EARLY STAGES OF NOTOCHORD AND FLOOR PLATE DEVELOPMENT IN THE CHICK-EMBRYO DEFINED BY NORMAL AND INDUCED EXPRESSION OF HNF-3-BETA

We have cloned a cDNA encoding the chick HNF-3 beta gene and have used RNA and antibody probes that detect HNF-3 beta to monitor the normal and induced expression of the gene in early embryos. HNF-3 beta is exp ressed in Koller's sickle, at the onset of primitive streak formation, and later in Hensen's node. At neural plate and neural tube stages, H NF-3 beta is expressed transiently in the notochord and is then expres sed by door plate cells. Prospective floor plate cells that are locate d in the epiblast immediately anterior to Hensen's node prior to its r egression do not express HNF-3 beta, providing evidence that floor pla te fate is normally determined only after these cells populate the mid line of the neural plate and overlie the notochord. Removal of the not ochord in vivo prevents floor plate development and in this condition HNF-3 beta is not expressed by cells at the ventral midline of the neu ral tube. Notochord grafts induce ectopic floor plate development and ectopic neural expression of HNF-3 beta. In vitro, neural plate explan ts are induced to express HNF-3 beta by notochord cells in a contact-d ependent but cycloheximide-resistant manner, providing evidence that e xpression of HNF-3 beta is a direct response of neural plate cells to notochord-derived inducing signals. (C) 1995 Academic Press, Inc.