, 2002 and Shu and Richards, 2001). Cingulate cortex pioneer neurons are generated from radial glial progenitors in the medial cortex, and these progenitors also form the glial wedge. These pioneer neurons are the first cells to project across the midline and serve as a critical scaffold for the remainder of the callosal axons to successfully cross (Rash and Richards, 2001 and Shu and Richards, 2001). Thus, the interaction of cingulate neurons and the apposed meningeal tissues could be Olaparib important for formation

of the callosum. Importantly, unveiling further mechanisms regulating corpus callosum formation may provide important insights into callosal agenesis in humans. We had the opportunity to develop and test this hypothesis in the course of examining a mouse model we generated which expresses more meningeal secretory molecules due to meningeal overgrowth. We used Msx2-Cre to generate excess meninges around the cortex and cortical midline and discovered that this leads to defects in callosal formation. This suggests that the meninges

produce factors that prohibit midline crossing. In support of this idea, ablation of midline meningeal cells at midcorticogenesis leads to an expanded corpus callosum. We then directly tested the functions of meningeal-secreted factors on corpus callosal crossing and have identified a cascade of signals that regulates callosum development. Specifically, we identified a complex interplay between BMP7 secreted Androgen Receptor Antagonist by the meninges and Wnt3 produced by the callosal pioneer neurons that coordinates the timing of corpus callosum formation. To investigate the effect of meningeal secretory molecules on embryonic brain development, we set out to generate a transgenic mouse line with expanded meninges by using a

meninges-specific cre mouse line. We decided to try using an Msx2-Cre mouse line which uses the 439 bp 5′ flanking region of the mouse Msx2 gene and crossed the line with exon 3 floxed β-Catenin (Ctnnb1lox(ex3), shortened Adenosine as bE3 in the figures) to express stabilized constitutively active β-catenin in the meninges ( Harada et al., 1999 and Sun et al., 2000). The native Msx2 gene is normally expressed in the meninges ( Rice et al., 2003); however, the transgenic Msx2-Cre line drives recombination not in the meninges but instead in the skin, revealed by crossing with the ROSA-YFP CRE reporter line ( Figures 1A’and 1A″ and low magnification in Figure S1B, available online) and the Rosa-LacZ CRE reporter line ( Figure S1C). There was no yellow fluorescent protein (YFP) or lacZ expression in the meninges or any regions of the brain. Msx2-Cre drove recombination starting from E13.5, and the YFP expression persisted postnatally ( Figure S1; data not shown). At E16.5, the Msx2-Cre;Ctnnb1lox(ex3) mutant embryos were smaller, the skin over the head was thinned, and the skull was malformed ( Figures 1B and 1B′).