Mechanisms of retinal angiogenesis. Graphic depiction of the cellular mechanisms governing retinal angiogenesis. (A) A stable, quiescent vessel with aligned endothelial cells (EC), united by vascular endothelial cadherin–rich junctions, and covered by pericytes. (B) On stimulation by angiogenic factors, a cascade of events ensues leading to pericyte detachment, basement membrane degradation, and endothelial junction loosening. Determination of stalk versus tip cell phenotypes is achieved through Notch-dependent signaling. VEGF through VEGFR2 induces the Notch ligand Dll4 on tip cells, which subsequently activates Nocth in adjacent endothelial cells and specifies their stalk cell phenotype. Conversely, the Notch ligand Jagged1 is highly expressed by stalk cells and antagonizes Dll4. This promotes VEGFR2 expression and renders ECs more responsive to VEGF and thus more susceptible to form tip cells. (C) Once the vessel has sprouted, stalk cells behind the tip cell divide and assemble to form the lumen of the neovessel. Pericytes are then recruited, and basement membrane is laid down. (D) To reach its final destination, the growing neovessel must navigate through the tissue by responding to a series of diffusible and membrane-bound attractive and repellent guidance cues. (E) Confocal image of sprouting retinal vessels with filopodia-rich tip cells (stained with isolectin B4) from P4 mouse retinas. Scale bar, 10 μm.