Growth and development of pattern in the cranial neural epithelium of rat embryos during neurulation

The pattern of growth and morphogenesis of the cranial neural epithelium of rat embryos during neurulation is described. Transverse sections of the midbrain/hindbrain neural epithelium at different stages (0-14 somites) show a constant area and cell number throughout neurulation, even though there is a high level of mitosis. Mitotic spindles are orientated parallel to the long axis of the embryo, so that increase in cell number occurs in this direction only. Growth is expressed only as an increase in size of the forebrain, which projects rostrad to the tip of the notochord. In the midbrain/upper hindbrain regions, cellular organization of the neural epithelium changes from columnar to cuboidal to pseudostratified, while its shape changes from flat to biconvex to V shaped. Closure is immediately preceded by neural crest cell emigration from the lateral edges. Throughout neurulation the cranial notochord develops an increasingly convex curvature in the rostrocaudal plane. The attached neural epithelium curves with the notochord (forming the primary cranial flexure) so that as its lateral edges move dorsomedially they form a more distant concentric arc with that of the notochord, and are hence stretched during the final closure period. The whole rat embryo culture technique was used to investigate the morphogenetic role of proteoglycans during neurulation, neural crest cell emigration and other events in the lateral edge region prior to closure, and the importance of microfilament contraction during concave curvature of the neural epithelium.