Fig. 6. spg/pou2 requirement in the neuroectoderm. (A) Transplanted wild-type cells (brown) in spg embryos express gbx2 cell autonomously. All blue cells carry the brown transplantation marker. The right half of the embryo serves as a control: it is devoid of wild-type cells. As is normally seen in spg mutants (compare with Fig. 5D), gbx2 expression is found only in the mesendoderm, but not the overlying neuroectoderm. Dorsal view of a spg chimera, anterior is upwards. The white line indicates the plane of the transversal section in B along the gbx2 domain. (B) Cross-section of the embryo in A showing that the transplanted wild-type cells expressing gbx2 (bracket; arrow indicates the unaffected non-neural ectoderm domain, see also Fig. 5D') are located in the neuroectoderm. Other cells that are only brown lie outside the normal domain of gbx2 expression. (C,D) Transplanted wild-type cells (brown) in spg embryos also express pax2.1 normally at the MHB. Arrows point to the residual pax2.1 expression at the MHB which is retained in spg embryos until late stages of somitogenesis. (E) Clones of wild-type cells within the mesoderm cannot restore gbx2 expression in spg mutant embryos at the tailbud stage. The plane of section is similar to B. Arrows point to the unaffected non-neural ectoderm domain. (F) Mouse Oct3/Oct4/Pou5f1 is globally expressed within the neural plate at day 8.0 p.c. (dorsal view, anterior to the left). (G) spg/pou2 might be required to activate Fgf8-dependent gbx2 expression either for a planar or vertical signal. The transplantation experiments presented here show a requirement in the neuroectoderm. (H) Mouse Oct3/Oct4 mRNA and lacZ mRNA were co-injected into one cell of a two-cell stage zebrafish embryo. pax2.1 expression can be restored in spg mutant embryos by mouse Oct3/Oct4 mRNA (arrow, lacZ expression is indicated by the brown color) (Burgess et al., 2002). nec, neuroectoderm; mes, mesendoderm; tb, tailbud stage.