Relationship between abnormal somite development and axial skeletal defects in rats following heat exposure

The effects of in vivo heat exposure on gestation day (GD) 10 rat embryos were evaluated on GD 11 to determine the relationships between morphological sequelae following in vivo and in vitro exposures and between effects detected on GD 11 and those observed in postnatal day (PND) 3 pups. Anesthetized rats were exposed to 42 degrees C in a warm air incubator until their rectal temperatures reached 41 degrees C or until a rectal temperature of 42-42.5 degrees C had been maintained for 5 minutes. Heat-exposed embryos exhibited a significant decrease in growth parameters including head length, somite number, and protein content/embryo versus controls. These changes correlated well with in vitro effects from an earlier study (G.L. Kimmel et al., '93). Among the morphological endpoints which were slightly delayed in development were the caudal neural tube, branchial bars, forelimb and hindlimb. The only effect on the embryos that could not be explained as a transient delay in development induced by heat was the induction of unsegmented somites. Additional embryos were exposed to 42 degrees C for 15-20 min in vitro and examined specifically for unsegmented somites, which were observed in 47% of embryos exposed to 42 degrees C in vivo or in vitro. This phenomenon was observed in somites 9-20, i.e., those that give rise to cervical and thoracic vertebrae and ribs. These results correlated well with the axial skeletal malformations observed in PND 3 pups exposed to the same heat treatment (C.A. Kimmel et al., '93).