Vasculogenic potential of peripheral blood (PB)– versus cord blood (CB)–derived endothelial progenitor cells (EPCs). PB-EPCs and CB-EPCs were mixed with 10T1/2 cells in a collagen gel, and implanted into cranial windows in severe combined immunodeficient (SCID) mice. Images were taken at periodic time points with multiphoton laser scanning microscope for in vivo dynamics of vascularization by the implanted endothelial cells. PB-EPCs formed vascular-like structure 4 days after implantation and some of them became perfused at day 11. The PB-EPC–derived blood vessels were transient and almost completely disappeared by day 21 (A). There was no significant difference in the mean (± SEM) density of functional vessels derived from PB-EPCs between groups implanted with PB-EPCs only and PB-EPCs with 10T1/2 cells (B) (n = 4 for each group and experiments were performed with 3 different batches of adult peripheral blood). In some animals, there were still some sparse but functional blood vessels 27 days after implantation (C). In contrast, CB-EPCs formed a uniformly dense network of functional blood vessels (D). Implantation of CB-EPCs alone led to a rapid regression of the implanted cells, while coimplantation of CB-EPCs and 10T1/2 cells resulted in a stable and functional vasculature (E) (n = 4 for each group and experiments were performed with 3 different batches of human umbilical cord blood). The CB-EPC–derived vascular network was stable and functional for more than 119 days in vivo (F). Green indicates PB- or CB-derived endothelial cell expressing enhanced green fluorescent protein (EGFP); red, functional blood vessels contrast-enhanced by rhodamine-dextran. Scale bars represent (A,D) 50 μm; (C,F) 100 μm.