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

Disclosures

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

In This Article

Discussion

The human species has a wide range of coloration. Various theories have been proposed to explain the complex biological phenomenon of complexion coloration. One ecogeographical rule recognizing correlations between complexion coloration and climatic or latitudinal gradients stemmed from the nineteenth century when Golger, a naturalist, stated that individuals living close to the equator had dark skins, and those living distances from the equator had lighter skin colour. Golger's rule explained well the distribution of skin colour for denizens of Europe and Africa, but was unable to describe well the distribution of skin colour in Asia, where the people exhibit a homogeneous yellow-brown colour in the skin. Needless to say, this is an oversimplification of the phenomenon of melanin, skin pigmentation and complexion coloration. It has been shown that within the epidermis the quantity and type of melanin, and the pattern of melanosome distribution, rather than the number of melanocytes, comprise the primary determinants of colour.[12,13,14,15] The distribution patterns of melanosomes in dark and light skin as in African/American individuals and Caucasians are well documented and have been described by various authors in a matter-of-fact manner. However, with increasing miscegenation and hence an expanding complexity in skin pigmentation, it would seem illusory to depict the nature of ethnic variation of skin colour by mere extrapolation from the previous generally qualitative impressions that we have regarding light and dark skin.

By using a quantitative approach, our study elaborated the gradient in the relative proportion of individual vs. clustered melanosomes across different ethnic groups, placing the Asian skin midway between the polar extremes found in that of African/American individuals and Caucasians. In Asian skin, the distribution pattern of melanosomes consisted of a combination of both individual and clustered melanosomes. The number of individual melanosomes within keratinocytes of Asian skin outnumbered the clustered melanosomes at 62.6% vs. 37.4%. This distribution pattern was intermediate between the distribution pattern of the dark skin of African/American individuals, which had predominantly individual melanosomes, and the light skin of Caucasians in which the melanosomes were distributed mainly in clusters. One point worth mentioning is that melanosomes were not exclusively single in African/American individuals and clustered in Caucasians as reported in previous findings.[5,6,7,8]

The gross differences in melanosome size that have previously been observed between different skin types show that melanosomes tend to be largest in African/American individuals, and are generally arranged singly within secondary lysosomes of keratinocytes (> 1 µm), whereas melanosomes are smaller (< 1 µm) in light skin of Caucasians and tend to form groups of two or more in secondary lysosomes.[6,16,17,18] We were able to determine the variation in size of melanosomes within the keratinocytes from various skin types reliably by using the Zeiss LSM Image Browser version 2.30.011. Instead of measuring the diameter and length of melanosomes, the Zeiss LSM Image Brower measures the area of the melanosomes as an indicator of size. A variation of melanosome size between and within individuals was noted, possibly due to the variation in the spatial orientation of the melanosomes and because we were only measuring the size of melanosomes at a two-dimensional aspect. Nevertheless, we were able to observe a progressive increase in mean melanosome size when moving from the most lightly pigmented to the most darkly pigmented skin types by using this new methodology.

Much has been offered to explain the different distribution of melanosomes in various ethnic groups. It remains unclear whether melanosomes are transferred from melanocytes to keratinocytes individually or in membrane-bound clusters. The exact mechanism of melanosome transfer from melanocytes to keratinocytes is also not fully understood. The mechanisms suggested include cytophagocytosis, endocytosis and direct transfer/inoculation.[19] One concept states that the actual size of melanosomes could be an important indicator of melanosome distribution,[20] possibly via a size-dependent phagocytosis process.[21,22] The findings in our study seem to support this hypothesis, as we found a significant difference between the size of the individual melanosomes and clustered melanosomes in all study groups. Our findings seemed to be opposed to the conclusion of Minwalla et al., who stated that melanosome size was not related to whether the melanosomes were distributed individually or in clusters.[8] The explanation for this phenomenon would be that the regulatory mechanisms for melanosome transfer in an in vivo system may be somewhat different from that in a cell culture system. We believe that the controlling factors in the distribution patterns of melanosomes within keratinocytes are so complex that further investigation should be performed before the true nature of this regulatory system can be unravelled.

In conclusion, we have demonstrated quantitatively the distribution pattern of melanosomes in keratinocytes of Asian skin using electron microscopy, in addition to determining the melanosome size within the keratinocytes of Asian skin and comparing the data with other skin types. We have illustrated the presence of a size gradient of melanosomes in these groups, which corresponds at least partially to the colour gradient across racial groups using a different methodology. The manner in which the melanin is dispersed, either as individual melanosomes or as aggregated clusters, and the melanosome size observed between different ethnic skin types, suggest that these may have significant roles to play in the determination of racial coloration. The approach undertaken in this study may be used to provide a new interpretation of the position of ethnic groups apart from African/American individuals and Caucasians in the model for global pigmentation.

Future studies will aim to achieve a better understanding of the relative importance of these biological components and the underlying regulatory mechanisms governing the racial difference of skin coloration.


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