Figure 3. Comparison of the architectonic structures of the embryonic and mature cortices in three representative species.

Schematic illustrations of the embryonic and adult cortical organizations in reptiles, rodents and primates. (a) In reptiles, neurogenesis occurs primarily by RGPs in the ventricular zone (VZ). Some TBR2+ IPs also exist in the VZ, but there is no obvious subventricular zone (SVZ). The mature cortex is a tri-laminar structure with one prominent excitatory neuron layer in the middle. (b) In rodents, in addition to the VZ consisting of RGPs, the SVZ is prominent with ample IPs. The extensive indirect neurogenesis through IPs contributes to the production of both DL and SL neurons, and the formation of a six-layer cortex. (c) In primates, in addition to the increase in RGPs in the VZ, the SVZ is further expanded, including IPs and oRGs that predominantly constitutes the inner (iSVZ) and outer subventricular zone (oSVZ), respectively. oRGs also provide additional scaffolds supporting and guiding neuronal migration. Variations in neural progenitor cell types and consequently neurogenesis, as well as cell migration, may lead to a distinct functional spatial organization of the cortex in different species. For example, the neural map of orientation selectivity in the visual cortex shows a ‘salt-and-pepper’ organization in the mouse, whereas it is ‘clustered’ in primates and other higher mammals. IZ, intermediate zone; CP, cortical plate.