X irradiation in G₂ phase of two-cell mouse embryos in vitro: Cleavage, blastulation, cell kinetics, and fetal development

Two-cell mouse embryos were X-irradiated in vitro at late G₂ phase with 0.12, 0.24, 0.47, 0.94, and 1.88 Gy and cultured to the hatching stage of blastocysts. Doses of 0.94 and 1.88 Gy impaired hatching of blastocysts and reduced the average cell number per embryo at the end of the in vitro incubation. However, some embryos survived and recovered after irradiation with these doses. They showed cell numbers in the range of control values (about 100 cells). Furthermore, expanded blastocysts microscopically unaffected by irradiation (0.94 and 1.88 Gy) were able to develop into normal, healthy mice after transplantation into foster female mice. Immediately after irradation, 1.88 Gy led to a G₂ block of two-cell embryos (about 4 hr). Due to the G₂ block the embryos proceeded with a delay through the cell cycle which followed irradiation. Fifteen hours after irradiation the cell cycle distribution of the irradiated cells differed from that of the controls. G₁ cells increased and S-phase cells decreased. In the first/second and third/fourth postradiation cell cycles, hypoploidic cells (DNA content < 2c) were found. It is assumed that these cells which lost DNA by forming micronuclei contributed to cell loss. Cell loss apparently occurred at the morula stage (about 20 cells per embryo) and later in the majority of embryos irradiated with 0.94 and 1.88 Gy.