For experiments at E11.5, the procedure was altered in that injections were guided by ultrasound visualization (Vevo 770, scanhead RMV711, Visualsonics) and electrode pulses were adjusted to 25 mV. Embryo positioning was identified by ultrasound Androgen Receptor activity signal and visually using a highpower lightsource. All procedures were performed in accordance with protocols approved by the institutional animal care. Mouse brains were fixed in 4% paraformaldehyde overnight at 4°C followed by cryoprotection

in 30% sucrose until they sunk to the bottom. Coronal sections were prepared using a cryostat (Leica Micro-systems). Brain sections were permeabilized with 0.1% Triton X-100 for 15 min and then incubated with blocking solution (5% normal goat serum, 0.1% Triton X-100, and 5% BSA in PBS) for 1 hr at room temperature, followed by the incubation of primary antibody at 4°C overnight. The following primary antibodies have been used: goat anti-PP4c (1:50, Santa Cruz), mouse anti-γ-Tubulin (1:1,000, Sigma), mouse anti-N-Cadherin (1:500, BD Biosciences), rabbit anti-PH3 (1:300, Millipore), rabbit anti-Pax6 (1:300, Covance), mouse anti-Tuj1 (1:1,000, Covance), rabbit anti-Caspase-3 (1:300, Cell Signaling), chicken anti-GFP (1:1,000, Abcam), rabbit anti-Stab2 (1:300, Abcam), rabbit

anti-Brn2 BAY 73-4506 concentration (1:200, Santa Cruz), rabbit anti-Tbr1 (1:250, Abcam), rabbit anti-Tbr2 (1:300, Abcam), and mouse anti-Ki67 (1:100, Cell Signaling). After incubation with the primary antibody, sections were washed in PBS, isothipendyl followed by the incubation with appropriate fluorescence-conjugated

secondary antibodies for 1 hr at room temperature before mounting. mRNA was isolated from both control and PP4cfl/fl;Emx1Cre cerebral cortex using TRIzol reagent (Invitrogen) and cDNA was synthesized from 3 μg of total RNA using Superscript II with random primers (Invitrogen). Real-time PCR was performed on a C1000 Thermal Cycler (Bio-Rad). Quantification was performed using CFX Manager software (Bio-Rad) with data normalized to the level of Actin mRNA. The following primer sequences were used: Actin Forward: 5′-TTTGCAGCTCCTTCGTTGC-3′, reverse: 5′-CCATTCCCACCATCACACC-3′ and Hes1 Forward: 5′-TCCAAGCTAGAGAAGGCAGACA-3′, reverse: 5′-CGCGGTATTTCCCCAACA-3′. Brain sections were stained with N-Cadherin to outline the cell shape and PH3 to identify the anaphase and early telophase dividing cells. γ-Tubulin was used to mark centrosomes. Images of z stack sections were taken by Zeiss LSM780 confocal microscopy and 3D reconstruction of the confocal stacks was done with IMARIS software (BITPLANE scientific software) as described previously in Postiglione et al. (2011). Briefly, we define x, y, and z coordinates of the two centrosomes and five points within the ventricular surface of the 3D-rendered mitotic progenitors. These five points are used to determine the best-fitting plane by orthogonal distance regression.