BMP signaling is required locally to pattern the dorsal telencephalic midline.

BMPs have been proposed to pattern the medial-lateral axis of the telencephalon in a concentration-dependent manner, thus helping to subdivide the embryonic telencephalon into distinct forebrain regions. Using a CRE/loxP genetic approach, we tested this hypothesis by disrupting the Bmpr1a gene in the telencephalon. In mutants, BMP signaling was compromised throughout the dorsal telencephalon, but only the most dorsalmedial derivative, the choroid plexus, failed to be specified or differentiate. Choroid plexus precursors remained proliferative and did not adopt the fate of their lateral telencephalic neighbors. These results demonstrate that BMP signaling is required for the formation of the most dorsal telencephalic derivative, the choroid plexus, and that BMP signaling plays an essential role in locally patterning the dorsal midline. Our data fail to support a more global, concentration-dependent role in specifying telencephalic cell fates.