Influence of gestational age at exposure on the prenatal effects of gamma-radiation

The abdominal region of pregnant Swiss albino mice was exposed to single dose of 0.5 Gy gamma-radiation at gestation days from 1.5 to 17.5 days post-coitus (p.c.). The animals were sacrificed on day 18 p.c. and foetuses were examined for resorption and embryonic death, foetal death, growth retardation, small head, low brain weight, microphthalmia and any other gross morphological abnormalities. The period of maximum sensitivity for each effect varied. The only demonstrable effect of irradiation during the preimplantation period was an increase in prenatal mortality. Resorptions were maximal after exposure between days 2 and 4 p.c. The pre-implantation irradiated embryos which survived did not show any major foetal abnormalities. These results confirm earlier mouse studies using higher doses of X-rays. Small head, low brain weight and microphthalmia were prominent after exposure during the late organogenesis period, especially between days 9 and 13 p.c. But no other externally visible anomalies were detected. These findings demonstrate that mouse organogenesis is very sensitive to radiation-induced retardation of development, even at doses < I Gy. One exencephaly, one cleft palate and two cases of open eyelids were observed in the foetuses exposed on days 14.5 and 15.5 p.c.; the number of these cases was too small to indicate a causal relationship with exposure.