Irradiation induced cell death as related to cell cycle.

Non-viable cells can be identified by their low density. In a Percoll density gradient we found in ascites tumour cells three fractions of cells. The first of these fractions at a low density of 1.020 g/ml contained morphologically dead cells and cells with a plating efficiency of less than 10 per cent, while the other two fractions contained pure viable cells. The proportion of cells in the cell cycle of the three fractions was measured using DNA flow cytometry. After whole-body irradiation with 5 Gy roentgen rays of ascites-bearing mice we calculated the dead cells in the various parts of the cell cycle up to 72 hours. The proportion of non-viable cells increased from about 5 per cent to about 20 per cent 20 to 25 hours after irradiation. The majority of these cells was in G1 and released from the irradiation induced G2-blockage. In addition, an increased number of non-viable S-phase cells preceded this G1 cell by some hours. These cells may represent cells damaged in G1 and S-phase at the time of irradiation and incapable of progressing further in the cell cycle.