, 2005). We found that 92% of all YFP positive cells located in layer V of the cortex colabeled

with CTIP2 while there was no colabeling with the transcription factor SATB2, which is expressed exclusively by callosal projection neurons in the cortex SCH727965 ( Alcamo et al., 2008 and Britanova et al., 2008) ( Figure 2C). Collectively these findings suggest that Shh is expressed by a significant portion of subcortical projection neuron subtypes. Previous studies have shown that cortical Shh expression peaks approximately at the second postnatal week of development and is downregulated and maintained at a lower expression level in the adult cortex (Charytoniuk et al., 2002). This pattern coincides with the period of peak dendritogenesis and synaptogenesis in the mouse cerebral cortex (Micheva PD173074 nmr and Beaulieu, 1996). To assess Shh function in the developing cortex, we utilized a conditional loss of function approach by specifically removing Shh from cortical pyramidal neurons without affecting patterning and specification in the early developing nervous system (Ericson et al., 1995, Roelink et al., 1995, Xu et al., 2005 and Xu et al., 2010), by crossing animals with an Emx1-ires-Cre knocked into the Emx1

locus ( Gorski et al., 2002) with animals carrying a conditional null allele of Shh ( Dassule et al., 2000) (ShhcKO). ShhcKO mice are viable with Phloretin no gross defects in the patterning or morphology of the brain. While the gross morphology of the brain is indicative of normal patterns of proliferation, we chose to investigate the possibility of a more subtle phenotype. While cortical neurogenesis is nearly complete before birth, gliogenesis continues on through postnatal development ( Ivanova et al., 2003). To assess whether cortical Shh had any role in the production or survival of glial cells during early postnatal development,

we administered a pulse of BrdU between postnatal day 1 to postnatal day 3 (P1–P3) and examined the number of labeled cells in both the cortex and spinal cord ( Figures S2C–S2E). We observed no change in cell death or proliferation in the postnatal brain and spinal cord of ShhcKO animals. We also analyzed whether loss of cortical Shh had a cell autonomous effect on the formation or maintenance of corticospinal axonal projections and found no differences between the conditional mutants and control animals ( Figure S2A), indicating that cortical Shh did not play a significant role in the maintenance or survival of neurons or glia in these regions during this window of neural development. To assess the involvement of Shh in the regulation of neuronal growth and synaptogenesis, we performed Golgi analysis on P21–P28 brains of ShhcKO mice and wild-type control littermates ( Figures 3A–3D).