Chapter 12 Neural Induction: Past, Present, and Future

Abstract

Despite the recent studies on avian and mammalian embryos, much of the knowledge on neural induction is still based on experimentation with amphibian embryos. This chapter focuses on experimental work on amphibian embryos. The first determinative events in an amphibian embryo take place very early: at the eight-cell stage, separation and subcultivation of the four animal and the four vegetal blastomeres result in different types of cytodifferentiation. The presumptive mesodermal and endodermal zones are determined as early as the morula stage. The presumptive neuroectoderm becomes determined relatively late, and even during early gastrulation, it retains several developmental alternatives. In accordance with the biological data reviewed, the first fractionation experiments on heterotypic inductors had already revealed results that clearly suggested the existence of two chemically different signal substances with different inductive actions, one neuralizing factor and another factor with a spinocaudal inductive action. The crucial question related to neural induction still unanswered is how an external trigger acts upon a cell with several developmental options so as to channel it into one of them. Several interesting hypotheses have been put forward in recent years, but the experimental evidence is still meager and inconclusive.