Fractionated-dose effect of X-irradiation on the induction of neural tube defects in mice.

This experiment examined whether mouse embryos have a radio-adaptive response to the induction of malformations. Pregnant Slc:ICR mice were exposed to 0.05 Gy X-radiation followed by 0.5 or 1.0 Gy 4 hr later on day 8 of pregnancy. To compare this with the effect of single doses, four other groups were exposed to 0.5, 0.55, 1.0 or 1.05 Gy. All dams were put to death on day 18, and fetuses were examined for malformations. The frequency of neural tube defects induced by the fractionated irradiation into 0.05 Gy and 1.0 Gy was significantly higher than that induced by single doses of 1.0 and 1.05 Gy. Among groups exposed to a dose split into 0.05 and 0.5 Gy, and single doses of 0.5 and 0.55 Gy, there was no significant difference in incidence of the malformation. The present findings suggest that mouse embryos are not adapted to X-irradiation, but become more susceptible following a low-dose exposure.