Recently, the transdifferentiation debate has been thrown open with the observations that chromosome transfer (and therefore Y-chromosome positivity in the nucleus of female cells) can arise as a result of fusion of cells rather than transdifferentiation, as has been assumed and frequently reported. This evidence comes from experiments in two independent laboratories and is based on observations made in vitro. Mouse embryonic stem cells were cocultured with either neurospheres (Terada et al., 2002) or bone marrow-derived cells (Ying et al., 2002), and using clearly distinct genetic markers, in both cases apparently spontaneous fusion led to the establishment of hybrid cell colonies. It must be emphasized that there is as yet no indication that this phenomenon occurs in vivo, and it is not yet certain how this would translate into the likelihood of such an event in vivo. However, the presence in, e.g., the livers of female bone marrow transplant recipients receiving male-derived donor bone marrow
of hepatocytes with Y-chromosome signals could in theory arise as a result of cell fusion. This clearly requires careful scrutiny and indicates that genotyping of cells suggested to have transdifferentiated may be the only way to exclude cell fusion as an entity and indeed before any final conclusions can be drawn on the extent of cell fusion ves transdifferentiation phenomena.
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