(A) Ventricular zone stem cells or radial glia (RG) function as the embryonic sources of glial progenitors. Two separate waves of oligodendrocyte progenitor cells (OPCs) arise from specific ventricular zone regions at E12 and E15, and migrate in streams to populate the telencephalon. Radial glia shift from neurogenesis to gliogenesis at E16 to generate intermediate glial progenitors. The majority of gliogenesis occurs during the first 3 weeks postnatally. The predominant source of oligodendrocytes in the postnatal brain comes from a third wave of OPCs that appears in the cortex around birth, expands, and differentiates into myelinating oligodendrocytes. Intermediate glial progenitors migrate into the cortex and differentiate into astrocytes. These differentiated astrocytes also undergo symmetric division during the first three weeks of age. In the adult, differentiated astrocytes can generate additional astrocytes, especially in response to injury. New oligodendrocytes are formed in the adult brain from resident OPCs in the cortex as well as OPCs arising from intermediate progenitors of the subventricular zone.