MSCs or CMs in monolayer were stained for proteins characteristic of MSCs (CD73, CD90, and CD105), as well as proteins characteristic of CMs (sarcomeric myosin (MF20)). Cell cultures were fixed with 4% paraformaldehyde for 10 min, followed by two washes with phosphate buffered saline (Fisher Scientific). Cells were probed with goat anti-CD73 (V-20, Santa Cruz Biotech, Santa Cruz, CA, USA), rabbit anti-CD90 (RB3970, Abgent, San Diego, CA, USA), goat anti-CD105 (GKY02, R&D Systems, Minneapolis, MN, USA), and mouse anti-MF20 (IgG2b, Developmental Studies Hybridoma Bank, Iowa City, IA) diluted 1 : 25, 1 : 50, 1 : 50, and no dilution, respectively, in diluting buffer (5% BSA (Fisher Scientific), 0.02% NaN3-(Acros Organics) in phosphate buffered saline (Fisher Scientific)) and incubated for 30 min at room temperature or overnight at 4 • C, followed by incubation with fluorescent secondary antibodies: donkey anti-goat Alexa Fluor (AF488, Invitrogen), goat anti-rabbit Alexa Fluor (AF647, Invitrogen), and donkey anti-mouse (AF546, Invitrogen) at a 1 : 200 dilution in preadsorption solution (90% diluting buffer, 5% human serum (Pelfreez, Brown Deer, WI, USA), and 5% mouse serum (Equitech-Bio, Inc, Kerrville, TX, USA)) for 45 min at room temperature. Samples were counterstained with DABCO/DAPI mounting solution. Fluorescence emission was detected on an IX71 inverted deconvolution fluorescence microscope (Olympus). Images were acquired with a 20X UPlanFluor objective (NA = 0.5), and analyzed using MSC-CM or vMSC-CM cocultures were probed with antibodies against CM marker (MF20) and MSC marker (CD105) to evaluate the morphology of fusion products and the phenotype of cells within coculture. Positive events for fusion were calculated as the percentage of CD105 and MF20 positive cells containing a nucleus divided by total number of nuclei obtained from analysis of at least eight optical fields per sample. Fields (3-10 fields) were selected based on cell number (minimum of 3 cells) and position within the wells (center of wells) n = 2.