The origin and migration of cortical neurones: new vistas.

The principal neuronal types of the cerebral cortex are the excitatory pyramidal cells, which project to distant targets, and the inhibitory nonpyramidal cells, which are the cortical interneurones. This article reviews evidence suggesting that these two neuronal types are generated in distinct proliferative zones. Pyramidal cells are derived from the neuroepithelium in the cortical ventricular zone, and use the processes of radial glia in order to migrate and take their positions in the cortex in an 'inside-out' sequence. Relatively few nonpyramidal cells are generated in the cortical neuroepithelium: the majority is derived from the ganglionic eminence of the ventral telencephalon. These nonpyramidal neurones use tangential migratory paths to reach the cortex, probably travelling along axonal bundles of the developing corticofugal fibre system.