A Comparison of the Melanocyte Response to Narrow Band UVA and UVB Exposure In Vivo

Abstract

The visible cutaneous pigmentary response to ultraviolet-A (UVA) is immediate and, following sufficient exposure, may persist, whereas ultraviolet-B (UVB)-induced pigmentation appears after a delay of several days. We compared the in vivo response of melanocytes to single and multiple exposures of narrow band UVA and UVB irradiation which produced visibly equal increases in pigmentation. Using a xenon-mercury source matched to a monochromator, human volunteers were exposed to 304 (+/- 5) and 365 (+/- 10) nm radiation. Biopsies were performed 1, 7, and 14 days after irradiation. For each biopsy, the number of melanocytes per square millimeter of epidermis was determined using L-3,4-dihydroxyphenylalanine (dopa)- and tyrosine-incubated split epidermal preparations. Vertical sections were also examined. At days 7 and 14, after both 304 and 365 nm radiation, melanocytes were more intensely dopa-positive than in unirradiated controls, and demonstrated enlarged perikarya and a greater number of enlarged dendrites. Following both 304 and 365 nm radiation the number of dopa-positive melanocytes was increased at days 7 and 14 by 44% and 58%, respectively. Tyrosine positivity, an indicator of enhanced tyrosinase activity and increased melanin formation, was absent in controls and at day 1, and became positive in all but one sample at day 7 and day 14. Therefore, one day after UVA exposure, visible pigmentation but not tyrosinase activity was increased. At day 7, the number of tyrosine-positive melanocytes approximately equaled the number of dopa-positive melanocytes. Although UVA and UVB induce different pigmentary responses, their effects on melanocyte number and function were indistinguishable.