STEM CELLS IN THE CNS: INTRINSIC AND EXTRINSIC MECHANISMS
Embryogenesis involves the proliferation of stem cells, followed by differentiation and migration of the resultant cell types. In the CNS, derived from the neural tube, neuronal stem cells persist in the subventricular (or subependymal) zone of the lateral ventricles (I).
Waves of neuronal proliferation, differentiation, and migration occur during prenatal CNS development. After birth, stem cells in the subventricular zone continue to proliferate and produce granule cells (neurons) for many brain regions; this process is driven by postnatal environmental stimuli. Throughout adulthood, in the sub-granular zone of the dentate gyrus, radial glial-like cells give rise to neuroblasts that contribute new granule cell neurons (II). In addition, oligodendroglial progenitor cells throughout the CNS can proliferate and then differentiate into mature oligodendrocytes (III). This process can occur after a demyelinating lesion and helps to remyelinate CNS axons (e.g., after a multiple sclerosis lesion).