Chapter 21 - The Expansion of the Cortical Sheet in Primates

Abstract

The cortical sheet has expanded extensively during mammalian evolution. It is characterized by a large number of neurons arranged in six layers. While there are significant regional differences, the upper layers have been particularly enlarged in primates. The neurons and glia of the mature neocortex are generated by different types of progenitor cells during development. Primates have an expanded and particularly diverse pool of progenitors. Here we describe how the evolutionary emergence of two types of progenitor cells, intermediate progenitors and outer radial glia cells, has contributed to the expansion of the cortical sheet. We also hypothesize that their organization into discrete progenitor zones has important implications for their regulation.