Cell cycle analysis and stimulation of rat kangaroo cells (PtK 2 ) after pulse labeling.

The phases of the cell cycle of Potorous tridactylus (PtK2) cells were determined in vitro by analysis of labeled mitoses for 37-½ hours after a tritiated thymidine pulse. The mean duration of DNA synthesis (tS) was 8 h. The mean duration of the gap phase before appearance of labeled mitoses was 5 h. Whereas the duration of the cell cycle (tC) based on analysis of labeled mitoses was 30 h, the doubling time (tD) derived from cell counts in the same cultures was only about 23 h. The analysis of the indices of labeled nuclei and mitoses suggests a stimulation of cells at the time of pulse labeling, which was maximal after beginning of the gap phase before DNA-synthesis, and possibly caused the observed difference between tC and tD.