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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.
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