Furthermore, our study opens the possibility that changes in cell migration may more generally participate in the evolution of brain connectivity. Consistently, LY294002 supplier it was shown that the formation of the corpus callosum, a mammalian-specific tract, relies on the migration of guidepost neurons (Shu et al., 2003a and Niquille

et al., 2009). Indeed, the mammalian telencephalon is characterized by a complex choreography of tangential neuronal migrations originating in both dorsal and ventral regions, which is essential for its functioning because cell migration defects have been involved in the etiology of several pathologies (McManus and Golden, 2005). Thus, our study raises the intriguing possibility that tangential neuronal migrations may have promoted the evolution of the telencephalon at the expense of its developmental robustness. Wild-type and GFP-expressing transgenic mice (Hadjantonakis et al., 1998), maintained in Swiss OF1 background, were used for expression analysis and tissue culture. Slit1−/−, Slit2−/−, and Slit1−/−;Slit2−/− mutant embryos were obtained by crossing Slit1+/−, Slit2+/−, and Slit1−/−;Slit2+/− parents ( Plump et al., 2002) maintained in B6D2 background.

Robo1−/−, Robo2−/−, and Robo1−/−;Robo2−/− were obtained by crossing Robo1+/−, Robo2+/−, and Robo1+/−;Robo2+/− BI-6727 parents ( Grieshammer et al., 2004, Long et al., 2004 and Ma and Tessier-Lavigne, 2007), which were maintained in CD1, C57BL/6, and mixed CD1–C57BL/6 backgrounds, respectively. Animals were kept under French and EU regulations. Chinese soft-shelled turtle embryos (Nagashima et al., 2009) and corn-snake embryos (Gomez et al., 2008) were fixed

in 4% paraformaldehyde (PFA) for 24–48 hr, kept in methanol, rehydrated, and cut into 100 μm thick sections on a vibratome. Sheep embryo was obtained by permission from a slaughterhouse in Cartagena (Spain), perfused with 4% PFA, Non-specific serine/threonine protein kinase postfixed overnight, embedded in paraffin, and cut into 8 μm sections. Human embryos were obtained from legal abortions (procedure approved by the French National Ethical Committee CCNESVS), staged, fixed in 4% PFA, cryoprotected, and cut into 12 μm thick sections as described previously (Verney et al., 2001). For in situ hybridization, mouse or chicken brains were fixed overnight in 4% PFA in PBS. Twenty micrometer frozen sections or 80 μm free-floating vibratome sections were hybridized with digoxigenin-labeled probes as described before (Lopez-Bendito et al., 2006). For immunohistochemistry, cultured slices/explants and mouse or chicken embryos were fixed in 4% PFA at 4°C for 30 min and for 2–3 hr, respectively. Immunohistochemistry was performed on culture slices, Matrigel pads, and 100 μm free-floating vibratome sections as previously described (Lopez-Bendito et al.