Adaptive response in embryogenesis: I. Dose and timing of radiation for reduction of prenatal death and congenital malformation during the late period of organogenesis.

An adaptive response was demonstrated during embryogenesis in mice. Whole-body irradiation at a dose of 0-50 cGy was given to condition pregnant ICR mice on day 9 to day 11 of gestation. Then their whole bodies were exposed to a challenging dose of 5 Gy on the next day. The numbers of living fetuses, prenatal deaths and living fetuses with external gross malformations were determined on day 19. A conditioning dose of 30 cGy on day 11 significantly increased the rate of living fetuses and reduced the incidence of congenital malformations induced by a 5-Gy dose on day 12. This indicates the existence of a critical dose and timing for administering a conditioning dose for radioadaptation during the late period of organogenesis in mice. The possible mechanisms involved are discussed.