MORPHOLOGICAL AND MAPPING STUDIES OF THE PARANODAL AND POSTNODAL LEVELS OF THE NEURAL PLATE DURING CHICK NEURULATION

The morphology of the paranodal and postnodal levels of the neural plate as well as the fate of its cells was examined in chick embryos at stages 3-11. The morphology of the paranodal and postnodal levels of the neural plate closely resembles that of the prenodal neural plate. Furthermore, during shaping and bending of the neural plate, these levels undergo changes similar to those of the prenodal level. In short, the paranodal and postnodal levels of the neural plate consist of a pseudostratified columnar epithelium that thickens dorsoventrally and narrows mediolaterally and then undergoes localized furrowing and folding. Fate mapping revealed that at mid-neurula stages, the prospective hindbrain and spinal cord levels of the neuraxis flank the primitive streak. Hensen's node moves caudally with respect to these future neuraxial levels as it regresses during the latter stages of gastrulation. Cells of the medullary cord, the rudiment of the secondary portion of the neural tube, arise in the vicinity of the cranial portion of the primitive streak, near the caudal end of the postnodal levels of the neural plate. Thus, during stages of gastrulation and primary neurulation, the precursor cells of the primary and secondary portions of the neural tube (spinal cord) lie in close proximity to one another. This study provides new information on the morphology and extent of the paranodal and postnodal levels of the neural plate, the changes these areas undergo during shaping and bending of the neural plate, and the contributions of its cells to the primary and secondary levels of the neural tube, increasing our understanding of the complex events underlying avian gastrulation and neurulation.