Immunoblotting was performed as described previously [44 (link)]. Equal amounts of tissue lysates were used for immunoblotting. Blots were incubated with specific antibodies to BMP-1 and latent TGFβ1-binding protein-1 (LTBP-1) (ab205394 and ab78294; Abcam), Flag (F3165; Sigma-Aldrich), ALK5 (MAB5871; R&D Systems), Col3a1 and Fn1 (NB600 and NBP1-91258; Novus Biologicals). β-Actin (A2228; Sigma-Aldrich) was used as a loading control. Immunofluorescence was performed as described in detail previously [44 (link)]. We used specific antibodies to CD34 (553731; BD Bioscience or ab54208; Abcam), Nkx2.1 (ab76013; Abcam) and VE-cadherin (562243; BD Biosciences). The nuclei were stained with 4′,6-diamidino-2-phenylindole (D9564; Sigma-Aldrich).