The symmetric division of neural stem cells (NSCs) underlies their ability to self-renew and serves to maintain the NSC population. In contrast, asymmetric mitosis produces one NSC and one neural progenitor cell (NPC), daughter cells with restricted differentiation capacity for neuronal or glial lineages. In addition, terminal asymmetric division generates two neural progenitor cells but does not contribute to maintaining the NSC pool. Extrinsic factors believed to be essential for NSC maintenance and proliferation include epidermal growth factor (EGF), fibroblast growth factors, Sonic Hedgehog (Shh), and members of the Wnt family.
There are four types of neural progenitor cell. Neurogenesis in mammals commences with the induction of the neuroectoderm, which is followed by the formation of the neural plate, which folds to form the neural tube. These structures are composed of neuroepithelial progenitors (NEP) that are responsible for neurogenesis in the neural tube. NEPs also give rise to two other types of neural progenitor cell, radial glia and basal progenitors. Radial glia are the dominant progenitor cell type in the developing brain whereas basal progenitors are specifically located at the subventricular zone (SVZ) in the developing telencephalon. In the adult brain, adult progenitors in the subgranular zone (SGZ) of the dentate gyrus in the hippocampus and the SVZ of the lateral ventricles, are thought to functionally contribute to brain plasticity and repair.