Cocultures of CMs with MSCs or vMSCs were maintained in culture medium for 4 h after incubation with fusion medium, followed by imaging and flow cytometry. Images were acquired with a 20X UPlanFluor objective (NA = 0.5), FITC and Texas Red filters, on an IX71 inverted deconvolution fluorescence microscope (Olympus Center Valley, PA, USA) and analyzed with Slidebook software (Intelligent Imaging Innovations Denver, CO, USA) and ImageJ (Fiji; open source software, http://pacific.mpi-cbg.de/wiki/index.php/Fiji). Images were normalized using unstained controls. Cells were analyzed at the UWCCC Flow Cytometry Facility on a FACSCalibur flow cytometer (BD Biosciences). Events were live/dead gated with forward scatter and side scatter plots. Fusion products were quantified by gating the region positive for FL1 and FL2 channels, corresponding to CellTracker Green CMFDA and Red CMTPX, respectively.