A ROLE FOR HGF SF IN NEURAL INDUCTION AND ITS EXPRESSION IN HENSENS NODE DURING GASTRULATION/

It was previously shown (Roberts, C., Platt, N., Streit, A., Schachner , M. and Stern, C. D. (1991) Development 112, 959-970) that grafts of Hensen's node into chick embryos enhance and maintain expression of th e L5 carbohydrate in neighbouring epiblast cells, and that antibodies against L5 inhibit neural induction by such a graft. We now show that L5 is initially widely expressed in the epiblast, but as neural induct ion proceeds it gradually becomes confined to and up-regulated in the early neural plate. L5 can therefore be considered as a marker for cel ls that are competent to respond to neural induction. We also show tha t Hepatocyte Growth Factor/Scatter Factor (HGF/SF) promotes the expres sion of L5 by extraembryonic epiblast in collagen gels after overnight culture. Explants cultured for several days in the presence of HGF/SF , as well as explants of prospective neural plate, can differentiate i nto cells with neuronal morphology expressing neuronal markers. To inv estigate whether HGF/SF is expressed in the chick embryo at appropriat e stages of development, we produced specific cDNA probes and used the m for in situ hybridization. We find that at the primitive streak stag e, HGF/SF is expressed specifically in Hensen's node. We therefore pro pose that HGF/SF plays a role during the early steps of neural inducti on, perhaps by inducing or maintaining the competence of the epiblast to respond to neural inducing signals.