EFFECTS OF X-IRRADIATION ON PREIMPLANTATION MOUSE EMBRYOS CULTURED IN VITRO

Summary. Irradiation of two-cell mouse embryos inhibits the differ-entiation of the blastocyst. Ifmorulae are irradiated, blastocyst formation occurs normally, but the embryos fail to hatch and become cytolytic in the zona pellucida. Russell (1950) and Russell & Russell (1954) have clearly shown that the in¬ cidence of embryonic death in irradiated pregnant mice is higher during the preimplantation period than during organogénesis. Irradiation during the preimplantation period causes neither neonatal death nor gross abnormalities in survivors. These results and others were recently reviewed (Russell, 1965) and cor¬ related with the effects on both the mother and the embryo (appearance of extranuclear chromatin or 'subnuclei', chromosome aberrations and sex-chromosome loss). It seems that the majority of the prenatal deaths occurring after irradiation of eggs cleaving in situ takes place before or at implantation. More recently, Snow (1973a) has shown a radiobiological effect, like nuclear fragmentations, of tritiated thymidine ([3H]TdR) on preimplantation mouse embryos. Most of the blastocysts grown in 0-05 µ methyl-T-TdR/ml totally lack an inner cell mass, these 'blastocysts ' being capable offurther differentiation but only as invasive trophoblast cells (Snow, 1973b). Few data are available on the radiosensitivity of preimplantation mam¬ malian embryos, especially regarding differentiation. The present study concerns the effects of X-rays on the primary differentiation of mouse eggs cleaving in vitro. Virgin albino random-bred female mice were induced to superovulate by intraperitoneal injection of 5 i.u. PMSG (Gestyl, Organon) followed 48 hr later by 5 i.u. HCG (Pregnyl, Organon) (Edwards & Gates, 1959). Ovulation was assumed to occur 12 hr (Time 0) after the second injection. Embryos were removed from the oviducts 35 hr after Time 0 (two-cell eggs) and were cultured in drops of culture medium under paraffin oil, as described by Brinster (1963). The culture medium was that of Whittingham (1971). Irradiation of the eggs was performed with an X-ray generator (Machlett