Regionalization of the cerebral cortex: developmental mechanisms and models.

The cerebral cortex is composed of functionally specialized areas that have unique connections with other cortical targets and subcortical nuclei. The developmental mechanisms responsible for the formation of discrete regions must include the regulation of the expression of genes encoding proteins that control axon guidance and targeting. New data on patterns of gene expression demonstrate the early appearance of such guidance molecules, thus reflecting the early emergence of regional specification within the cortex. Transplant and cell culture studies suggest that the decisions made by neuronal progenitor cells to express region-appropriate phenotypes is controlled by the capacity of the cells to respond to and have access to specific signals. The key to understanding cortical specification may lie in determining the factors that control receptor diversity on progenitors and the temporal and spatial distribution of inductive signals within the forebrain.