An unusual radiation-induced g₂ arrest in the zygote of the BALB/c mouse strain

Female mice of the BALB/c strain were superovulated, mated with males of the same strain, and irradiated with 1 Gy of X-rays at hourly intervals during the first cell cycle of the embryos. Two types of effects were found in the embryos, depending on the time of X-irradiation. When irradiation was delivered between 14 and 21 h after human chorionic gonadotrophin (hCG) injection, cultured two-cell embryos developed normally up to the morula stage, where a high mortality occurred. On the other hand, when irradiation was given between 17 and 24 h after hCG injection, a high proportion of the eggs was unable to cleave and remained blocked at the one-cell stage. Cytofluorometric analysis of the pronuclear DNA content of uncleaved zygotes showed that DNA synthesis was unaffected by X-irradiation, and that they were blocked in G₂ phase of the first cell cycle. Similar studies on other strains, as well as reciprocal crosses between BALB/c and F₁(♀BALB/c X♂C₅₇ BLACK) mice showed that the 'one-cell block' is determined by the maternal genotype and results most probably from a direct action of X-rays on a radiosensitive cytoplasmic factor necessary for the first embryonic cell division, and appearing 17 h after hCG injection. A high proportion of blocked zygotes (30-40 per cent) recovered partially, cleaved with a delay of about 20 h, and died soon after, almost none of them being able to reach the blastocyst stage. At the time of maximum radiosensitivity, the LD₅₀ for development up to the blastocyst stage was 0·95 Gy.