Glial nature of neural stem cells (NSCs) in development and in the adult. Neuroepithelial cells in early development divide symmetrically to generate more neuroepithelial cells. Some neuroepithelial cells likely generate early neurons. As the developing brain epithelium thickens, neuroepithelial cells elongate and convert into radial glial (RG) cells. RG divide asymmetrically to generate neurons directly or indirectly through intermediate progenitor cells (nIPCs). Oligodendrocytes are also derived from RG through intermediate progenitor cells that generate oligodendrocytes (oIPCs). As the progeny from RG and IPCs move into the mantel for differentiation, the brain thickness, further elongating RG cells. Radial glia have apical-basal polarity: apically (down), RG contact the ventricle, where they project a single primary cilium; basally (up), RG contact the meninges, basal lamina, and blood vessels. At the end of embryonic development, most RG begin to detach from the apical side and convert into astrocytes while oIPC production continues. Production of astrocytes may also include some IPCs (see ) not illustrated here. A subpopulation of RG retain apical contact and continue functioning as NSCs in the neonate. These neonatal RG continue to generate neurons and oligodendrocytes through nIPCs and oIPCS; some convert into ependymal cells, whereas others convert into adult SVZ astrocytes (type B cells) that continue to function as NSCs in the adult. B cells maintain an epithelial organization with apical contact at the ventricle and basal endings in blood vessels. B cells continue to generate neurons and oligodendrocytes through (n and o) IPCs. This illustration depicts some of what is known for the developing and adult rodent brain. Timing and number of divisions likely vary from one species to another, but the general principles of NSC identity and lineages are likely to be preserved. Solid arrows are supported by experimental evidence; dashed arrows are hypothetical. Colors depict symmetric, asymmetric, or direct transformation. IPC, intermediate progenitor cell; MA, mantle; MZ, marginal zone; NE, neuroepithelium; nIPC, neurogenic progenitor cell; oIPC, oligodendrocytic progenitor cell; RG, radial glia; SVZ, subventricular zone; VZ, ventricular zone.