[en] Investigations in plants suggest that cytoplasmic growth during mitotic delay induces an early attainment of terminal differentiation and cessation of mitotic activity. In mammals a direct demonstration of these processes is difficult. Plants and mammals show, however, a common phenomenon: Polyploidy does not usually reduce radiosensitivity as drastically as predicted by genetical considerations and certain experimental results. In root meristems of barley it is shown that cytoplasmic growth during mitotic delay increases the amount of cytoplasma per nuclear genome to approximately the same levels in tetraploid as in diploid cells. This results in the same loss, for both ploidy levels, of meristematic cells due to early differentiation. Apparently, under usual conditions, polyploidy is unable to significantly reduce radiosensitivity because the induction of differentiation processes is more important to radiation damage than the direct effect of genetic damage. Since the same basic principles also occur in mammals, it is suggested that early differentiation, and thereby cell sterilization, are induced in mammalian cells by the same mechanism as in plants. (Auth.)

[en] The sensitivity of CFU-S 7D mammalian cells to 0.1 Gy γ-irradiation was studied following pre-irradiation with the same dose. It was observed that with a time interval of 30 minutes between two exposures to 0.01 and 0.1 Gy, enzyme inactivation and recovery was enhanced. With an interval of 4 hours the cells responded to the second irradiation as if they had not been exposed. There was thus a radiation induced resistance against a second dose - regarding inactivation of thymidine kinase. With an interval of 12 hours, the cells reacted to the second dose again, similarly to a single dose. (UK)