The effect of Co60‐radiation on development and DNA synthesis in the 11‐day rat embryo

Female albino rats of a Wistar strain were exposed to either 135 r or 270 r of Co60‐radiation on the morning of day 11 (Witschi stage 17). Autopsy on day 20 revealed that the lower dose produced a slight increase in the degree of resorption, no effect on fetal weight and no malformations in the survivors. The higher dose, on the other hand, produced a large increase in the degree of resorption, a pronounced effect on fetal weight and many malformations. Radioactive phosphorus (P32) was injected into irradiated and non‐irradiated pregnant females to study DNA synthesis in individual litters after irradiation. Isotopically‐labeled DNA was extracted from embryo homogenates by a modified Schmidt‐Thannhauser procedure. DNA synthesis was suppressed in both series of irradiated embryos at 3, 6 and 12 hours after irradiation, the higher dose producing the larger amount of suppression. At 24 hours after irradiation, no significant difference was observed in the degree of DNA synthesis in control and irradiated embryos. No difference was observed between the specific activity of P32 in the acid‐soluble fractions of placenta and embryo in control and irradiated litters, indicating that suppression of DNA synthesis reflected a change in the cellular population which is synthesizing DNA. Histological criteria indicate that this initial effect is cellular death and that the embryo compensates for this damage by regulatory processes.