Fig. 8. Schematic summary of relationships between hedgehogs, Foxf1 and Bmp4 in different tissues. Expression of Bmp4 precedes that of Foxf1 in mesoderm formed in the primitive streak area during gastrulation; data from Xenopus (Tseng et al., 2004) indicate that Bmp4 activates transcription of Foxf1 at this stage. As the posterior mesoderm migrates out of the primitive streak and associates with the endoderm in the lateral plate and yolk sac, maintenance of Foxf1 expression depends on hedgehog ligands (Ihh and Shh) secreted by the endoderm. In the murine allantois, which lacks endoderm, Foxf1 expression remains high in the absence of hedgehog signaling. Maintenance of robust Bmp4 expression in the mesodermal cells, after exit from the primitive streak, requires Foxf1. Bmp4 induces the formation of endothelial tubes from mesodermal progenitors.