Fig. 9. RA, Hh and FGFR signaling pathways can crossregulate each other at the transcriptional level. (A) High dose RA treatments (10 µM) strongly repress Shh expression in the anterior midline (indicated by the yellow bracket) both at neurula and stage 33. The same RA doses cause fusion of the two stripes of FGF8 expression in the ANR and the anteroventral ectoderm in neurula stage embryos, while at stage 33 FGF8 expression is strongly downregulated in the whole head region. Low dose RA treatments (0.1 µM) have weaker effects on both Shh and FGF8. (B) Unilateral injection of 500 pg bhh mRNA laterally expands FGF8 expression in the anterior neural ridge but does not affect Raldh2 expression at neurula stages. The inset shows FGF8 expression in the ANR of an uninjected embryo at the same stage. The same dose of bhh upregulates both FGF8 and Raldh2 in the eye region at stage 33. (C) Unilateral injection of 4 pg iFGFR1 mRNA, followed by induction with AP20187 at stage 12.5/13, expands Shh expression in the prospective hypothalamic region at neurula, and activates Shh in the eye regions at stage 33. At doses of 4 and 2 pg, iFGFR-1 mRNA has no significant effect on Raldh2 expression at neurula stages and stage 33, respectively.