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

Results

Figure 1 demonstrates the difference in melanosome distribution pattern within keratinocytes among African/American, Asian and Caucasian skin types. Melanosomes within keratinocytes of dark skin from an African/American (Figure 1a) were individually distributed throughout the cytoplasm, being predominantly localized apically over the nucleus. In contrast, melanosomes in the light skin from a Caucasian (Figure 1c) were almost exclusively distributed in membrane-bound clusters that were also localized at a supranuclear position. As shown in Figure 1(b), the distribution pattern of melanosomes in the recipient Asian keratinocytes was shown to be intermediate between the African/American and Caucasian skin, with a combination of both individual and clustered melanosomes. Subsequently, melanosomes were assessed as being either individually distributed or in membrane-bound clusters of two to three, four to six, or greater than six as described in Materials and methods ( Table 1 ). In Asian skin, 62.6% of melanosomes were individually distributed and 37.4% were clustered (n = 15). In addition, 74.6% of the membrane-bound clusters presented as clusters of two to three melanosomes. There was no statistically significant difference in the distribution pattern of melanosomes among the various age and sex groups in the Asian population. A variation in the distribution pattern between individuals from each group was noted. Table 1 demonstrates the distribution pattern of melanosomes within keratinocytes from various skin types. In Caucasian skin, 15.5% of melanosomes were distributed individually. The remaining 84.5% of the melanosomes in Caucasian skin were clustered. In contrast, 88.9% of the melanosomes in African/American skin were distributed individually and only 11.1% were clustered. There was a statistically significant difference in the distribution pattern of melanosomes among the three skin types ( Table 1 ).

Differences in the distribution of melanosomes within keratinocytes of African/American, Asian and Caucasian skin. Melanosomes in dark skin from an African/American (a,d) are predominantly distributed individually, with a few as membrane-bound clusters (arrowheads) throughout the cytoplasm of epidermal keratinocytes. Melanosomes in light skin from a Caucasian (c,f) are distributed as membrane-bound clusters with a few as individuals (arrows). Melanosomes in Asian skin (b,e) showed a combination of individual (arrows) and clustered (arrowheads) distribution pattern intermediate between the African/American and Caucasian skin. Melanosomes in all types of skin frequently aggregated apically over the nucleus. (a-c) Low magnification; (d-f) higher magnification. Bars: (a-c) 5.2 µm; (d-f) 2.0 µm.

There also appeared to be a skin type-related pattern to the degradation of melanosomes in the higher levels of the squamous epithelium. As keratinocytes undergo terminal differentiation and migrate to the stratum corneum, transferred melanosomes are generally degraded.[11] Melanosomes are completely degraded and thus absent from the corneocytes of light skin, whereas some intact melanosomes remain in the corneocytes of dark skin (Figure 2). Corneocytes of Asian skin appear intermediate between these two conditions in that relatively few melanosomes exist in the stratum corneum of Asian skin. In addition, it appears that individual melanosomes are more prominent than clustered melanosomes in the keratinocytes of the stratum granulosum of the Asian skin (Figure 2c), indicating that clustered melanosomes are degraded more efficiently than individual melanosomes during terminal differentiation of the epidermis.

Differences in the presence of melanosomes at the higher levels of the squamous epithelium. Interface between the stratum granulosum and the stratum corneum demonstrating few melanosomes (arrows) remaining in corneocytes/keratinocytes of dark skin (a), occasional melanosomes (arrows) remaining in the corneocytes (b) and keratinocytes (c) of the stratum granulosum of Asian skin, and no apparent melanosomes remaining in corneocytes/keratinocytes of light skin (d). Bar = 5.0 µm.

The mean size of individual and clustered melanosomes was determined as described in Materials and methods. The Zeiss LSM Image Browser version 2.30.011 allows the calculation of the area within a circled region using a stylus. The outer membrane of the melanosome defined that region. We observed a high degree of variation within each individual. Table 2 shows the mean areas of 200 random (individual and clustered) melanosomes per group. There was no statistically significant difference in the mean size of random melanosomes in the various age and sex groups of Asian skin. The sizes of 100 melanosomes that were distributed individually and of 100 melanosomes that were clustered within keratinocytes were subsequently determined for each group, as shown in Table 2 . The mean ± SD size of melanosomes that were distributed individually (1.49 ± 0.13 µm2 x 10-2) in all Asian groups was significantly larger that of melanosomes that were distributed in clusters (0.88 ± 0.20 µm2 x 10-2). No statistical difference was noted in the sizes of individual or clustered melanosomes among the different age and sex groups in the Asian population. The mean sizes of random individual and clustered melanosomes from African/American and Caucasian skin were determined as shown in Table 2 . There was a statistically significant difference between the sizes of individual and clustered melanosomes in both the African/American and Caucasian groups. The mean ± SD size of random melanosomes from African/American skin (1.44 ± 0.67 µm2 x 10-2) was significantly larger than that from Caucasian skin (0.94 ± 0.48 µm2 x 10-2). The mean size of the random melanosomes of Asian skin (1.36 ± 0.15 µm2 x 10-2) lay between values for African/American and Caucasian skin and was statistically different from the Caucasian skin but not the African/American skin. There seemed to be a trend indicating a progressive increase in mean melanosome size when moving from the Caucasian to the African/American skin types.

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