Austin Smith and colleagues at the University of Edinburgh, Scotland, and Oxford University, England, co-cultured differentiated mouse brain cells (from embryos and from adults) with embryonic stem cells. They wound up with pluripotent cells that carry markers identifying them as descended from the mouse brain cells, but also markers specific to the embryonic stem cells. Moreover, the researchers found genetic material from both lines, indicating the new cells were hybrids.

Similarly, Naohiro Terada and associates at the University of Florida, in Gainesville, followed up on studies showing that transplanted bone marrow cells could be reprogrammed into various cell types, including neurons. They cultured adult mouse bone marrow cells together with mouse embryonic stem cells. As in the other study, what appeared to be the de-differentiation of bone marrow cells into stem cells was, in fact, the fusion of marrow and embryonic cells. Some of these hybrids contained as many as three sets of genetic material from both sources.

Echoing the cautionary comments of Smith et al., Terada et al. note that, "Contribution of cell fusion to apparently transdifferentiated cells in vivo is presently pure speculation; however, our data raise a warning to the overzealous trend in stem cell research to conclude trans-differentiation or de-differentiation of cells without careful examination of genotypes."-Hakon Heimer.

References:
Ying Q-L, Nichols J, Evans E, Smith AG. Changing potency by spontaneous fusion. Nature 2002 Mar 13;DOI 10.1038/nature729. Abstract

Terada N, Hamazaki T, Oka M, Hoki M, Mastalerz DM, Nakano Y, Meyer EM, Morel L, Petersen BE, Scott EW. Bone marrow cells adopt the phenotype of other cells by spontaneous cell fusion. Nature. 2002 Apr 4 ; 416(6880):542-5. Abstract

ARF: Tetraploid neurons were seen in hippocampal and basal forebrain neurons Alzheimer's (Yang et al. 2001), probably because these neurons re-enter the cell cycle and duplicate their DNA. Could these tetraploid neurons also be the result of fusion between neurons and stem cells migrating to the site of neurodegeneration?
Sugaya: I think the explanation Karl Herrup made in his paper is more likely, because if the neural stem cells could spontaneously fuse to the brain cells it would be quite dangerous. We might end up having too many tumors. Though I would be open to this idea. Affected areas of AD brain produce a lot of cytokines, like IL-3, which Terada used for induction of the fusion. Anything could happen in science.