An important issue in stem cell biology relates to mechanisms of cellular plasticity. Specifically, could any observed multipotency of, e.g., adult stem cells arise from true transdifferentiation or as a result of cell-cell fusion? We studied this issue using a culture paradigm of astrocyte monolayers and multipotent neurospheres generated from neonatal cerebellar cortex and the subventricular zone (SVZ). Based on fluorescence in situ hybridization (FISH), cells from these cultures were found to contain an abnormal number of sex chromosomes, suggesting that cellular fusion is a common in vitro occurrence. A Cre/lox recombination method was also exploited to further confirm the evidence of fusion. Next, we assessed the potential of fusogenic microglial involvement by combining CD11b immunolabeling with FISH sex chromosome analysis. Differentiating neurospheres were also studied from the PU.1 knockout mouse that lacks cells of myeloid origin, presumed to be a source of central nervous system microglia. Very few cells immunopositive for the microglial marker CD11b were found to be aneuploid, and there was no difference in fusion frequency between PU.1+/+ and PU.1-/- neurospheres. These results, together, suggest that stem and/or progenitor cells that generate neurons and glia in culture possess the ability to generate fused polyploidal cells, but microglial participation is not a requirement for fusion to occur. In addition to caution that should be exerted during the interpretation of in vitro neural cell plasticity, the data also suggest that novel therapeutic treatments could be designed that exploit cellular fusion in rescue paradigms for degenerating neuronal populations.