Fig. 4. Prosencephalic markers expand posteriorly in spg embryos. (A) emx1 is expressed in telencephalic precursors from end of gastrulation onwards in wild-type embryos. The posterior transverse expression domain marks the di-mesencephalic boundary. pax2.1 expression at the anterior MHB is shown in red. (A') In spg embryos, defined by the impaired expression of pax2.1 at the MHB, emx1 expression is generally elevated but reduced in its spatial lateral extent. The posterior border of emx1 expands caudally. (B) anf1, like emx1, is expressed at the anterior neural border with a patch of expression centering around the midline of the neuroectoderm. gbx2 expression at the posterior MHB is shown in red. (B') anf1 is lost within the midline expression domain in spg embryos, defined by impaired gbx2 expression (see Fig. 3 for gbx2 expression). (C) pax6 is initiated within the forebrain at the end of gastrulation. pou2 expression at the MHB is seen in red. (C') spg embryos, identified by loss of pou2 expression, show a posterior expansion of pax6 expression into the territory of the prospective MHB. (D,D') Double in situ hybridization with fgfr3 (blue) and en3 (red) at the 10-somite stage show the hindbrain domain of fgfr3 (arrow in D,D') is fused with the diencephalic domain of fgfr3, particularly at its ventral aspect. The MHB marker en3 is restricted to a dorsal patch in mutant embryos (D'). (E) During somitogenesis, besides its expression in the forebrain, pax6 is also expressed within the hindbrain and spinal cord in wild-type embryos. (E') The prosencephalic and the rhombencephalic domain nearly fuse in spg mutant embryos mainly owing to strong posterior expansion of the posterior border of the prosencephalic domain of pax6. (F,G) Anterograde filling of whole eyes with DiI (green fluorescence) or DiO (red fluorescence) shows a proper contralateral retinotectal mapping of RGC axons in spg embryos (F). The chiasma opticum is properly formed in spg mutant embryos (G).
