Fig. 6. HH stage 4+ prechordal mesendoderm is a potent inducer of floor-plate character in vivo. (A) Schematic, showing in vitro recombination experiments to determine the ability of HH stage 4+ prechordal mesoderm to induce floor-plate character. (B) Recombinate of HH stage 4+ chick prechordal mesoderm (Cpm) and E9.5 rat intermediate neural plate (np). After 48 hours in culture, rat neural plate expresses the floor-plate marker FP3, as assessed by immunohistochemistry. (C,D) Recombinate of HH stage 4+ quail prechordal mesoderm (Qpm) and HH stage 4 chick `area a'. After 24 hours in culture the position of the prechordal mesoderm was analysed by immunohistochemistry with an antibody against the quail specific nuclear marker QCPN (C, prechordal mesoderm delineated by broken lines). Analysis of HNF3ß expression by immunohistochemistry in an adjacent section reveals induction of floor plate cells in `area a' tissue that immediately abuts a small region of prechordal mesoderm (D). (E) Schematic and whole-mount views (inset) of in vivo grafts of mesodermal tissue. Grafts were inserted between the neural plate and the adjacent pre-somitic mesoderm in the posterior neuropore region of HH stage 10 embryos. (F) Control embryo. HNF3ß expression is restricted to the floor-plate region lying at the ventral midline of the neural tube. (G) Graft of a notochord next to the lateral wall of the neural tube (white asterisk in G) induces an ectopic floor plate as indicated by the localised expression of HNF3ß directly adjacent to the graft. (H) Graft of a HH stage 4+ prechordal mesendoderm (white asterisk) induces floor-plate character throughout the ipsilateral neural tube and also on the contralateral side of the neural tube (white arrowhead in H).