Fig. 1. Neural stem cell formation and neuronal differentiation. (A) Drosophila neurogenesis. Neuroectodermal cells (NE; yellow, the apical surface is uppermost) give rise to neuroblasts (NBs; green) by delamination, and each NB divides in a stem cell mode to bud off a chain of ganglion mother cells (GMCs; orange) from its basal surface. GMCs are intermediate precursors that typically generate two postmitotic neurons (n; red). Larval NBs are closely associated with glia (blue). Thoracic and brain NBs become mitotically quiescent in late embryos and resume proliferation during larval stages. Approximate cell cycle times or quiescence times are given in hours (h). epi, embryonic epidermis. (B) Mammalian embryonic cortical neurogenesis. Initially the cortex has only neuroepithelial cells (NE; yellow, the apical surface is uppermost), which mature into radial glia (RG; green). Radial glia and neuroectodermal cells generate basal progenitors (BP; orange), which are intermediate progenitors that generate a limited number of neurons (n; red). NE and RG can also generate neurons. VZ, ventricular zone, adjacent to the lumen; SVZ, subventricular zone; MZ, marginal zone.