Transplantation of CFU-ECs and ECFCs into NOD/SCID mice. (A) Photomicrographs (original magnification, × 20) of cellularized grafts and surrounding murine tissue 28 days after implantation into NOD/SCID mice. Left and middle panels show consecutive sections of the same ECFC graft stained with anti–murine CD31 (mCD31) and anti–human CD31 (hCD31) to identify either murine or human blood vessels, respectively. mCD31 (left) does not cross-react with human endothelial cells within the graft and hCD31 (middle) does not cross-react with murine endothelial cells in the vessels outside the graft. Murine vessels were never identified in the cellularized graft (n = 18). Right panel shows a CFU-EC graft stained with anti–human CD31. Arrows indicate positive antigen staining. Results represent 9 other ECFC grafts and 2 other CFU-EC grafts. (B) Photomicrographs (original magnification, × 100) of ECFC and CFU-EC (far right) cellularized grafts stained with anti–human CD31 28 days after implantation. Vessels and capillaries in ECFC grafts are perfused with murine red blood cells (arrows) indicating anastomoses with murine blood vessels. CFU-EC grafts fail to form vessels or capillaries. Results represent 9 other ECFC grafts and 2 other CFU-EC grafts. (C) Quantitation of capillary density within ECFC (□) and CFU-EC (▪) cellularized grafts 28 days after implantation. Results represent the average number of capillaries containing murine red blood cells/mm² of graft tissue ± SEM of 10 ECFC and 3 CFU-EC grafts. *P < .05 by Student unpaired t test.