The Control of Neuronal Differentiation by Intracellular Sodium

Abstract

It has long been held that the first event in the development of the nervous system in the amphibian embryo is an interaction between mesoderm cells, brought into position by gastrulation movements, and the overlying dorsal ectoderm cells. This event has been called primary induction and is discussed elsewhere in this volume. Recently this view has been challenged by Marcus Jacobson (1982 and this volume), who argues that the nervous system is derived from 7 compartments, whose founder cells are established at the early-mid blastula stage. Following gastrulation the neural tube is constructed as a result of an extensive series of morphogenetic movements as the dorsal surface of the ectoderm develops first the neural groove and then the neural folds. The area encompassed by the neural folds, the neural plate, will form the central nervous system, both neurones and supporting cells, while the edges of the neural folds are destined to give rise to the neural crest. As the neural groove deepens and the neural folds lift, the folds from either side are brought together in the mid-line, where the ectoderm fuses to form a continuous sheet and the neural tube becomes a separate entity. Neural crest cells then begin their characteristic migration away from the dorsal surface of the neural tube to form the sympathetic and sensory ganglia, Schwann cells and a variety of other derivatives. Work in a number of laboratories has shown that the fate of neural crest cells is dependent upon the environment in which they eventually find themselves.