The effects of psoralen and UVA (PUVA) on epidermal melanocytes of the tail in C57BL mice.

The morphological and numerical changes in epidermal melanocytes in tail skin of C57BL mice after exposure to psoralen followed by UVA irradiation were studied. The dorsal aspect of the tail was painted daily for 5 days a week with 0.1 ml methoxsalen (8-methoxypsoralen) in acetone, followed by UVA irradiation (peak 365 nm) after 2 hr with increasing energies up to a maximum of 2.0 J/cm2 daily. Control animals received UVA or acetone treatment only. The total duration of the experiment was 8 weeks. Biopsies taken after 1 and 3 days, 1, 2, 4, 6 and 8 weeks were studied by the combined skin-splitting DOPA techniques. After 2 weeks of PUVA treatment the original brick like arrangement of melanocytes was obliterated and replaced by a confluent layer of melanocytes, and after 4 weeks the number of melanocytes had tipled and these cells also appeared on the epithelium covering the pilosebaceous units. UVA treatment alone did not show these effects. Inflammation and denuding of the skin after 4 weeks was followed by regeneration of the epidermis and the melanocyte system, in spite of continuing PUVA treatment.