Ethnic variation in skin properties is an important topic because those variations may influence clinical differences in dermatologic disorders. Part I of this piece, published in the April issue of C&T magazine, began an update on the quantity and quality of dermatological knowledge on the skinmetrics of racial differences in skin structure and function. Skinmetrics such as transepidermal water loss, blood vessel reactivity, pH and morphology were discussed. Part II completes this update and offers an interpretation in reference to skin care for different ethnic groups.
It should be noted that because the definitions of race, ethnicity or skin of color are not well-established, different researchers may use different terms to describe ethnicity, complicating the interpretation and comparison of studies on ethnic differences. In this review article, the terms used by the authors of each referenced study have been maintained and thus these terms are not necessarily consistent from one study to the next.
Vellus Hair Follicles
Because follicular morphology and distribution may affect penetration of topical medications and consequent treatment response, Mangelsdorf et al.1 investigated vellus hair follicle size and distribution in Asians and African-Americans as compared to Whites.2 The distribution of follicle density at different body sites was the same, with the highest average density on the forehead and the lowest on the calf for all groups. However, follicular density on the forehead was significantly lower in Asians and African-Americans. These two groups also exhibited smaller values for potential penetration surface and follicular orifice on the thigh and calf regions.
The authors concluded that the significant ethnic differences in follicle structure and pattern of distribution emphasize the need for skin absorption experiments on different skin types to develop effective skin treatments.
Ethnic differences in the number of melanocytes, number of melanosomes, and morphology of melanosomes has been of great interest in working toward the development of objective definitions of skin color.
It is generally accepted that differences in skin color are caused more by differences in melanosome distribution, size and content than by differences in melanocyte number.3,4 Melanosomes in black skin have been observed as being larger and more individually dispersed than in lighter skin;5 large melanosomes are taken up individually by keratinocytes while small melanosomes are incorporated as aggregates.6
Thong et al.7 quantified the ethnic differences in melanosome size and distribution, finding an inverse relationship between clustering and the size of melanosomes; i.e., as melanosome size decreased, the percentage of clustered melanosomes versus individual melanosomes increased. At one extreme, African-American skin showed larger melanosomes; at the other, Caucasian skin showed smaller melanosomes, predominantly clustered. Asian skin displayed intermediate results. Alaluf et al.8 confirmed this trend by showing a progressive increase in melanosome size as ethnic skin went from lighter to darker in European, Chinese, Mexican, Indian and African subjects. Furthermore, dark skin contained more total melanin and a larger fraction of dihydroxyindole-enriched (dark-colored) eumelanin.
Although there is evidence for objective differences in skin color, it remains unclear what role these differences in melanin and melanosomes play in dermatologic disorders. Clinical implications of these skin properties are discussed in the following two sections.
Antimicrobial properties: In 2001, Mackintosh9 reviewed evidence that melanization of skin and the innate immune defense system are genetically and functionally linked, postulating that the evolution of black skin could represent high pressures from infection. Citing evidence that melanocytes and melanosomes exhibit antimicrobial activity and are regulated by known mediators of inflammatory response, he asserts that a major function of melanocytes, melanosomes and melanin is to inhibit the proliferation of bacterial, fungal and other parasitic infections.
People of African descent have been shown to be less susceptible than Whites to scabies, fungal dermatophytosis, cutaneous Candida albicans infections and bacterial pyodermas.9 Additionally, although Rebora and Guarrera10 demonstrated increased skin microflora in Blacks, they found that the severity of dermatitis in these subjects was significantly less, suggesting the possibility of increased barrier defense. This evidence may also explain the existence of melanocytes and melanization among different parts of the body independent of sun exposure, as in the genitalia.
The presented evolutionary data is compelling and indicates a necessity for controlled studies to clarify whether melanization can affect the antimicrobial properties of skin and influence the propensity for certain skin diseases.
Photodamage: The most extensively studied function of darker skin color has been in resistance to photodamage from UV radiation. End effects of photodamage include skin cancer, which are well-documented as affecting lighter-skinned individuals more than those with darker skin.
In determining a relationship between melanosome groupings and sun exposure, studies have shown that dark-skinned Whites have nonaggregated melanosomes when exposed to sunlight, whereas light-skin, unexposed Whites have aggregated melanosomes; a similar trend is found in Asian skin.11,12 Alaluf et al.8 noted an increase in melanosome size directly correlating with epidermal melanin content in photoexposed skin versus photoprotected skin in all ethnic groups, suggesting increased melanogenesis in photoexposed areas. Van Nieuwport et al.13 demonstrated that with increased melanogenesis, light skin melanosomes show no significant change in surface area, while dark skin melanosomes show an increase in surface area and volume. Thus, although all skin types show an increase in epidermal melanin with sun exposure, both distribution and morphology may influence unequal filtering between light and dark skin types.
Ethnic differences in patterns and kinetics of DNA damage in response to UV radiation have also been observed. For example, Rijken et al.14 found DNA damage in epidermal and dermal cells, an influx of neutrophils, active photo-aging-associated proteoytic enzymes, and keratinocyte activation in white skin after UV exposure; meanwhile, aside from DNA damage in the suprabasal epidermis, there were no changes found in black skin. Similarly, Tadokoro et al.15 found that immediate DNA damage levels after irradiation were higher in Whites and Asians than in Blacks and Hispanics. However, the kinetics of DNA damage removal differed among individual subjects, showing no association between melanin content or ethnic group and DNA repair rates.
The authors noted that other properties of melanin, such as antioxidant properties and radical scavenging properties, may play roles in minimizing the ultimate level of UV damage. Ethnic differences in expression of receptors involved in melanosome uptake and melanocyte-specific proteins, both before and after UV exposure, are also being investigated.
Conclusion and Treatment Protocols
The US Census Bureau estimates that the population is composed of 12.1% Black or African-American, 13.9% Hispanic or Latino, and 11.9% other non-Whites.16 It has been predicted that individuals with skin of color will constitute a majority of the United States and international populations in the 21st century.17 In light of these statistics, objective investigation of relationships between ethnicity and differences in structure and function of skin becomes important for developing appropriate treatment protocols.
A summary of the conclusions drawn from Parts I and II is presented in Table 1. However, it is sometimes difficult to interpret studies on ethnic differences because each study may use different definitions of race or ethnicity. Race seems to encompass genetic variations selected to facilitate adaptations to a particular environment, which include but are not limited to pigmentation.18,19 Ethnicity, on the other hand, is a more general term, defined as how one sees oneself and how one is seen by others as part of a group on the basis of presumed ancestry and sharing a common destiny, often with commonalities in skin color, religion, language, customs, ancestry and/or occupation or region.19 Thus, ethnicity overlaps with race but also depends on more subjective and cultural factors, while race seems to encompass genetic variations based on natural selection.20 Nevertheless, studies have shown differences based on ethnic categorizations.
The FDA currently recommends inclusion of more ethnic groups in dermatologic trials, citing evidence that physiological differences in skin structure between ethnic groups can result in varying efficacies of dermatologic and topical treatments.16 Objective data on ethnic differences in skin properties not only emphasizes the value of investigation of disease processes and treatment responses in ethnic skin, but also highlights the growing list of physiologic variables involved. Future therapeutic studies could be strengthened by the inclusion of detailed definitions of how subjects are designated to a particular race or ethnic group in addition to skin phototype and would enable more reliable comparisons of results from different studies.
Taken together, these tantalizing observations and metrics suggest that the time for ethnic-related, evidence-based skin care is at hand. A focused research group could reach this goal within the decade; the fundamentals are becoming clearer and should permit such development.
1. S Mangelsdorf, N Otberg, HI Maibach, R Sinkgraven, W Sterry and J Lademann, Ethnic variation in vellus hair follicle size and distribution, Skin Pharmacol Physiol 19 159–167 (2006)
2. N Otberg, H Richter and H Schaefer, Variations of hair follicle size and distribution in different body sites, J Invest Dermatol 122 14–19 (2004)
3. S Alaluf, K Barrett, M Blount and N Carter, Ethnic variation in tyrosinase and TYRP1 expression in photoexposed and photoprotected human skin, Pigment Cell Res 16 35–42 (2003)
4. T Tadokoro, Y Yamaguchi, J Batzer, SG Coelho, Z Zmudzka, SA Miller, R Wolber, JZ Beer and VJ Hearing, Mechanisms of skin tanning in different racial/ethnic groups in response to ultraviolet radiation, J Invest Dermatol 124 1326–1332 (2005)
5. G Szabo, AB Gerald, MA Pathak and TB Fitzpatrick, Racial differences in the fate of melanosomes in human epidermis, Nature 222 1081–1082 (1969)
6. K Konrad and K Wolff, Hyperpigmentation, melanosome size, and distribution patterns of melanosomes, Arch Dermatol 107 853–860 (1973)
7. HY Thong, SH Jee, CC Sun and RE Boissy, The patterns of melanosome distribution in keratinocytes of human skin as one determining factor of skin colour, Brit J Dermatol 149 498–505 (2003)
8. S Alaluf, D Atkins, K Barrett, M Blount, N Carter and A Heath, Ethnic variation in melanin content and composition in photoexposed and photoprotected human skin, Pigment Cell Res 15 112–118 (2002)
9. J Mackintosh, The antimicrobial properties of melanocytes, melanosomes and melanin and the evolution of black skin, J Theoretical Biology 211(2) 101–113 (2001)
10. A Rebora and M Guarrera, Racial differences in experimental skin infection with Candida albicans, Acta Derm Venereol (Stockh) 68 165–168 (1988)
11. S Taylor, Skin of color: Biology, structure, function, and implications for dermatologic disease, JAAD 46(2) S41–S62 (2002)
12. G Richards, C Oresajo and R Halder, Structure and function of ethnic skin and hair, Dermatologic Clinics 21(4) 595–600 (2003)
13. F Van Nieuwpoort, N Smit, R Kolb et al, Tyrosine-induced melanogenesis shows differences in morphologic and melanogenic preferences of melanosomes from light and dark skin types, JID 122(5) 1251 (2004)
14. F Rijken, L Bruijnzeel, H van Weelden and R Kiekens, Responses of black and white skin to solar-simulating radiation: differences in DNA photodamage, infiltrating neutrophils, proteolytic enzymes induced, keratinocyte activation, and IL-10 expression, JID 122(5) 1251–1255 (2004)
15. T Tadokoro, N Kobayashi, BZ Zmudzka, S Ito, K Wakamatsu, Y Yamaguchi, KS Korossy, SA Miller, JZ Beer and VJ Hearing, UV-induced DNA damage and melanin content in human skin differing in racial/ethnic origin, FASEB J 17(9) 1177–1179 (2003)
16. US Census Bureau, Profile of general demographic characteristics (2003)
17. Populations Projections Program, Population Division, US Census Bureau. Projections of the resident population by race, Hispanic origin, and nationality, Middle series 2050–2070 (2000)
18. J Chan, A Ehrlich, R Lawrence, A Moshell, M Turner and A Kimball, Assessing the role of race in quantitative measures of skin pigmentation and clinical assessments of photosensitivity, JAAD 52(4) 609–615 (2005)
19. GM Oppenheimer, Paradigm lost: race, ethnicity, and the search for a new population taxonomy, Am J Public Health 91(7) 1049–1055 (2001)
20. NO Wesley and HI Maibach, Racial (ethnic) differences in skin properties: the objective data, Am J Clin Dermatol 4(12) 843–860 (2003)