hMSCs stabilized engineered blood vessels in vivo. EGFP-labeled HUVECs were either implanted alone (A) or coimplanted with hMSCs (B) or 10T1/2 cells (C) in a collagen gel onto cranial windows in SCID mice. Images were taken at periodic time points with multiphoton laser scanning microscope to monitor the in vivo dynamics of vascularization by the implanted endothelial cells. Green indicates HUVECs expressing EGFP; red, functional blood vessels contrast-enhanced by rhodamine-dextran. Scale bar represents 50 μm.