Teratogenic and lethal effects of long-term hyperthermia and hypothermia in the chick embryo.

The teratogenic effect of maternal hyperthermia is well known in laboratory animals and is presumed to exist also in humans. The aim of our study was to describe the embryotoxic effect of long-term higher and lower incubation temperatures on the chick embryo. Chick embryos were incubated within days 1 to 9 at 12 different incubation temperatures ranging from 31 to 42 degrees C. On the basis of our results, we estimated that there are three upper and lower critical thresholds of the incubation temperature: the first thresholds are 31 and 42 degrees C, at which all embryos died; the second thresholds are 32 and 41 degrees C, at which all living embryos were malformed; the third thresholds are 33 and 40 degrees C, at which some of the living embryos were without structural malformations, but their weight was shifted down and up with lower and higher temperature, respectively. The incubation temperature of 37 to 38 degrees C was optimal. Typical malformations detected on day 9 of incubation were microphthalmia, gastroschisis, caudal regression syndrome, and hyperlordosis, all of which occurred in dead embryos several times more frequently than in living embryos. CNS malformations were only sporadically present on day 9, as most of specimens bearing CNS defects died during the first days of incubation.