The development of the brain follows a stereotyped pattern beginning with the expansion of neuroepithelial cells, which go on to form distinct pools of progenitors in a number of anatomical locations (Chenn and McConnell, 1995). In the cortex, most γ-aminobutyric acid (GABA)-producing local inhibitory neurons arise in the ventral telencephalon and migrate long distances to the dorsal cortex (Wonders and Anderson, 2006). On the other hand, excitatory projection neurons, which are glutamatergic and project to other cortical areas as well as subcortical targets, originate from the dorsal telencephalon in the germinal zone lining the ventricles, called the ventricular zone (VZ) (Hendelman, 1991). Radial glial cells (RGs) are bipolar neural precursors whose cell body resides in the VZ; they are bona fide neural stem cells (NSCs) in that they can divide symmetrically to self-renew or asymmetrically to produce more differentiated progeny (Götz and Huttner, 2005; Kriegstein et al., 2006). Initially, RGs generate neurons directly (‘direct neurogenesis’), but begin producing a second population of neural precursors called intermediate progenitors (IPs) around E13.5 (Hendelman, 1991). The late phase of corticogenesis is dominated by
