Fig. 3. Smad2 phosphorylation is absent in endothelial cells of Eng mutant embryos. Transverse sections of E9.5 embryos stained with PECAM1 antibody showing endothelial cells in the brain (A), dorsal aorta and cardinal vein (B), heart (C), and vitelline artery and umbilical vein (D). Transverse sections of E9.5 wild-type embryos stained with PSmad2 (PS2) antibody showing positive endothelial cells in the brain (E,E'), dorsal aorta and branchial arch artery (F,F'), endothelium of the heart (G,G'), dorsal aorta and vitelline artery (H,H'). In the endoglin null embryos, PSmad2 was not detectable in endothelial cells of arteries and veins (I,J,I',J') but was observed in the endothelium of the heart (K,K'). (L,L') Scale bars: 0.5 mm. Red arrowheads in I',J',L' indicate negative ECs. Black arrowheads in K' indicate PS2-positive ECs. Abbreviations: bba, branchial arch artery; cv, cardinal vein; da, dorsal aorta; da/baa, communication between da and baa; fg, foregut, ht, heart; nt, neural tube; pvp, perioptic vascular plexus; s, somite; uv, umbilical vein; va, vitelline artery.