Early neurogenesis in Amniote vertebrates

Labelling of Hensen's node in a B-somite stage chick embryo by the quail/ch ick chimera method has revealed that, while moving caudalwards as the embry o elongates, the node leaves in its wake not only the notochord but also th e floor plate and a longitudinal strand of dorsal endoderm. The node itself contains cells endowed with the capacity to yield midline cells (i.e, noto chord and floor plate) along the whole length of the neural axis. Caudal no de 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 sinc e, if removed, neither the notochord nor the floor plate, are formed caudal ly to the ablation. The embryo extends however in the absence of midline ce lls and a neural tube develops posterior to the excision. Only dorsal molec ular markers are detectable on this neural tube (e.g. Pax3 and Slug). The p osterior region of the embryo in which the structures secreting Shh are mis sing undergo cell death within the 24 to 48 hours following its formation. Unpublished results indicate that rescue of the posterior region of the emb ryo can be obtained by implantation of Shh secreting cells. One of the crit ical roles of floor plate and notochord is therefore to inhibit the cell de ath programme in the axial and paraxial structures of the embryo at gastrul ation and neurulation stages.