Cell Recovery in Foetal Brain after Ionizing Radiation

Summary

Timed-pregnancy rats were exposed on gestation day 13 to single doses of gamma (⁶⁰Co)-irradiation of from 25 to 200 R in increments of 25 R, and to split doses of 100 R plus a second dose of from 25 to 150 R in increments of 25 R, with a 9-hour interval between exposures. Additional animals were exposed to the same split-dose regimens but with induction of hypothermia-hypercapnia after the initial dose of 100 R (deep colonic temperature about 20°c) and recovery to normal temperature levels in about 5 ½ hours. Others received a single dose of 100 or 150 R followed by induction of hypothermia-hypercapnia but without exposure to a second radiation dose. The incidence and severity of tissue alterations on gestation day 19 generally varied directly as dose-level. Such alterations were clearly greater in single-dose than in split-dose groups at comparable total dose-levels. In split-dose groups with induction of hypothermia-hypercapnia between exposures, tissue damage was greater than in the split-dose group but somewhat less than in the single-dose group. Dose-response curves based on semi-log plots of mean cross-sectional area of cortical plate revealed a threshold (shoulder) zone on both the single-dose and split-dose curve. The latter suggests the possibility of cell recovery in the 9-hour interval between exposures.

Values for cross-sectional area of cortical plate in animals subjected to hypothermia-hypercapnia in the 9-hour interval between exposures were lower than those of the split-dose series. This suggests that repair processes in the interval between exposures may have been inhibited by the hypothermia-hypercapnia. Additional factors which may account, wholly or in part, for the observed results are discussed.