G₂ arrest in mouse zygotes after x-irradiation: Reversion by caffeine and influence of chromosome abnormalities

The effect of caffeine was studied on mouse zygotes blocked in the G₂ phase of the first cell cycle after X-irradiation. Caffeine (2 mm) effectively reversed the G₂ arrest when zygotes were incubated in its presence at the time when first mitosis normally takes place. This effect of caffeine was inhibited by cycloheximide (5 μg ml^-1). In embryos escaping the G₂ arrest the frequencies of chromosome aberrations varied as a function of the time of irradiation, showing a clear relationship with the varying rates of lethality occurring from the morula stage. Blocked zygotes suffered major chromosome damage: however, this did not appear to be the only cause of the G₂ arrest. Triploid zygotes were preferentially blocked, suggesting that nuclei contain the target for this X-ray effect. Mouse zygotes are a useful model for studies on G₂ arrest and normal cell division regulation.