This may reflect a deficiency in the production of intermediate progenitor (Tbr2+) cells, which were noticeably scarce in human SFEBq aggregates compared to mouse. In humans and in mice, Tbr2 deletion causes microcephaly (Baala et al., 2007 and Sessa et al., 2008), and the deficiency in neurogenesis is most pronounced in upper layers (Arnold et al., 2008). Beyond

its well-appreciated role in transit amplifying cells, Tbr2 is also required for the proper differentiation of upper layer neurons (Arnold et al., 2008). What elements selleck chemicals do telencephalic SFEBq aggregates lack that might impact the scarcity of Tbr2+ cells? Tbr2+ cells produce chemokines that recruit migrating interneurons from the ventral telencephalon (Sessa et al., 2010), a mechanism for balancing excitatory and inhibitory neuron numbers that may also regulate Tbr2+ cell numbers. Hippocampal transit amplifying cells receive GABAergic and peptidergic inputs that regulate their proliferation and differentiation (Tozuka et al., 2005 and Zaben et al., 2009); the cortex may very well employ similar mechanisms. Tbr2+ cells also interact with the vasculature in embryonic mouse cortex (Javaherian and

Kriegstein, 2009 and Stubbs et al., 2009). These interactions between Tbr2+ Screening Library supplier cells and their environment may be more acutely required in the human cortex, which takes several weeks to accomplish neurogenesis, compared to the mouse cortex, which takes only days. In addition, there are fundamental differences in the cellular mechanisms by which human and mouse cortices produce upper-layer neurons, to which we will now

turn our attention. The developmental guideposts we have discussed for differentiating pluripotent cells to cortical neurons have been established mainly in mouse models of cortical development. The human cortex, however, is structurally more complex and thousands MTMR9 of times larger than the mouse. As our knowledge of human brain development increases, we should expect to encounter distinct cellular mechanisms, reflected at the level of neural progenitor cells, that facilitate the development of a larger cortex with more complex circuitry. Here we will discuss recently characterized progenitor cell populations that are thought to account for the enormous increase in cell numbers that underlies the expansion of the human cortex, and the prospects for generating these cell types from pluripotent stem cells. In the embryonic mouse cortex, neurogenesis occurs only in the periventricular region. The radial glia (RG) that function as neural stem cells divide at the ventricular surface, producing neuronal progeny that often divide again in the subventricular zone before migrating radially to the cortical plate (Haubensak et al., 2004 and Noctor et al., 2004).