40 B. E. Johnson of accommodated skin is greater than that of the protein constituents of the horny layer. MELANIN The major contributory factor in the natural protection of human skin against uvr is the complex, indole type polymer called melanin. The relationship between sunlight and melanin pigmentation has been most recently reviewed by Pathak et al. (9), and Quevedo et al. (101), Szabo (102), and Pathak (103) present good coverage of the subject matter as known at that time. The development of a suntan in normally lightly pigmented skin, resulting from a uvr-stimulated redistribution of preformed melanosomes in the epi- dermis and the tyrosinase controlled production of melanised melanosomes in the melanocytes with subsequent transfer to and distribution in the keratinocytes of the hyperplastic epidermis, obviously affords protection against further uvr exposure. The genetically controlled variation in number, form and distribution of melanosomes, mainly responsible for racial variation in skin colour, results also in variations in the skin responses to uvr. In fair-skinned Celtic peoples, Albinos and in vitiliginous skin, where little or no melanin is present, the skin reactions are severe. As the degree of melanisation increases, so the intensity of skin reactions decreases and in deeply pig- mented Negroids and Australoids, it is difficult to elicit reactions other than the IPD and skin cancer due to uvr does not occur. It is thought that the racial variation in human skin colour may represent an evolutionary compromise between the need to protect against the carcinogenic action of uvr and the requirement of photochemically-produced vitamin D in the viable layers of the epidermis (104, 105). The transmission of 280-320 nm radiation through negro horny layer is significantly less than through that from white skin, but there is no significant difference in the thickness of this layer (106, 107). The only difference appears to be the presence of melanin as demonstrated by Kligman (108). The mechanisms by which melanin exerts its protective function apparently vary. It may act as a neutral density filter in the outermost layers of the epidermis where it is present in a disperse, amorphous form. The absorbed energy is dissipated harmlessly as heat. Moreover, in this form it acts as an excellent scattering screen for the wavelengths involved. The scattering produced by the larger melanin granules, usually observed in a supranuclear distribution in the basal and suprabasal layers, is thought to involve a high degree of forward direction, and if this is so the function of the superanuclear caps in protecting the germinal DNA is a little difficult to accept. Stable free radicals are present in the melanin polymer and may act as electron traps in a uvr-excited system. However, the spatial relationship between the potentially susceptible biological substrate, for instance DNA, and the melanin would have to be rather more intimate than it appears in electron microscope pictures for this protective mechanism to function efficiently. It is also thought that melanin may act as a semi-conductor material, in this way diverting electron energy away from susceptible biological material. The exact mechanisms involved in melanin protection against uvr damage, apart from the action as a filter, are obviously not as yet worked out, but it would seem that they do exist and are effective. CONCLUSION In summary, the reactions of human skin to the uvr component of solar radiation may vary according to the intensity and wavelength of the incident radiation from the mildest

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