Analysis of cell kinetics during ultraviolet light-induced epidermal carcinogenesis in hairless mice.

plastic epidermis and in lesions of squamous cell carcinoma. The growth fraction in control unirradiated mice was also 100%. The duration of TV and S decreased progressively with induction and progression of the tumor. For normalappearing epidermis of irradiated mice and epidermis of control mice, TVand S were approximately 95 and 6.2 hr, respectively. With the induction of hyperplasia, TV and S were reduced to 43.75 and 4.45 hr, respectively, while even lower values were obtained for squamous cell carcinoma where TV = 16 hr and S = 3.05 hr. The labeling index in normal epidermis was 6.75% and increased progressively in hyperplastic epidermis (10.17%) and squamous cell carci noma (18.98%). These results indicate that ultraviolet light-induced epidermal tumor production in hairless mice is associated with a progressive shortening of the cell cycle.