Vascular Dll-Notch signaling. In fetal arteries (left), only Dll1 and not Dll4 appear to signal through Notch1 receptors to block the venous program and to mediate arterial gene expression in endothelia and in the surrounding smooth muscle layer in a VEGF-independent manner. This contrasts with sprouting angiogenesis in the capillary bed (right), where Dll4 is the essential ligand that mediates the arterial program in stalk cells. VEGF induces this pathway and thereby reduces VEGF sensitivity in stalk cells. Professional illustration by A. Y. Chen.