RSV stimulates capacities of PT-ECFCs to form functional blood vessels in vivo. To determine the proangiogenic effect of RSV on PT-ECFCs in vivo, 2 groups of 5-week-old nu/nu mice were injected with a mixture of ECFCs and SMC (ratio 75:25) resuspended in growth factor-reduced Matrigel supplemented with either DMSO or 1µM RSV. Implants were harvested after 7 days and stained with hematoxylin and Fuschin (A) or labeled by fluorescent staining (B) as described in “Patients, materials, and methods.” (A) Representative images of luminal microvessels in sections of Matrigel plugs (original magnification ×20; scale bars, 50 µm). H&F staining showing vessel-like structures in plugs unfilled in the absence (i) or presence (ii) of 1µM RSV and filled with CT-ECFCs (iii-iv), PT-ECFCs (v-vi), and PT-ECFCs treated with 1µM RSV (vii-viii). Images (v,vii) or (vi,viii) were representative of the same sample in the absence or presence of RSV. (B) Representative images of the human vasculature (shown in red by staining against hCD31) and the perfusion in the plugs (shown in green by infusion of Alexa 488–dextran). Nuclei were counterstained with DAPI (blue). Staining showing that vessel-like structures were representative of plugs filled with CT-ECFCs (i-iii), and showing PT-ECFCs in the absence (iv-vi) or presence (xii-ix) of 1 μM RSV. Subpanels iv-ix were representative of the same sample in the absence or presence of RSV (original magnification ×20; scale bars, 50 µm). (C) Quantification of the overall microvascular density of the human vessels per square millimeter (n = 5-7 mice). (D) Quantification of large microvessel density per square millimeter (n = 5-7 mice). (E) Quantification of perfused human vessels per square millimeter, as shown by Dextran labeling (n = 5-7 mice). (F) Quantification of the average diameter of the human vessels (n = 5-7 mice). (C-F) Means ± SEM from 5 nonoverlapping microscopic fields in 5 separate sections per plugs for 5-7 plugs per groups. *P < .05; **P < .01; ***P < .001.