A detailed study has been made of the proliferation kinetics (in particular of the cell cycle time) of chemically induced squamous carcinomas and of the normal epithelium of the cheek pouch of the Syrian hamster. Several different methods, using tritiated thymidine labeling in vivo and standard autoradiographic techniques, were employed to measure the parameters of the cell cycle of the normal and malignant cells both before and after irradiation. The mean length of the cell cycle of the tumor cells was found to be approximately 12 hours and the mean growth fraction was estimated as 31%. From a comparison of the potential doubling time of the tumors with their measured doubling time, a mean cell loss factor of 0.75 was estimated. The range of values obtained for the cell cycle time of the normal epithelial cells was 130 to 152 hours. Following irradiation of the tumors with doses of 500 or 1000 rads it was found that the cell cycle time of the proliferating cells was slightly lengthened, the elongation being primarily a result of increases in the durations of the G1 and G2 phases. On the other hand, the same doses delivered to the normal epithelium produced a 20-35% shortening of the cell cycle which persisted from 3 days to approximately 3 weeks after irradiation. This shortening was almost entirely due to a reduction in the length of the G1 phase of the cell cycle.

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