Towards a cellular and molecular understanding of neurulation.

Neurulation occurs during the early embryogenesis of chordates, and it results in the formation of the neural tube, a dorsal hollow nerve cord that constitutes the rudiment of the entire adult central nervous system. The goal of studies on neurulation is to understand its tissue, cellular and molecular basis, as well as how neurulation is perturbed during the formation of neural tube defects. The tissue basis of neurulation consists of a series of coordinated morphogenetic movements within the primitive streak (e.g., regression of Hensen's node) and nascent primary germ layers formed during gastrulation. Signaling occurs between Hensen's node and the nascent ectoderm, initiating neurulation by inducing the neural plate (i.e., actually, by suppressing development of the epidermal ectoderm). Tissue movements subsequently result in shaping and bending of the neural plate and closure of the neural groove. The cellular basis of the tissue movements of neurulation consists of changes in the behavior of the constituent cells; namely, changes in cell number, position, shape, size and adhesion. Neurulation, like any morphogenetic event, occurs within the milieu of generic biophysical determinants of form present in all living tissues. Such forces govern and to some degree control morphogenesis in a tissue-autonomous manner. The molecular basis of neurulation remains largely unknown, but we suggest that neurulation genes have evolved to work in concert with such determinants, so that appropriate changes occur in the behaviors of the correct populations of cells at the correct time, maximizing the efficiency of neurulation and leading to heritable species- and axial-differences in this process. In this article, we review the tissue and cellular basis of neurulation and provide strategies to determine its molecular basis. We expect that such strategies will lead to the identification in the near future of critical neurulation genes, genes that when mutated perturb neurulation in a highly specific and predictable fashion and cause neurulation defects, thereby contributing to the formation of neural tube defects.