Fig. 2. Notochord-derived Shh::GFP ligand concentrates at the apical region of the neural tube. Neural tube sections of embryos of genotypes and stages indicated. (A-C) Shh::gfp expression visualized by RNA in situ hybridization. (D-F) Shh::GFP distribution directly visualized by confocal microscopy. Shh::GFP concentrated in an apical region of ventral midline cells overlying the notochord (white arrows in D-F) and basolateral region (red arrow, E). (G-I) Immunostaining showing Shh-dependent neural progenitor domains p3 (Nkx2.2) and pMN (Olig2). (J) Immunostaining for Shh in wild-type mouse embryos using an Shh antibody. Note apical signal (arrow). (K,L) A Shh-driven GFP::Cre transgene (Shh^GFP:::Cre) (Harfe et al., 2004) is only detected in the Shh-expressing notochord (arrow in K). Wnt1::GFP, following ShhGFP::Cre-mediated activation of a ubiquitous Wnt1::gfp expression allele (Carroll et al., 2005) specifically in the notochord, is detected in the neural tube (arrow in L). Note that the most dorsally positioned Shh::GFP ligand signal in D is punctate and is adjacent to the most dorsally positioned pMN cell in G (blue arrows).