To identify functional roles for the γ-Pcdhs in cortical development, we crossed Pcdh-γfcon3 conditional mutant mice ( Prasad et al., 2008) with a line expressing Cre from the Emx1 locus (see Figures GABA inhibitor drugs S1A–S1G available online). The Emx1-Cre line has been used extensively to excise floxed alleles in progenitors that give rise to primary glutamatergic neurons as well as astrocytes in the cortex, while sparing ganglionic eminence-derived GABAergic cortical interneurons ( Gorski et al., 2002). We confirmed that Emx1-Cre efficiently recombined the Pcdhγfcon3 allele

by immunostaining in neonatal Emx1-Cre; Pcdh-γfcon3/+ brains ( Figures S1A–S1G). Emx1-Cre; Pcdh-γfcon3/fcon3 mutants were born in Mendelian ratios and were viable and fertile. Gross examination of the brain revealed no obvious abnormalities or changes in overall size. Comparison

of sections through the primary somatosensory cortex (S1), however, revealed that the mutant cortex was thinner than that of controls. Close examination showed that this was due entirely to a reduction of the superficial, cell-sparse layer I: layers II–VI were remarkably similar in side-by-side micrographs of controls and mutants ( Figures Vorinostat mouse 1A and 1B), and quantitative analysis of a variety of cortical layer markers indicated no difference in cell number or in lamination ( Figures S1H–S1M). Layer I thinning occurred between postnatal day 18 (P18) and P28, with the distance between layer II and the pia reduced by 42% (n = 48 total measurements before from three animals per genotype; Figure 1C). Apoptosis was similarly low in control and mutant cortex throughout the postnatal period ( Figures S1N and S1O), and loss of the γ-Pcdhs in the primary neurons of the cortex also did not affect the numbers of cortical interneurons ( Figures S1P and S1Q; data not shown). Because cortical layer I is composed mainly

of apical dendritic tufts of deep-layer pyramidal neurons, loss of layer I in the mutants could be due to defects in dendrite arborization. To investigate this, we crossed Emx1-Cre; Pcdh-γfcon3 mice with the Thy1-YFPH transgenic line ( Feng et al., 2000), in which a population of layer V neurons throughout the cortex strongly expresses yellow fluorescent protein (YFP). We analyzed confocal stacks from 100 μm vibratome sections of Emx1-Cre; Pcdh-γfcon3/fcon3; Thy1-YFPH mutants and littermate controls between P18 and 3 months of age. As in controls, mutant layer V pyramidal neurons extended apical dendrites into layer I ( Figures 1D and 1E), and their axons correctly exited the cortex through the internal capsule to form the corticospinal tract (data not shown). Individual neurons were reconstructed through confocal stacks by using a program (Neuromantic) to disambiguate processes from those of any neighboring YFP+ cells.