Fig. 8. Three actions of sonic hedgehog on embryonic telencephalon development. (A) Early in development (E9.0-E12.5), Shh forms a reciprocal gradient with the repressor form of Gli3. The relative expression levels of these two signals then work to establish dorsoventral patterning of the telencephalon. (B) Through prenatal development and possibly throughout life, Shh supports the expansion of radial glial stem cells (blue), promoting overall growth of the developing brain. (C) During neurogenesis in the MGE, Shh maintains the expression of Nkx2.1 (green nuclei) in progenitor cells (blue) that asymmetrically divide into an additional radial glial cell and a transient-amplifying progenitor (red). In the presence of Shh signaling (right pathway), these progenitors migrate into the SVZ where they continue to divide to generate striatal and cortical interneurons. In the absence of Shh (left pathway), the progenitors continue to divide (albeit with reduction of progenitor self-renewal), but Nkx2.1 expression is lost, as is their fate to become Pv- or Som-expressing interneurons.