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Dose-Dependent Shift of Apoptotic and Unaltered Melanocytes into the Dermis after Irradiation with UVA 1

Background: Intense UVA irradiation induces an increase in number, size and activity of intraepidermal melanocytes. The number of melanocytes and the activity of melanogenesis return to normal after cessation of irradiation. Objective: In this study, we aimed to clarify the mechanism of reduction of melanocytes by apoptosis to prevent an uncontrolled increase in melanocytes within the epidermis. Methods: The position of the melanocytes before and after UVA 1 irradiation was controlled by electronmicroscopy and histochemistry using Fontana-Masson staining. The status of apoptosis was demonstrated immunohistologically by the use of p53 and bcl2. Results: A dose-dependent shift of melanocytes into the corium could be demonstrated. At low irradiation doses (20 J/cm2) pendulous melanocytes protrude into the dermis without losing contact to the dermoepidermal basement membrane. Higher irradiation doses (60 J/cm2) lead to a total elimination of fibrillary degenerated, apoptotic or even morphologically intact melanocytes into the corium. Once transported into the corium, the melanocytes can be detected there for more than 4 years. Conclusion: This shift mechanism seems to regulate and control UV-induced proliferation of epidermal melanocytes.