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

Summary

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 spent 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 differential 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.