Implanted donor (green) endothelial cells (ECs) form vascular tubes with lumens that must anastomose with host vessels to become part of the functional vasculature. The host vessel in this image is composed of endothelial cells (brown), basement membrane (blue), and pericytes (red) and contains flowing red blood cells. The donor ECs wrap around the host vessel to displace the pericytes and proteolytically attack the basement membrane of the host vessel endothelium via matrix metalloproteinases-9 and -14. Subsequently, the donor ECs may now gain access to the host endothelium and displaces some cells to integrate into the endothelial intima and divert blood into the donor vasculature. See Figure 6 in Cheng et al, page 4740.