Ontogenesis of neocortical asymmetry: a [3H]thymidine study.

Previous research has demonstrated that symmetric regions in one brain are, on the whole, larger than their asymmetric counterparts in another brain, and that side differences in the volumes of homologous architectonic areas are the result of a decrease in neuronal number in the smaller of the two areas. Therefore, understanding mechanisms by which neuronal numbers are regulated during development may be essential to the investigation of the ontogeny of asymmetry. The radial unit hypothesis of Rakic postulates four factors that determine the number of neurons within a neocortical region: (i) early progenitor cell division; (ii) late cell division; (iii) the effect of thalamocortical and corticocortical afferents, which govern, in part, boundary placement; and (iv) ontogenetic cell death. We report here on experiments that address the development of anatomical asymmetry in the light of this hypothesis. Pregnant Wistar rats were injected with [3H]thymidine on several dates during embryogenesis and their pups killed at several postnatal ages. An estimate of the total number of neurons contained within area 17 and area 18a of each hemisphere was determined and the percentage of those which were labeled was calculated. There were no side differences in this measure between either symmetric or asymmetric architectonic areas although there were consistent differences between areas 17 and 18a. This indicated that while late neuroblast division may be important for cytoarchitectonic differentiation, it may play little or no role in interhemispheric asymmetry.