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

<b>Background:</b> 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. <b>Objective:</b> 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. <b>Methods:</b> 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. <b>Results:</b> 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. <b>Conclusion:</b> This shift mechanism seems to regulate and control UV-induced proliferation of epidermal melanocytes.