Many studies have demonstrated that stem cell programming is influenced by the microenvironment [56] [57] [58] . To determine whether the phenotypic fate of vMSC-CM fusion products could be regulated by the microenvironment, following treatment with fusion media, we cultured vMSC-CM fusion products under either MSCspecific or CM-specific culture conditions and examined the incidence of fusion and morphology of MSC-CM fusion products. At days 5 and 7 following the induction of cell fusion, cocultures were probed with CM and MSC specific antibodies (anti-MF20 and anti-CD105, respectively, n = 1 replicate per sample per trial for 3 trials). At day 5, vMSC-CM cocultures contained a relatively high number of cells that expressed both MF20 and CD105 and the percentage of MF20 + /CD105 + cells relative to the total cell number was significantly greater than that of MSC-CM cocultures for both culture conditions (P < 0.005) (Figure 3(a) ). Of note, the percentage of MF20 + /CD105 + cells was much higher than the percentage of double positive cells detected using CellTracker dyes and flow cytometry (Figure 2(b) ). This could reflect the loss of VSV-G sustained by cell harvest, the different analytical approach (i.e., flow cytometry versus image analysis) and/or the behavior of fusion products between day 1 and day 5 (i.e., proliferation). By day 7, the percentage of MF20 + /CD105 + cells decreased to levels not statistically different from controls for both culture conditions. At the same time, the number of cells expressing MF20 alone increased substantially for both culture conditions. The change in percentage of MF20 + /CD105 + cells from day 5 to day 7 could reflect death of fusion products, or programming of the MSC fusion partner to a cardiomyocyte phenotype or both. If death of fusion products occurred, one would expect unfused CMs and MSCs to proliferate to fill the voids of the culture space. Interestingly, only the CM population increased from day 5 to day 7 and at rates significantly higher than that of control cultures, suggesting at least a portion of fusion products were maintained, and ultimately adopted a cardiomyocyte-like phenotype. This result was observed independent of the culture conditions. Of note, this experimental approach does not exclude the possibility that metaplasia rather than fusion occurred, that is, MSCs differentiate into CMs as a consequence of soluble factors in the coculture medium and maintain (at least transiently) expression of each cell type. However, MF20 + /CD105 + cells were rare in MSC-CM cocultures, suggesting metaplasia alone cannot account for coexpression of MF20 + /CD105 + or subsequent loss of MF20 + /CD105 + cells. In addition, MF20 + /CD105 + cells exhibited two distinct morphologies; some were long and spread, displaying MSC-like morphology (MSC medium = 16.59% ± 6.32%; CM medium = 14.03% ± 1.59%) while the majority (P < 0.05) were round and cobblestone-like, indicative of CM-like morphology (MSC medium = 80.49%±10.45%; CM medium = 85.97%± 1.60%) (Figures 3(b) and 3(c), Supplementary Figure 1B) . These results further support the possibility that CM nuclear material and cytoplasmic elements direct programming of MSC-CM fusion products independent of culture conditions.