Further evidence of extrinsic forces in bending of the neural plate

Abstract

Bending of the neural plate has long been considered to be driven by principally intrinsic forces generated by wedging of neurepithelial cells. Our previous studies have shown that during neural fold elevation, significant neurepithelial cell wedging occurs only within the median hinge point (MHP), the midline region of neural plate anchored to the notochord. We have also shown that neural fold elevation can still occur when MHP cells are prevented from becoming wedge-shaped but fails to occur when the neural plate is separated from lateral nonneurepithelial tissues, even though MHP cells still become wedge-shaped and the midline neural plate still furrows. Together, these results suggest that neural fold elevation, rather than being driven by neurepithelial cell wedging, is driven, at least in part, by extrinsic forces generated by lateral nonneurepithelial tissues. However, it could be argued that in the absence of localized neurepithelial cell wedging, compensatory and atypical cell wedging occurred uniformly throughout the neural plate, providing forces adequate for neural fold elevation. Likewise, it could be argued that in the process of separating the neural plate from lateral nonneurepithelial tissues, the neural plate was damaged to the extent that the neural folds were unable to elevate. To investigate the validity of these arguments, we removed the following tissues microsurgically prior to neural fold elevation: MHP cells, varying amounts of lateral neurepithelial cells (L cells), and the tissues directly underlying these two populations of neurepithelial cells. We found that the neural folds still formed and underwent elevation, convergence, and fusion, resulting in an essentially normal neural tube, even though MHP cells, the underlying notochord, and some L cells were absent for long craniocaudal distances. These results demonstrate that microsurgery alone does not damage the neural plate sufficiently to prevent neural fold elevation, convergence, and fusion. Moreover, the fact that each of the two persisting remnants of lateral neurepithelium generally remained straight and consistently changed their orientation from horizontal to vertical rather than curling suggests very strongly that bending of the neural plate in these embryos is not the result of compensatory and atypical cell wedging. Finally, the results provide further direct evidence of extrinsic forces in bending because the two remnants of lateral neurepithelium, which were oriented horizontally at the time of tissue extirpation, could not have become oriented vertically in the absence of such forces.