Early neurogenesis in Amniote vertebrates

Labelling of Hensen's node in a 6-somite stage chick embryo by the quail/chick chimera method has revealed that, while moving caudalwards as the embryo elongates, the node leaves in its wake not only the notochord but also the floor plate and a longitudinal strand of dorsal endoderm. The node itself contains cells endowed with the capacity to yield midline cells (i.e. notochord and floor plate) along the whole length of the neural axis. Caudal node cells function as stem cells. They are responsible for the apical growth of the cord of cells that are at the origin of the midline structures since, if removed, neither the notochord nor the floor plate, are formed caudally to the ablation. The embryo extends however in the absence of midline cells and a neural tube develops posterior to the excision. Only dorsal molecular markers are detectable on this neural tube (e.g. Pax3 and Slug). The posterior region of the embryo in which the structures secreting Shh are missing undergo cell death within the 24 to 48 hours following its formation. Unpublished results indicate that rescue of the posterior region of the embryo can be obtained by implantation of Shh secreting cells. One of the critical roles of floor plate and notochord is therefore to inhibit the cell death programme in the axial and paraxial structures of the embryo at gastrulation and neurulation stages.