Variation in Radiosensitivity during the Cell-cycle of Oedogonium Cardiacum

Summary

Measurement of cell-viability of Oedogonium cardiacum to ⁶⁰Co radiation at increasing time during several cell-cycles shows large cyclic variations as the cells pass synchronously through a radiation sensitive stage. The beginning of this radiosensitive stage, which is referred to as R₀, can be correlated with the first appearance of a ring which forms at the distal end of the cell. Stage R₀ lasts 4 to 5 hours and ends before prophase. Mitotic stages from prophase to telophase are less sensitive to radiation than the R₀ stage. Preliminary results from dose—survival curves indicate a six-fold increase in radiosensitivity between cells irradiated in the resting stage and those irradiated in the sensitive stage R₀. From kinetic studies of single-cell irradiations, it is shown that the number of times non-survivors divide depends on the stage of the cells at the time of irradiation. Cells in the R₀ stage divide about 1½ times on the average, compared with 2½ times for cells in other stages.

Results of irradiation of short chains up to 4-cells long suggests that there is a radiation coupling-effect between cells in a chain. Further, the probability of survival of a short chain is lessened if any cell in the chain is in the R₀ stage at the time of irradiation.