Endothelin-1 stimulated vasoconstriction of the engineered blood vessels. HUVEC/hMSC-derived engineered blood vessels were superfused with 100 nM endothelin-1 to stimulate vasoconstriction in vivo. (A) Representative multiphoton microscopy images during endothelin-1 superfusion. Green indicates EGFP-hMSCs; red, functional blood vessels contrast-enhanced by rhodamine-dextran. Scale bar represents 50 μm. (B) The blood vessel diameter was quantified over a 20-minute period (n = 4). The value was expressed as a fraction of the original diameter. Scale bar represents 50 μm.