The patterns of melanosome distribution in keratinocytes of human skin as one determining factor of skin colour

Background One determining factor of skin colour is the distribution pattern of melanosomes within keratinocytes. Melanosomes in keratinocytes of light skin as in Caucasians are distributed as membrane-bound clusters, whereas the melanosomes in keratinocytes of dark skin as in African/American individuals tend to be larger and distributed individually. It has been shown that melanin content, melanin composition and the size of melanosomes in the human epidermis vary considerably with both ethnicity and chronic sun exposure. Objectives To assess quantitatively the distribution pattern of melanosomes that have been transferred to keratinocytes in the photoprotected ( volar forearm) skin from normal Asian individuals and to compare these data with those from light-skinned Caucasian and dark-skinned African / American individuals. Methods Electron microscopy was used. Results We have demonstrated that melanosomes within keratinocytes of Asian skin are distributed as a combination of individual and clustered melanosomes with a proportion of 62.6% vs. 37.4%, respectively. This contrasts with dark and light skin keratinocytes where melanosomes are predominantly individual (88.9%) and clustered (84.5%), respectively. Analysis of mean +/- SD melanosome size also revealed a progressive variation in size with ethnicity, melanosomes in dark skin being the largest (1.44 +/- 0.67 mum(2) x 10(-2)) followed in turn by those in Asian skin (1.36 +/- 0.15 mum(2) x 10(-2)) and Caucasian skin (0.94 +/- 0.48 mum(2) x 10(-2)). In addition, it was shown that the melanosomes that are individually distributed tend to have a larger size than the clustered melanosomes. Conclusions The present data indicate that there may be a size gradient of melanosomes encompassing the global complexion coloration and that the melanosome distribution in keratinocytes of Asian skin is intermediate between that in light Caucasian and dark African / American skin.