity, although it is much less potent than MSH itself. Presumably the hormone acts like MSH, directly on the melanocyte. These 2 hormones darken the skin in man, and 1 or both is responsible for the well known pigmentation which follows bilateral adrenalectomy when replacement therapy is inadequate. The influence of thyroxine upon epidermal melanin is complex and varies from species to species. The action of androgens on melanogenesis is both complex and ill-understood. Adrenoline and noradrenaline inhibit the action of MSH on frog skin but there is no direct evidence that these hormones influence the mammalian melanocyte. Further investigations are needed.