An in vitro technique to detect dominant lethal mutations induced in mouse oocytes by ethyl methanesulphonate...

Administration of chemical mutagens to the female rodent can induce dominant lethal mutations in oocytes and affect embryo development after fertilization. Traditional in vivo dominant lethal assays cannot separate specific genotoxic effects on the embryo from generalized cytotoxic effects. We have used embryo culture, after in vivo exposure of oocytes, to separate the genotoxic effects of a chemical on oocytes from effects due to maternal toxicity. Pre- and post-implantation development in culture was monitored in embryos recovered at the two-cell stage from females dosed ip, 30-32 hr before ovulation, with 125 or 250 mg/kg ethyl methanesulphonate (EMS)/kg body weight. In vitro zygote development progressed from two-cell to trophectoderm outgrowth and inner cell mass formation. All stages of development were affected by the EMS treatment. The morula stage showed a dose-related decrease in development; blastula formation and inner-cell mass formation were also significantly decreased. This study indicates that an in vitro dominant lethal test can be useful in evaluating damage to metaphase-1 oocytes. The in vitro test can be used to study the effects of chemicals on all stages of zygote development thereby separating induced genotoxic effects from the possible effects of maternal toxicity on zygote development.