Fig. 3

FGF signaling is necessary to maintain paraxial mesoderm fate and inhibit a default endothelial fate.

Heat-shock induction of dnfgfr (B) or treatment with a MEK inhibitor (D) at the 12-somite stage causes an expansion of the endothelial marker etv2 into the pre-somitic mesoderm 5 hr later compared to controls (A, C). (F, F’) Transplanted HS:dnfgfr x fli1:GFP show a cell-autonomous shift from somite to endothelial fate when heat-shocked at the 12-somite stage, whereas fli:GFP transplants mostly contribute to muscle with minor endothelial contribution (E, E’). The same effect is seen with HS:dnfgfr x kdrl:GFP transplanted cells when heat-shocked at the 12-somite stage (G, H). NLS-kikume was injected into donor embryos to quantify cell fate changes. 12-somite stage FGF inhibition resulted in 31% kdrl:GFP-positive cells (13 embryos, 308 cells), compared to 0% in control transplants (seven embryos, 587 cells, p<0.0001) (I). Expansion of endothelium 5 hr after MEK inhibitor treatment is not due to an expansion of BMP signaling, as revealed by pSMAD 1/5/8 staining (K compared to J, green staining, red color is DAPI staining). (L–O) Similarly, treatment with the BMP inhibitor DMH1 does not prevent MEK-inhibitor-induced expansion of endothelium. Embryos were treated at the 12-somite stage and fixed 6 hr later. The expansion of the endothelial marker etv2 into the PSM after MEK inhibitor treatment (N) is not inhibited by the addition of DMH1 (O).