sDll4 induces tubule formation in vitro. (A) HUVECs were cultured on standard Matrigel in growth factor–deficient conditions in triplicates in 2 independent experiments with either sDll4 or VEGF for 24 hours. Shown are representative pictures from triplicate wells repeated twice. (B) Quantitative analysis for tube length and the number of junctions in sDll4-treated HUVECs (Bioquant Image Analysis; mean ± SEM from triplicate wells in 2 repetition experiments). Similar results were seen with human arterial endothelial cell assay (data not shown). *P < .05 compared to no growth factor. Photomicrographs in panel A were taken with a Nikon Plan Fluor ∞, 0.17, 4×/0.12 NA objective and 10× eyepiece and processed with Image-Pro Plus 6.0 (Media Cybernetics, Bethesda, MA).