Forebrain neurogenic compartments and cortical neurogenesis: comparative aspects

Neurons of the central nervous system are born at different stages of embryonic development in spatially discrete compartments that are largely transient. The first compartment to appear, the ventricular zone (VZ) lines the ventricles and is composed of neurogenic radial glia and short progenitors. A secondary compartment, the subventricular zone (SVZ) arises from the VZ, and is comprised of gliogenic and neuronic intermediate progenitors. Pallial (dorsal) VZ and SVZ give rise to the cerebral cortex, whereas subpallial (ventral) VZ and SVZ’s give rise to basal ganglia and interneurons that migrate to cerebral cortex. Pallial and subpallial VZ and SVZ can be further subdivided into distinct compartments by transcription factor expression, some of which in the subpallium correspond to transient bulges; the lateral, medial, and caudal eminences. The embryonic SVZ intermediate progenitor population expanded hugely and compartmentalised further during evolution and is a likely source of the preponderance of glia and interneurons in the human cortex. Two major neurogenic zones persist through adulthood: the subependymal zone (SEZ) adjacent to lateral ventricle ependymal cells, and the subgranular zone (SGZ) of the dentate gyrus. The SEZ is derived from a combination of pallial and subpallial progenitors. The SEZ is often termed “subventricular zone”, but the SEZ, at least in part, is derived from VZ radial glia. The extent to which intervening eSVZ cells may contribute to the SEZ is currently unknown. SEZ cells give rise to olfactory bulb interneurons, and SGZ cells give rise to hippocampal interneurons. Adult mammalian neurogenesis may also occur in the cerebral cortex and other brain areas, although compared to the SEZ and SGZ there is much less evidence for it. Rodent SEZ neurogenesis is quite robust, whereas human neurogenesis is markedly less. Many animal experiments and a few postmortem human studies have shown increased SEZ neurogenesis after injury, suggesting that persistent neurogenic compartments may have therapeutic relevance.