Epidermal morphology, cell proliferation and repopulation in mouse skin during daily fractionated irradiation.

In mouse, a 2 cm long field of both legs was irradiated daily with 3 Gy, five fractions per week for up to 3 weeks. Epidermal morphology, the circadian variation of the labelling index, and repopulation were determined at the end of each week. After a decrease of epidermal cellularity in the first week, excessive regeneration led to hyperplasia and hypertrophy, which continued throughout the experimental period while the residual tolerance of the epidermis to top-up irradiation decreased progressively. Although the repopulation rate was accelerated five-fold, it was unable to compensate the effects of more than two-thirds of the given dose. The hypertrophic and hyperplastic response is attributed to pronounced acceleration of abortive proliferation of doomed stem cells.