Complex signaling responsible for molecular regionalization of the cerebral cortex

The formation of discrete functional areas is a key event in the developmen t of the cerebral cortex. The expression patterns of several molecules asso ciated with axon guidance reveal specification of regional identity during fetal development within the cortex, with different area-specific features acquired at very early to later stages of corticogenesis. Cell culture expe riments suggest that complex mechanisms regulate the differentiation of reg ion-appropriate phenotypes. In all instances that we have examined thus far , however, the final phenotype adopted by cortical neurons is governed by t he capacity of the cell to respond to regionalizing signals; this is reflec ted in the heterogeneity of receptor expression by progenitors, and the tem poral and spatial distribution of such signals within the forebrain.