The Embryonic and Fetal Effects in ICR Mice Irradiated in the Various Stages of the Preimplantation Period

Pregnant ICR mice were irradiated with 0.1-2.5 Gy 137Cs 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/fetal 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 postconception 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 stages. These analyses demonstrated that the regression slopes were significantly positive for groups in all stages and increased with decreasing time after conception. Furthermore, the regression analyses suggested that the most sensitive stage for preimplantation death and embryonic 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 with 0.1 Gy at 2, 72 and 96 h postconception. In the same manner as embryonic mortality, the regression analyses suggested that the susceptibility of the mice irradiated at 2, 72 and 96 h postconception during preimplantation to external malformations was higher than that of the mice irradiated at 8 or 11 days of gestation, which is the period of organogenesis, and that the most sensitive stage for external malformations was 2 h postconception. However, no malformations were observed in the mice irradiated at 48 h postconception when the embryos were precompacted with four to eight cells.