On the morphogenesis of the early rudiments of the developing central nervous system

Neurulation consists of a complex series of events that result in formation of the neural tube, the rudiment of the entire adult central nervous system. This article discusses primary neurulation, a process that occurs in three major stages: formation of the neural plate; bending of the neural plate, with formation of the neural groove and neural folds; and fusion of the neural folds, with formation of the roof of the neural tube and overlying surface epithelium. In addition, new information is presented on the morphogenesis of the chick neuroepithelium during bending of the neural plate. These data show that bending of the neural plate occurs in two distinct steps at all craniocaudal levels: elevation of the lateral parts of the neural plate and the incipient neural folds, and formation and convergence of the definitive neural folds. Elevation is associated with the formation of a midline, ventral locus of bending, and convergence, with the formation of bilateral loci of bending on the future luminal sides of the dorsolateral walls of the neural groove. Several mechanisms possibly involved in the stages and steps of primary neurulation are discussed.