The Patterns of Melanosome Distribution in Keratinocytes of Human Skin as One Determining Factor of Skin Colour

H.-Y. Thong, S.-H. Jee, C.-C. Sun, R.E. Boissy


The British Journal of Dermatology. 2003;149(3) 

In This Article

Summary and Introduction

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 µm2 x 10-2) followed in turn by those in Asian skin (1.36 ± 0.15 µm2 x 10-2) and Caucasian skin (0.94 ± 0.48 µm2 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.

Complexion coloration in different ethnic groups varies dramatically from dark to light, as exemplified by the skin of central African and northern Scandinavian individuals, respectively, despite the fact that the density of melanocytes in the skin of these two extreme skin types is identical.[1,2] The biological basis of human skin colour variation has been studied by numerous investigators over the years. The intrinsic complexion coloration is determined by the amount and type of melanin in the skin.[3] However, an equally important determinant of skin coloration is the variation in the quantity, packaging and distribution of epidermal melanin within keratinocytes of different ethnic groups.[4] It is well documented that the melanosomes within keratinocytes of dark skin are distributed individually in the cytosol, predominantly over the nucleus of the keratinocytes, whereas the melanosomes within keratinocytes of light skin are clustered together in membrane-limited groups of two to eight melanosomes, also in the cytosol predominantly over the nucleus.[5,6,7,8] It is also known that there is a progressive variation in melanosome size with ethnicity, African skin having the largest melanosomes, European skin the smallest melanosomes, and the melanosomes in Indian, Mexican and Chinese skin being intermediate in size.[9] However, most of these previous reports describe the distribution patterns of melanosomes among different ethnic groups in a subjective, qualitative nature. The extent to which the distribution patterns of melanosomes in the skin of different ethnic groups differ has never been defined precisely. Therefore, the aim of the current study was to use a quantitative approach to assess the distribution patterns of melanosomes that have been transferred to keratinocytes in skin from Asian individuals and to compare these data with those for light-skinned Caucasian and dark-skinned African/American individuals. We also measured the area of melanosomes with the intention of elaborating the quantitative difference between the size of individual and clustered melanosomes. The quantitative approach used in our study may potentially permit a new interpretation of the position of Asian skin in the model for global pigmentation.