Fig. 1

Fgf signalling inhibits blood and endothelial gene expression in the heart field. (A) Schematic highlighting the anterior blood/endothelial precursor or haemangioblast population (yellow) rostral to cardiac precursors (purple) in 7-somite zebrafish embryos. Another haemangioblast population is found in the posterior lateral plate mesoderm (red). (B) Expression of the cardiac markers (brackets) nkx2.5, mef2c and hand2 is severely reduced in embryos treated with the Fgf inhibitor SU5402 compared with control DMSO-treated siblings. A few hand2⁺ cells remain in the cardiac mesoderm of inhibitor-treated embryos. Anterior mesoderm specification is unaffected as revealed by normal expression of the anterior lateral plate mesoderm (ALPM) markers nkx2.7 and gata6. (C) Blood and endothelial gene expression is upregulated and ectopic (brackets) in the ALPM of SU5402-treated embryos. Images show flat-mounted embryos, anterior to the top. Somites were stained with myoD or uncx4 (red) for staging. Rhombomeres 3 and 5 were stained with krox20 (red stripes) as a positional marker. Reduction in rhombomere 5 reveals that Fgf signalling is reduced, together with short body axis and posterior mesoderm defects. Scale bars: 100 μm.