LAUSR

&NA; The mammalian dorsal telencephalic neuroepithelium develops—from medial to lateral—into the choroid plaque, cortical hem, hippocampal primordium and isocortex under the influence of Bmp, Wnt and Notch signaling. Correct telencephalic development requires a tight coordination of the extent/duration of these signals, but the identification of possible molecular coordinators is still limited. Here, we postulated that Secreted Frizzled Related Protein 1 (Sfrp1), a multifunctional regulator of Bmp, Wnt and Notch signaling strongly expressed during early telencephalic development, may represent 1 of such molecules. We report that in E10.5‐E12.5 Sfrp1−/− embryos, the hem and hippocampal domains are reduced in size whereas the prospective neocortex is medially extended. These changes are associated with a significant reduction of the medio‐lateral telencephalic expression of Axin2, a read‐out of Wnt/&bgr;catenin signaling activation. Furthermore, in the absence of Sfrp1, Notch signaling is increased, cortical progenitor cell cycle is shorter, with expanded progenitor pools and enhanced generation of early‐born neurons. Hence, in postnatal Sfrp1−/− animals the anterior hippocampus is reduced and the neocortex is shorter in the antero‐posterior and medio‐lateral axis but is thicker. We propose that, by controlling Wnt and Notch signaling in opposite directions, Sfrp1 promotes hippocampal patterning and balances medio‐lateral and antero‐posterior cortex expansion.