The assay concept is embedding a 1:1 ratio of endothelial cells and fibroblasts in a fibrin gel to study vessel formation in vitro and in 3D. Fibrin density of the gel can be varied. After 7 days, vessel growth (ring) can be analyzed and imaged by staining the hydrogels for the endothelial marker CD31. Soluble VEGF produced during the ring assay can be measured using VEGF Human ELISA Kit. VEGF signaling can be inhibited by adding a small molecule inhibitor of VEGFR, SU-5402.