Changes in Cell Proliferation Produced by 12 Roentgens of 60 Co Gamma Irradiation per Day in the Intestinal Crypt Cells of 100-, 400-, and 825-Day-Old BCF 1 Mice

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Abstract
The effects of 1, 3, and 10 days of exposure to 12 R of continuous Co60 gamma-irradiation per day on the duration of the generation time (GT) and on the length of the four phases of the cycle-G1, S, G2, and M-in the crypts of the mouse small intestine were studied in 100-day, 400-day, and 825-day BCF1 hybrid mice. Estimates of the time spent in the total GT and its four phases by the percent labeled mitotic figure method following the labeling of those in S were made with 3HTdR [trltiated thymidine]. These estimates show that exposure to 12 R of 60Co gamma-rays per day for 1 to 10 days reduces the generation time of the intestinal crypt cells in all three age groups. The reduction is accomplished in less than 3 days in 100-day and 400-day animals but may require a longer interval to reach its maximum in the 825-day mice. The reduction in GT is greatest in the old mice, but this is primarily due to the longer preirradiation generation cycle. The GT lengthens between 3 and 10 days in the two younger groups but remains the same in the older animals. All the GT changes are related to changes in G1 in all three age groups. The very early reduction in GT in all three age groups of mice after only 3 days of exposure to 12 R/day eliminates the selection hypothesis.