[Irradiation of mouse embryos in pronucleus and 2-cell stages: dependence of micronucleus formation and cell proliferation upon DNA amount and cell cycle phase].

1-or 2-cell mouse embryos were X-irradiated with 1.88 Gy. At irradiation time both pronuclei of each 1-cell embryo (see introduction) had a haploid DNA-amount and were in G1-phase. In contrast the cell nuclei of the 2-cell embryos had a tetraploid DNA-amount and were in late G2. DNA-amount and the cell cycle phases respectively were determined cytofluorometrically. The blastocyst formation was more impaired after irradiation of 1-cell embryos (28%) than 2-cell embryos (73%; controls: 100%). Cell death, which was observed in the cell proliferation investigations, should be the most important reason for the impaired early embryonic development. The different extent of cell death can be explained with the different amount of micronucleus formation. This chromosomal damage, which leads to a hypoploid DNA-amount of the cell nuclei, was more bulky after irradiation of 1-cell embryos than 2-cell embryos. Mechanisms which could cause a higher micronucleus formation after irradiation of haploid cell nuclei (pronuclei) in G1-phase than after irradiation of tetraploid cell nuclei in G2-phase are discussed.