THE EMBRYONIC AND FETAL EFFECTS IN ICR MICE IRRADIATED IN THE VARIOUSSTAGES OF THE PREIMPLANTATION PERIOD

Pregnant ICR mice were irradiated with 0.1-2.5 Gy Cs-137 gamma rays at a dose rate of 0.2 Gy/min at 2, 48, 72 or 96 h postconception. In the mice irradiated during these stages of preimplantation, embryonic/fet al mortalities, incidence of external gross malformations, fetal body weight and sex ratio were observed at day 18 of gestation. There were significant increases in death in the preimplantation period compared to control levels after exposure to at least 0.25 Gy at 2 and 72 h pos tconception and 0.5 Gy at 96 h postconception. In contrast, a dose of 1.5 Gy was required at 48 h postconception. The frequency of embryonic death was analyzed using a logistic regression for comparing among st ages. These analyses demonstrated that the regression slopes were sign ificantly positive for groups in all stages and increased with decreas ing time after conception. Furthermore, the regression analyses sugges ted that the most sensitive stage for preimplantation death and embryo nic death was 2 h postconception, when embryos consisted of one cell. Many types of external gross malformations, such as exencephaly, cleft palate and anophthalmia, were observed even in the mice irradiated wi th 0.1 Gy at 2, 72 and 96 h postconception. In the same manner as embr yonic mortality, the regression analyses suggested that the susceptibi lity of the mice irradiated at 2, 72 and 96 h postconception during pr eimplantation to external malformations was higher than that of the mi ce irradiated at 8 or 11 days of gestation, which is the period of org anogenesis, and that the most sensitive stage for external malformatio ns was 2 h postconception. However, no malformations were observed in the mice irradiated at 48 h postconception, when the embryos were prec ompacted with four to eight cells. (C) 1997 by Radiation Research Soci ety.