Radiation-sensitivity Differences within Cell-division Cycles during Mouse Cleavage

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Abstract
Acute x-rays (100 and 200 r) were administered to pregnant females at one of three times on day 0 (day of the copulation plug) or one of two times on day 1. The effects studied were embryonic mortality and sex-chromosome loss in survivors. On day 0, sensitivity is very high shortly after sperm entry (completion of second meiotic division) and again in early pronuclear stage, then becomes low in later pronuclear stage (probably post DNA synthesis). On day 1, sensitivity is relatively low in resting 2-cell stage, then becomes very high at the beginning of the second cleavage. Results suggest that death occurs at about the time of implantation and that chance determines whether or not an implantation reaction will occur. From the high proportion of embryos showing sub-nuclei after irradiation, it was concluded that most, if not all, of the death is the result of aneuploidy. The mechanism for aneuploidy production may vary depending on nuclear state at the time of irradiation. This probably accounts for differences in frequencies of induced death. Parallelism with dominant-lethal type of death is striking. The present results are not in conflict with earlier indications that autosomes and sex chromosomes have similar sensitivities to radiation-induced loss in zygote stages.