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Variations in Pigmentation and Ultrastructural Skin Differences Among Ethnic Groups
By: Rupa Pugashetti, MD, Santa Clara Valley Medical Center, and Howard I. Maibach, MD, University of California School of Medicine
Posted: September 1, 2010, from the September 2010 issue of Cosmetics & Toiletries.
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In addition to structural factors that influence skin pigmentation, investigators recently have gained a better understanding for the genetic basis of normal variations in human skin pigmentation. Much of the research, carried out in animal models, has investigated genes involved in melanin synthesis. With the help of mouse coat color mutations, several biochemical melanin synthesis pathways have been studied and elucidated. In fact, more than 100 genes have been identified that affect mouse coat color,11 and many of these genes have corresponding human phenotypes.
One gene that affects normal variations in skin pigmentation is the melanocortin 1 receptor (MC1R) gene. Mutations in this gene affect pigmentation in humans, mice, cattle, horses, sheep, pigs and chickens, among other animals. Specifically, this gene product is in the melanocyte cell membrane and is the receptor for the α-melanocyte stimulating hormone. While polymorphisms in MC1R may play a vital role in shaping human pigmentation, this process is complex and multiple genes likely determine normal variation in skin pigmentation.12 In addition, some variation in human skin color is associated with variations in TYR and OCA2, two of the known pigmentation genes.13
More recently, studies have focused on SLC24A5, a putative cation exchanger that was originally studied in zebra fish. It is suggested to play a key role in human skin pigmentation.14 The SLC24A5 exchanger localizes to an intracellular membrane, likely the melanosome or its precursor. Variations in this gene may help to explain differences in pigmentation between European and African skin. Different variations of the exchanger may explain how melanosomes and melanosome clustering vary among different ethnicities.
Studies also have shown that the genes MATP, TYR and SLC24A5 may play a predominant role in the evolution of lighter skin in Europeans but not East Asians, suggesting there is a recent convergent evolution of lighter skin pigmentation in East Asians and Europeans.15 Interestingly, such data also suggests that European skin turned lighter approximately 6,000 to 12,000 years ago, contrary to the previous hypothesis of approximately 40,000 years ago.16
UV Radiation Response
Besides inherent pigment traits, mechanisms of skin’s response to external conditions were examined by Tadokoro et al., who looked at skin tanning in different ethnic groups.17 The effect of one minimal erythemal dose of UV radiation on skin was studied. Overall, the density of melanocytes present at the epidermal-dermal junction was not found to change significantly one week following UV light exposure, and this density was similar among different ethnic skin types. However, the distribution of melanin from the lower layers to the middle layers of the epidermis was more dramatic in darker skin than lighter skin following UV exposure.