Fig. 9. Differential effect of the mandibular-arch-derived signals Fgf8 and Bmp7. (A) Scheme of bead implantation at HH10, orientation as in Fig. 3A. (Ai,ii) Fgf8 expression at HH10 (Ai, dorsal view, anterior to the top) and HH19 (Aii, lateral view of right side, anterior to the top). Note persistent expression of Fgf8 in the pharyngeal endoderm and at HH19, upregulated expression in the oral ectoderm and in the endoderm plus overlying ectoderm of each pharyngeal pouch (Aii, arrows point towards expression associated with the mandibular arch). (Aii,iv) Bmp7 expression at HH10 (Ai, ventral view, anterior to the top) and HH19 (Aii, lateral view of right side, anterior to the top). Note the persistent expression of Bmp7 in the HH10 endoderm and heart; at HH19 expression is upregulated in the pharyngeal pouches, the oral ectoderm (arrows), and the in- and outflow tract of the heart. Fgf8 and Bmp7 are absent from the area of the lateral rectus EOM underneath the trigeminal ganglion. (B-E) Lateral views of embryos treated with BSA-loaded control beads, (F-O) Fgf8-loaded beads and (P-W) Bmp7-loaded beads; untreated sides of the embryos on the left, treated sides on the right, orientation as in Fig. 3B-E. The embryos were double labelled as indicated. Normal expression domains are marked by arrows, downregulated expression by open arrowheads, upregulated expression by filled arrowheads. The position of the beads is marked by asterisks. Note that Fgf8 beads placed into the head mesoderm did not influence positional values of the hindbrain or the neural crest cells, as evidenced by the unperturbed expression of Hoxa2 (F,G). However, Fgf8 downregulated the lateral rectus marker (H,I) and upregulated the mandibular arch muscle marker (L,M). Unlike neural tube-mesoderm separation experiments, however, Myf5 expression was also downregulated (J,K), while MyoR was upregulated (N,O; compare with Fig. 5). Bmp7 dowregulated Paraxis in the region of the lateral rectus EOM (P,Q) and Myf5 in the region of the lateral rectus and the mandibular arch muscles (R,S). En2 expression was downregulated in the distal part of the mandibular arch but not proximally close to the bead (T,U). MyoR was also downregulated, most prominently in the distal region of the arch (V,W). Scale bars: 250 µm in Ai,Aiii; 100 µm in Aii,Aiv; 200 µm in B-U. di, diencephalon; endo, endoderm; fn, frontonasal Fgf8 signal in ventral forebrain and surface ectoderm; hy, hyoid arch; i, isthmus; m, midbrain; ma, mandibular arch; ov, otic vesicle; pchp, prechordal plate; r, rhombomere; V, trigeminal ganglion; VII, facial nerve.