Studies on the Duration of DNA-synthesis and Mitosis in Irradiated and Regenerating Epidermis Cells in Mice, by Means of Tritium-labelled Thymidine

Abstract
1. The fraction of labelled mitoses was recorded in normal, irradiated and regenerating epidermis of the skin of a strain of hairless mice at short intervals after injection of tritiated thymidine. 2. Analysis of the results indicate that the G2 phase could be accomplished in about one hour. The time SDent in mitosis was about one hour for some cells, while the S phase required about 6 hours. 3. The total mean time spent in the S+G2+M phase appears remarkably similar in normal, irradiated and regenerating cells. This indicates that differences in the turnover-time of these cells are mainly related to changes in the G1 phase. Under conditions of rapid growth the G1 phase seems to be extremely short. 4. Analysis of different counts of labelled mitoses indicate considerable variations in the duration of the S, the G2 and the M phases. A number of cells appear to spend several hours in mitosis.

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