Histogenesis of Mammalian Neocortex

Abstract

Tilney (1933) suggested that the definitive six layered neocortex was formed by three consecutive mass migrations of cells forming the supragranular, the granular and the infra-granular layers respectively. Tilney’s interpretation remained unchallenged until recently when the attention of the present authors centred on this subject after a study of the effects of x-irradiation on foetal neocortex (Berry et al, 1963, Berry a. Eayrs, 1963). The results suggested that the infra-granular layers were formed first and that the granular and supragranular layers were produced later in ontogeny. This hypothesis was not only entirely contrary to that proposed by Tilney but also implied that the movements of cells were more complex than had previously been envisaged for, if the deep layers were established first, then cells forming the more superficial layers must migrate through the first formed laminae to attain their adult location. The technique of autoradiography was subsequently applied to the study of this problem (Berry et al, 1964, Berry a. Rogers, 1965). The results obtained confirmed the earlier findings and suggested an outline of the pattern of migratory movements of neuroblasts throughout cortical histogenesis. Standard histological preparations were also studied and a tentative theory was formulated to explain the mode of genesis and migration of neuroblasts. It was suggested that the nuclei but not the cytoplasm of germinal, or ependymal cells, divide. One nucleus moves along the superficially directed process of the ependymal cell to attain a subzonal or intrazonal position where cytoplasmic division occurs and the neuroblasts become independent. The nucleus remaining at the ventrical pole of the ependymal cell divides again to repeat the cycle.