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Fig. 7. FGF8 regulates the activity of activin, a potent modulator of RP
development. (A-C) Dynamic expression of activin B (arrowheads) in
the midbrain (A,B) and caudal forebrain (B,C) of HH10, HH12 (A,B, dorsal
views) and HH18 (C, lateral view) embryos. (D-H') Overexpression
of activin through bead insertion (blue dots) destabilizes the midbrain RP.
(D,E) Activin beads induce ectopic LMX1B expression 7 (D) and 23 (E) hours
after bead insertion. At 48 hours (F-H'), a thin row of GDF7-expressing
cells links the activin bead to the endogenous RP (arrowheads in F,H,H';
H' is a sagittal section through H). Activin also interferes with GDF7
expression in the endogenous caudal midbrain RP (arrows in G,H). (I-P)
follistatin expression is modulated by FGF8 signaling. At stage HH11
(I,J), follistatin is expressed in a bilateral domain that straddles
the midbrain-forebrain junction and flanks (arrowhead in I) the site of
initiation of Gdf7 expression on the dorsal midline of the anterior
midbrain (arrowhead in J). (K) Lateral view of a stage HH14 embryo
illustrating the subsequent posterior extension of follistatin
expression in the midbrain (posterior is leftwards, the arrowhead indicates
the midbrain-forebrain junction). In contrast to noggin-soaked beads
(Fig. 2A-A''), dorsal
follistatin-soaked beads (L, arrowhead) did not impair RP differentiation.
Compared with the contralateral control side (arrowheads in M),
follistatin expression was down- or upregulated, respectively, near
beads soaked in FGF8 (red dot in M) or the FGF8 signaling inhibitor SU5402
(black dot in N). (O,P) Stage HH13-14 heads were separated on the midline into
two halves. To remove the endogenous source of FGF8, the isthmic region was
removed from one half (as indicated by the broken line in O). The control (O)
and ablated (P) halves were cultured side by side on floating membranes for 6
hours and treated together for the detection of follistatin
transcripts. The level of expression of follistatin was higher on the
ablated side (arrows in O and P).
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