Anomalous X-ray Survival Kinetics in HeLa Cell Populations

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Abstract
Cultures of HeLa S3 cells which have grown for 3 days in a standard growth-medium containing human serum show a rapid drop in sensitivity to x-rays immediately after trypsinization. They subsequently undergo a further transient decrease in sensitivity lasting about one day before slowly returning to the pre-trypsinization level of sensitivity about 2 days later. If grown in medium lacking human serum but otherwise identical to the usual medium, only the transient sensitivity change is observed. The latter is accompanied by changes in the rate of DNA-synthesis and in mitotic index, and appears to result from a partial synchronization of cell-growth brought about by the trypsinization procedure, with consequent redistribution of cells among the various sensitivity states that are characteristic of different periods of the cell-division cycle. The longer-term increase in sensitivity that occurs in medium containing human serum is correlated with alterations in the duration of the various phases of the cycle of DNA-synthesis. Hence it, too, would appear to arise from a redistribution of cells among the various inter -phase sensitivity states, although changes in the absolute sensitivity of one or more of these states may occur as well. No evidence was found for the occurrence of cell interactions that could lead to cooperative killing of sub-lethally damaged cells.