Characterization of CB-EPC–derived vascular networks in vivo. Whole mount staining of the implanted collagen gel revealed that the CB-EPCs (EGFP+) at day 87 after implantation maintained the expression of CD31 (A) and had intact basement membrane of collagen type IV (B) in vivo (EGFP, green; CD31 and collagen type IV, red). CB-EPCs implanted alone at high cell density (5 million cells/mL at high density vs 1 million cells/mL at normal density) were not able to form long-lasting blood vessels (C). The blood-derived endothelial cells were exposed to serum-free media for 48 hours, and the cells were then stained and quantified for TUNEL, a marker for apoptotic cells (D). CB-EPCs exhibited a higher resistance to serum-free medium–induced apoptosis. The vascular permeability of the CB-EPC–derived blood vessels to albumin was measured (Table 1). CB-EPC–derived blood vessels had low vascular permeability with values similar to those of normal mouse pial blood vessels. Blood flow rate as measured by red blood cell (RBC) velocity was comparable between the CB-EPC–derived vessels and mouse brain vessels (E). The number of rolling leukocytes on CB-EPC–derived blood vessels was increased after induction of systemic inflammation with intraperitoneal injection of 100 ng IL-1β for 4 hours (F). #P < .001; *NS; **P < .05. Scale bars (A,B) represent 50 μm.