Based on the hypothesis that acceleration of repopulation in skin during fractionated irradiation is triggered by an inflammatory response of the dermis to radiation-induced epidermal hypoplasia, we produced a mild erythema by exposure to UVB radiation before applying different X-irradiation schedules. At different times ranging from 6 h to 14 days after a single exposure to UVB radiation which caused a distinct erythema, a 2-cm skin field on the legs of mice was irradiated with either different single doses or five daily fractions of 3 Gy followed by different single top-up doses of 300 kV X rays. Skin reactions were scored daily for 4 weeks and the occurrence of moist desquamation was taken to construct dose-response curves and to calculate ED50 values. Five days after exposure to UVB radiation and later, radioresistance of epidermis to single and fractionated X irradiation was significantly increased. Results were analyzed using the linear-quadratic formalism to identify possible mechanisms for this UV-radiation-induced radioresistance. The data suggest that exposure to UVB radiation led to a gradual increase in the number of epidermal stem cells and their repopulation rate.

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