After the neural tube closes, neuroepithelial cells begin expressing markers such as vimentin and nestin, marking the point at which radial glial cells appear in the cerebral cortex. Whether neuroepithelial cells represent truly unique neural stem cells that generate a distinct population of radial glial cells through division or whether neuroepithelial cells initiate the expression of new proteins at the onset of cortical neurogenesis remains to be determined. During early stages of cortical development, the cerebral cortex is composed almost entirely of proliferative radial glial cells that divide at the ventricular surface. At the onset of neurogenesis, radial glial cells begin generating both IP cells and cortical neurons, which migrate away from the ventricle. Neurons migrate to a superficial position to form the cortical mantle and IP cells migrate away from the ventricular surface and establish the SVZ as a distinct proliferative layer superficial to the VZ. The IP cells are mostly concentrated in the SVZ, but they are also distributed throughout the upper VZ and the lower intermediate zone.3- 5 The SVZ is initially seeded by IP cells generated by radial glial cells, but at later stages of cortical development the SVZ progenitor cell pool may expand through symmetric proliferative IP cell divisions.4 In addition, progenitor cells from the ventral telencephalon may migrate into the dorsal cortex to contribute to the cortical SVZ progenitor pool. Thus, from the midstages of cortical neurogenesis onward, the size of the SVZ expands. In contrast, the VZ reaches its peak size in the midstages of neurogenesis, after which it begins to shrink. When cortical neurogenesis is complete, radial glial cells transform into astrocytes and exit the VZ, which thins to a single layer of ependymal cells in the postnatal cortex.2 As a result, the proliferative IP cells become a progressively larger component of the cortical progenitor cell pool and represent the majority of mitotic progenitor cells by end stages of embryonic neurogenesis.1 Furthermore, while only a single layer of VZ cells remain in postnatal animals, large numbers of mitotic IP cells are present in the SVZ in postnatal animals and persist into adulthood.