The influence of g₂ progression on x-ray sensitivity of two-cell mouse embryos

Naturally synchronous, two-cell mouse embryos were X-irradiated in vitro. In experiment 1, irradiation was either in the early or in the late G₂ phase, which lasts about 14 hours. In experiment 2, irradiation of all the embryos was in late G₂ but embryos with different intervals between irradiation and the first mitosis after irradiation were separated and investigated independently. After 2 Gy the time interval between irradiation in late G₂ and the first mitosis post-irradiation was on the average about 9 hours; after irradiation in the early G₂ phase about 13·5 hours. Development (hatching of blastocysts) and cell proliferation (cell number per embryo at the stage of the hatched blastocyst) was most impaired and the frequency of micronuclei (determined in four- or eight-cell embryos) was highest in the case of a short interval between irradiation in G₂ and the first mitosis post-irradiation. It is concluded that a longer interval allows a longer period of DNA repair. The results also demonstrate a positive correlation between the extent of chromosomal damage (micronuclei) and the extent of cell death as well as the impairment of the development of the whole biological system.