*Revised with permission from: R Pugashetti and H Maibach, Pigmentation in Ethnic Groups, ch 52 in Aging Skin, MA Farage, KW Miller and HI Maibach, eds, New York: Springer (2010) pp 503–508.
The US Census Bureau estimates that by 2050, approximately half of the US resident population will be individuals of color.1 While human skin color is a trait that distinguishes people of different races, it remains unclear how variations in human skin pigmentation may contribute to differences in skin structure, function and pathophysiology. As formulators create products for varying ethnic backgrounds and with diverse skin types, an understanding of differences in pigmentation and skin structure and function becomes more important. This column reviews recent studies on the structural, genetic and ultraviolet (UV)-responsive differences in skin pigmentation to allow the formulator to create successful products for varying ethnicities and to accurately measure pigmentation.
Structural Differences in Pigmentation
Montagna and Carlisle examined the morphology of black and white facial skin among adult women ages 22–50,2 including factors such as atrophic spots, elastosis, fiber fragments, sweat glands and the condition of the stratum lucidum. Their observations were as follows:
Atrophic spots: Atrophic spots, or areas of the skin that appear thinner, were demonstrated frequently in white skin, while only 1 in 19 black women in the study demonstrated atrophic spots in the epidermis.
Elastosis: One of the key indices of dermal photodamage is the presence of elastotic material. Elastosis can be measured by looking at skin biopsy samples to identify the amount of elastotic fibers present. Black skin demonstrated minimal elastosis while white skin showed variable amounts of moderate to extensive elastosis. The authors speculated that greater numbers of melanosomes and distribution in black skin perhaps protect the epidermis from photodamage.
Fiber fragments: The dermis of black skin demonstrated more fiber fragments made of collagen fibrils and glycoproteins than white skin, as well as numerous and larger fibroblasts.
Sweat glands: Black skin showed more mixed apocrine-eccrine sweat glands and more blood and lymphatic vessels than white skin.
Stratum lucidum: Finally, Montagna and Carlisle2 also observed the stratum lucidum, the layer beneath the stratum corneum composed of 3–5 layers of keratinocytes, and found the stratum lucidum in black skin was unaltered by UV exposure, whereas white skin was usually distorted by UV exposure.
Melanosome size and distribution: Szabo et al.3 observed that overall, the most striking difference between black and white skin appears to be the size of melanosomes and their distribution pattern. When comparing the epidermis of black skin to white skin, black skin demonstrated more and larger singly distributed melanosomes in corneocytes and keratinocytes. In darkly pigmented skin, large melanosomes are surrounded by a membrane, whereas in lighter skin, smaller melanosomes are clustered together in a single membrane.3
Additionally, Toda et al. found that melanosomal packaging is closer to the basal layer in more darkly pigmented skin, compared with Caucasian skin.4 Regarding variations in human skin color, Szabo noted the density of pigment producing melanocytes in the skin, approximately 1,000/mm2, was not found to vary with ethnicity.5 Conversely, the ratio between eumelanin and pheomelanin synthesis was demonstrated by Thody to be higher in black skin than white skin.6
Thong et al. examined the patterns of melanosome distribution in keratinocytes in Asian skin, and compared these to light Caucasian skin and dark African-American skin.7 In this study, the distribution pattern of melanosomes transferred to keratinocytes in photoprotected skin (volar forearm) from normal Asian individuals was examined. Results demonstrated that melanosomes in the keratinocytes of Asian skin were distributed both as individual and clustered melanosomes; 62.6% as individual and 37.4% as clustered.
In dark skin keratinocytes, melanosomes were found to be predominantly individual (88.9%), whereas in light Caucasian skin keratinocytes, melanosomes were predominantly clustered (84.5%). Therefore, the melanosome distribution in Asian keratinocytes appeared to be an intermediate between light Caucasian and dark keratinocytes. In addition, the size of melanosomes appeared to vary with ethnicity; melanosomes in dark skin were the largest, followed by melanosomes of Asian and then Caucasian skin. Furthermore, melanosomes that were distributed individually tended to be larger than clustered, smaller melanosomes.
Keratinocyte regulation of melanosomes: Minwalla et al. examined in vitro the role keratinocytes play in regulating the distribution patterns of recipient melanosomes.8 Co-cultures of melanocytes and keratinocytes from different racial backgrounds were studied using electron microscopy. When keratinocytes from dark skin were co-cultured with melanocytes from either dark or light skin, the recipient melanosomes were found to be predominantly individual as opposed to clustered. However, when keratinocytes from light skin were co-cultured with melanocytes from dark or light skin, recipient melanosomes were predominantly clustered as opposed to individual. Therefore, recipient melanosomes are predominantly distributed in membrane-bound clusters from light skin keratinocytes and distributed individually by dark skin keratinocytes. Furthermore, melanosome size was not related to distribution. This work suggested that keratinocyte regulatory factors may determine how recipient melanosomes are distributed.
Epidermal melanin: Alaluf et al. examined the melanin content in photoexposed and photoprotected skin of various ethnic types including African, Indian, Mexican, Chinese and European.9 Lightly pigmented skin types—i.e., Mexican, Chinese and European—were found to have approximately half as much epidermal melanin as darkly pigmented skin types. Furthermore, the melanin composition among the lighter skin types was more enriched with lightly colored, alkali-soluble melanin pigments such as pheomelanin and eumelanin.
Epidermal melanin content was found to be greater in chronically photoexposed skin than photoprotected skin, regardless of ethnic background. This analysis, like previous studies, also demonstrated that melanosome size varies with ethnicity: African skin exhibited the largest melanosomes, followed by Indian, Mexican, Chinese and finally, European. Therefore, the amount and composition of melanin, and differences in melanosome size may all play roles in determining skin pigmentation.
Gluthathione: Halprin et al. reported that glutathione may play a role in the genetically determined differences in skin color among different races.10 This sulfydryl-containing epidermal compound plays a role in melanin formation. Halprin described that the tripeptide glutathione (γ-glutamyl-cysteinyl-glycine) is present in the human epidermis in sufficient concentrations to be the inhibitor of melanin formation from tyrosine by tyrosinase. Glutathione in its reduced state (GSH) as well as the enzyme glutathione reductase, which maintains GSH levels, were found in lower concentrations in African skin than in Caucasian skin.
Genetics of Skin Pigmentation
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.
Erythemal response also was examined in patients of different complexions by Olson.18 The minimal erythema dose (MED) was determined in Caucasian and in different complexions of African-American skin. Among light, medium, and dark complected African-American skin, no minimal erythema response typically was found. Instead, a spectrum of responses was found that was directly proportional to the degree of pigmentation. Additionally, the average MED of darker-complected African-American skin was 33 times greater than that of Caucasian skin. Caucasian skin had the smallest amount of pigment and smallest melanosomes, which were mostly contained within melanosome complexes.
Further findings included the observation that melanosome size is directly proportional to the intensity of skin pigmentation, and darkly pigmented subjects have larger, wider and denser melanosomes. Generally, with increasing pigmentation, the size of melanosomes, the proportion of singly dispersed melanosomes, and the MED were all shown to increase. Authors suggested that the increased resistance of darker skin to the damaging effects of UV radiation may therefore be due to larger, more light-absorbing, individually dispersed melanosomes. Furthermore, melanosomes in darkly pigmented skin are degraded less by lysosomes, resulting in more light-absorbing bodies in the stratum corneum.19
Measuring Skin Color
The assessment of skin color or the effects of treatments on skin color has advanced with improvements in measurement devices, as are described here. Most of these devices are based on the L*, a*, b* system to determine skin color. As is generally known, L* represents skin reflectance or lightness, a* measures color saturation from red to green, and b* measures color saturation from yellow to blue.
Chromameter: Lee et al. evaluated a chromametera for the objective measurement of periocular and facial pigmentation in African-American, Caucasian and Hispanic subjects.20 Using the L*, a*, b* system, significant differences in L* were observed among all ethnic groups, while a* and b* were less sensitive to pigmentation differences. Additionally, the L* value demonstrated significant differences between Fitzpatrick skin types III–VI, the more heavily pigmented groups. The chromameter was found to reliably measure facial pigmentation; in addition, it showed good inter- and intra-instrument reliability.
Tristimulus colorimeter vs. narrow-band reflectometer: Shriver and Parra compared two methods to measure pigmentation in both the skin and hair of European-Americans, African-Americans, South Asians and East Asians.21 A tristimulus colorimeterb, again based on the L*, a*, b* color system, was compared to a narrow-band reflectometerc, which measures pigment in terms of erythema and melanin indices. Both instruments provided accurate estimates of pigment level in the skin. However, measurements performed by the narrow-band reflectometer were less affected by the greater redness of specific body sites due to increased vascularization. Thus, while both approaches were found to be accurate, pigmentation measurements using the narrow-band reflectometer may be more useful.
Tristimulus colorimeter: Alaluf et al. examined the impact of epidermal melanin in different ethnic skin types on the objective measurements of human skin color22 taken by a tristimulus chromameter. L*, a*, b* measurements were made in European, Mexican, Chinese, Indian and African subjects. Overall, darker skin types tended to have lower L* values, higher a* values and higher b* values, compared with constitutively lighter skin types. Results demonstrated that total epidermal melanin is the primary determinant of L* values. Melanosome size also had a significant influence on L* values, and larger melanosomes are associated with a darker skin color, as previously discussed. Based on the strength of correlations observed in this study, epidermal melanin content still appears to play a greater role in determining skin color than melanosome size.
Many advances have been made in understanding the genetic, molecular and cellular differences underlying normal variation in human skin pigmentation. However, further studies must be carried out to investigate the genetic pathway underlying melanin synthesis, the role of genetic variation in epidermal pigmentation, and to elucidate differences in skin pathophysiology among humans from different ethnic backgrounds. Understanding the differences in pigmentation and skin structure and function among varying ethnic types can assist chemists and formulators in developing products to target the different needs of diverse skin types. Reproduction of the article without expressed consent is strictly prohibited.
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