Early persistent UVA-pigmentation: ultrastructural and morphometric analyses

UV-A-induced skin pigmentation was investigated morphologically in semithin and thin sections from 11 volunteers, using different irradiation modalities (single doses of 10, 50 and 100 J/cm2). Visible skin pigmentation was observed in all but two probands, and erythema in two; pronounced pigmentation was present after repeated irradiation only. Contralateral non-irradiated, UV-B-irradiated and suntanned skin specimens UV-B-irradiated and suntanned skin specimens were used as controls. There was an increase in the number of clear cells in the basal layer (x1.6) and particularly of large clear cells (x1.7) after repeated irradiation. Also, the number of melanosomes in melanocytic dendrites (x2.8) increased after repeated irradiation. The number, size and shape of the melanosome complexes in both basal and suprabasal keratinocytes remained unchanged, even when a distinction was made between central and peripheral location. In contrast, suntanned skin showed an increase in melanosome complexes in basal (x5.8) and suprabasal (x3.7) keratinocytes. It is concluded that UV-A-induced skin pigmentation differs in some ways from UV-B or sun-induced melanogenesis, and that the clinical grade of tanning cannot accurately be determined by ultrastructural methods.