The human leukocyte test system. X. higher sensitivity to X-irradiation in the G0 stage of the cell cycle of early as compared to late replicating cells.

Leukocyte cultures were set up with X-irradiated whole blood (200 R). Cells starting with their DNA synthesis between 25 and 35 h after cultureinitiation ("early replicating cells") were pulse-labeled with tritiated thymidine ([3H]TdR). Mitoses were collected with colcemid in adjacent intervals from 36 up to 72 h after culture initiation. At fixation times of 50, 56, 62, and 72 h enough mitoses for a determination of the frequencies of chromosomal aberrations (dicentric and ring chromosomes) were found. After that the preparations were processed for autoradiography. All mitoses analyzed for chromosomal aberrations were re-analyzed for labeling, and the frequencies of chromosomal aberrations in labeled (="late replicating cells") mitoses were compared. At all fixation times, higher frequencies of dicentric chromosomes were found in labeled as compared to unlabeled mitoses, indicating a higher sensitivity of early replicating cells to X-irradiation in the G0 stage of the cell cycle.