Defining subregions of Hensen's node essential for caudalward movement, midline development and cell survival

Hensen's node, also called the chordoneural hinge in the tail bud, is a gro up of cells that constitutes the organizer of the avian embryo and that exp resses the gene HNF-3 beta. During gastrulation and neurulation, it undergo es a rostral-to-caudal movement as the embryo elongates, Labeling of Hensen 's node by the quail-chick chimera system has shown that, while moving caud ally, Hensen's node leaves in its wake not only the notochord but also the floor plate and a longitudinal strand of dorsal endodermal cells. In this w ork, we demonstrate that the node can be divided into functionally distinct subregions, Caudalward migration of the node depends on the presence of th e most posterior region, which is closely apposed to the anterior portion o f the primitive streak as defined by expression of the T-box gene Ch-Tbx6L, We call this region the axial-paraxial hinge because it corresponds to the junction of the presumptive midline axial structures (notochord and floor plate) and the paraxial mesoderm, We propose that the axial-paraxial hinge is the equivalent of the neurenteric canal of other vertebrates such as Xen opus, Blocking the caudal movement of Hensen's node at the 5- to 6-somite s tage by removing the axial-paraxial hinge deprives the embryo of midline st ructures caudal to the brachial level, but does not prevent formation of th e neural tube and mesoderm located posteriorly, However, the whole embryoni c region generated posterior to the level of Hensen's node arrest undergoes widespread apoptosis within the next 24 hours. Hensen's node-derived struc tures (notochord and floor plate) thus appear to produce maintenance factor (s) that ensures the survival and further development of adjacent tissues.