In vitro and in vivo X‐irradiation of preimplantation mouse embryos

Two‐day‐old strain 129/SvSl mouse embryos obtained by induced superovulation were exposed to various doses of X‐rays in vitro and in vivo and cultured from the 4‐cell to the blastocyst stage. The LD 50 for blastocyst development 48 h after in vitro irradiation was 300 R and following in vivo irradiation was 600 R. Blastulation occurred in over 80% of the cultured sham‐irradiated control embryos. Treated and control embryos that blastulated after 48 h of culture were transplanted to nulliparous adult 2‐day pseudo‐pregnant 129/SvSl females and examined 15 days later. Approximately 70% of implanting control blastocysts developed into fetuses. Significantly fewer fetuses developed after in vitro exposure to more than 73 R. No fetal development occurred following in vitro irradiation with more than 170 R. No indication of implantation was observed for embryos irradiated in vitro with more than 267 R. Fetal development occurred following in vivo irradiation with 388 R. It appears that it required twice as much in vivo irradiation as in vitro irradiation to produce equivalent lethality for the early mouse embryo. No significant gross abnormalities were observed in preterm fetuses following embryonic irradiation in vitro or in vivo, but there was significant weight‐gain impairment in fetuses developing from transplanted embryos irradiated in vitro with 73 R or more.