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Unpublished results indicate that rescue of the posterior region of the emb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.