Fig. 5. Onset of RA synthesis is promoted by canonical Wnt signalling once FGF signalling has declined. (A-B') Raldh2 is weakly expressed in caudal presomitic mesoderm (CPM) explants (drawn in A) cultured for 8 hours with control cells (B), whereas it is activated in presence of Wnt8c cells (B'). (C-F) Embryos cultured in control media (C,E) or media with LiCl (D,F). The Wnt responsive gene Lef1 is upregulated by LiCl, whereas Raldh2 expression is not consistently upregulated. (G-J) Comparison of embryos treated with DMSO (control media G,I) or SU5402 (H,J). SU5402 inhibits the expression of the FGF responsive gene Sprouty2 but Raldh2 is not affected by this treatment. (K,L) Compared with embryos in DMSO media (K), embryos cultured in presence of SU5402 and LiCl combined (L) show enhanced Raldh2 expression, and in some cases (L) this expands caudally. Arrow, node; arrowhead, last somite formed. Scale bars: 50 µm in B for B,B'; 100 µm in C for C-L.