Hyperthermia-induced exencephaly in mice: effect of multiple exposures.

Pregnant LM/Bc female mice were given a 10-minute hyperthermic exposure in a 43 degrees C waterbath during the period of neural tube closure. On day 15.5 of gestation, the females were killed, and the fetuses were examined for exencephaly. Following a single treatment on day 8.0, 8.5, 8.75, or 9.0 of gestation 1.7, 13.6, 2.9, and 0.8% of the respective fetuses displayed exencephaly. With two treatments, one on each of gestational days 8.5 and 8.75, or three treatments, one on each of gestational days 8.5, 8.75, and 9.0, the percentage of exencephalic fetuses increased to 28.3 and 59.3%, respectively. The increased response to multiple treatments was not due to an increase in the number of susceptible embryos but rather was due to the increased number of treatments. The results of this study suggest that with increasing numbers of treatments, the embryo's ability to recover from the hyperthermic exposure is lessened, resulting in an increase in exencephaly.