Notch inhibition and VEGF stimulation synergize in lymphatic sprouting in vitro. (A) LEC spheroids were treated with Dll4-Fc (20 μg/mL), VEGF (10 or 50 ng/mL), or their combination as indicated and stained for PECAM-1. Note the synergistic induction of LEC sprouting by Dll4-Fc and VEGF in a dose-dependent manner, as quantified in panel C. GFs indicates growth factors. (B) Similar titration of VEGF-C in the presence or absence of Dll4-Fc (20 μg/mL). (D) Quantification of the sprout number in the experiment shown in panel B. Scale bar, 100 μm. Data represent means ± SEM from 2-3 independent experiments with n = 6-13 per group per experiment; *P < .05; **P < .01; ***P < .001.