PHOTOAGING IN SKIN OF COLOR
UV Reactivity and Photoprotection
The photoprotection conferred by melanin in darkly pigmented skin greatly influences the UV-induced differences observed in black and white skin. Epidermal architecture in black and white subjects supports this notion. One study demonstrated an intact, compact stratum lucidum in sun-exposed black skin, in contrast to a swollen, cellular stratum lucidum in sun-exposed white skin. Black skin rarely exhibited atrophy, while white skin had numerous focal areas of atrophy, necrosis, vacuoles, and dyskeratosis.23
Melanin clearly offers protection from UV light. It acts as a neutral density filter to reduce penetration of all wavelengths of light equally.76 In a study using skin samples from blacks and whites, investi- gators found that 5 times as much UV light reached the upper dermis of white skin when compared to black skin. The authors determined that the main site of UV filtration in white patients was the SC, compared to the malpighian layer in black patients. The average protection offered by melanin in black skin wascalculated to be equivalent to a sun pro- tective factor (SPF) of 13.4 compared to 3.4 for white skin. They concluded that the photoprotection observed in black skin was due to both increased melanin content and the unique distribution of melanosomes in dark skin.76
Another study examined biopsies in black and white skin before and after solar-simulating radiation (SSR). After SSR, white skin displayed epidermal and dermal DNA damage, an influx of neu- trophils, active proteolytic enzymes, and diffuse keratinocyte activation. Black skin only demonstrated DNA damage in the suprabasal dermis. This study of acute changes after SSR con- firms the significance of UV protection imparted by melanin.77
Racial differences in MED have been described. Darkly pigmented black skin has been determined to have an MED up to 33 times greater than that of individu- als with white skin.70 In Japanese sub- jects, MED has also been correlated with skin color. In this study, the investigators found that the greater the epidermal melanin content, the less severe the reac- tion to the sun.78 It is important to note, however, that darker skin is not always predictive of MED. As mentioned previ- ously, in darker skin, pigmentation does not consistently correlate with MED.4 Other factors may influence the ability to tan or burn in skin of color.
The process of skin tanning in differ- ent racial ethnic groups has been stud- ied. The most significant change noted after 1 MED exposure was an upward shift in the distribution of melanin to the middle layers of the epidermis. This change was most dramatic in darker skin. Such data provide the basis for a better understanding of tanning in the darker skin types.79
One study examined skin in Korean subjects and the cumulative response to sun exposure. Investigators compared constitutive and facultative (acquired) pigmentation in different age groups. Facultative pigment of sun-exposed skin in Caucasians appears to reflect cumula- tive lifetime UV exposure. In this study, constitutive pigment was highest during the first decade of life, decreased during the second decade, and was maintained during the third decade of life in Korean subjects. In contrast to Caucasians, fac- ultative pigmentation did not increase with age.80
Histologic Findings
Despite the photoprotection conferred by darker skin, chronologic aging has
been observed in black skin. In one study, older black subjects demonstrated flattening of the dermal–epidermal junc- tion when compared to younger sub- jects.73 Elastic fiber degeneration and an increase in the superficial vascular plexus were also noted in the aged group. The skin of older black subjects was also characterized by a decrease in the number of melanocytes.73
A study in older Thai patients with a history of high sun exposure also showed epidermal atrophy, cell atypia, poor polarity, and disorderly differen- tiation.81 In a study of Japanese patients, the relationship between skin phototype and facial wrinkling was examined. As expected, higher scores were recorded for deep wrinkles in individuals with Fitzpatrick SPT I. Interestingly, the same tendency was not demonstrated for fine wrinkle scores.82
Clinical Findings
The clinical signs of aging in skin of color have been described. In a study of French Caucasian and Chinese subjects, wrinkle onset was delayed by approxi- mately 10 years in Chinese women. Hyperpigmentation was a much more important sign of aging in Chinese women.83
In a study of Japanese and Caucasian patients, young Japanese patients had significantly lower wrinkle scores. The sagging score was also significantly lower in Japanese subjects older than 40 years when compared with Caucasian subjects. Lower face aging was more common in Caucasian subjects.84
In African Americans, the clinical signs of aging are less pronounced and tend to be delayed at least a decade when compared to Caucasian skin. Many patients complain of dark circles and hollowing beneath the eyes, while others experience lower eyelid bags. Lower-eyelid signs of aging usually start with midface aging during the 30s. In midface aging, the malar fat pad descends from its location overlying the infraorbital rim and accumulates along the nasolabial fold. This can lead to a hollowed appearance beneath the eyes and an apparent deepening of the nasolablial fold.85 It is important to note that these changes occur with intrinsic aging and are less related to photodam- age. Photoaging in darker skin is mani- fested primarily by uneven pigmenta- tion, which is one of the most common cosmetic complaints in skin of color. The presence of seborrheic keratosis and der-
matosis papulosa nigra is another com- mon clinical sign of aging in patients with skin of color.
Based on these studies, photoaging, although delayed, does occur in skin of color. Despite the significant protection offered by melanin in darker skin types, these data suggest that photoprotection should still be emphasized in patients with skin of color.