Mechanism of hyperthermia effects on CNS development: Rostral gene expression domains remain, despite severe head truncation, and the hindbrain/otocyst relationship is altered

To study the mechanism of hyperthermia on the development of the rostral ne ural tube, we used a model in which closely-staged presomite 9.5-day rat em bryos were exposed in culture to 43 degrees C for 13 min, and then cultured further for 12-48 hr. This treatment had little effect on the development of the rest of the embryo, but resulted in a spectrum of brain defects, the most severe being a lack of all forebrain and midbrain structures. Whole-m ount in situ hybridisation was used to monitor the expression domains of Ot x2, Emx2, Krox20, and hoxb1. These showed that there were no ectopic expres sion patterns, for any gene at any stage examined. Even in those embryos wh ich apparently lacked all forebrain and midbrain structures, there were exp ression domains of Otx2 and Emx2 in the most rostral neural tissue, and the se retained their nested dorso-ventral boundaries, showing that cells fated to form rostral brain were not wholly eliminated. Thus, heat-induced rostr al neural tube truncation is of a quite different mechanism from the respec ification proposed for retinoic acid, despite their very similar phenotypes . In the hindbrain region of treated embryos, we observed decreased intensi ty of Krox20, staining and an abnormal relationship developed between the p osition of hoxb1 expression and the otocyst and pharyngeal arches. In the m ost extreme cases, this domain was shifted to be more caudal than the rostr al edge of the otocyst, while the otocyst retained its normal position rela tive to the pharyngeal arches. We interpret this as a growth imbalance betw een neuroepithelium and overlying tissues, perhaps due to a disruption of s ignals from the midbrain/hindbrain boundary. (C) 1999 Wiley-Liss, Inc.