In recent years, extensive claims of stem cell plasticity have generated much excitement. The hypothesis posits that progenitor cells that already have a certain degree of specificity could be induced to "dedifferentiate"-to backtrack, as it were-to a wider phenotypic potential, or that they could even be reprogrammed (i.e., transdifferentiated) directly into progenitor status in a different lineage, without having to revert to an earlier, multipotent state. Some of the experimental support for this assumed plasticity came from findings that bone marrow stem cells, which are the progenitors of blood cell lineages, can be induced to produce other cell types, including neurons (Mezey et al., 2000) when transplantated to a new tissue environment.

Cautionary flags went up last year, when two papers showed that, in vitro, stem cells could fuse with differentiated cells, producing cells with two or even three sets of genetic material (see ARF related news story). In their paper describing fusion of embryonic neural stem cells with adult neurons, Austin Smith and colleagues remarked, "Our data raise a warning to the overzealous trend in stem cell research to conclude transdifferentiation or dedifferentiation of cells without careful examination of genotypes." (Ying et al., 2002) But these authors also noted that their findings were speculative and needed replication.

Support now comes from two independent in-vivo studies in a mouse model of a disorder involving the destruction of liver cells, which can be reversed by transplantation of haematopoietic stem cells. The renewed population of liver hepatocytes (which divide to repopulate the liver) has now been shown to be derived largely-perhaps even exclusively-from fused donor and recipient cells. Interestingly, the study by Wang et al. suggested that it is not the donor stem cell itself, but one of its differentiated progeny (possible macrophages or T or B cells) that actually fused with a local cell.

While the disorder in this model could be corrected, these results raise the sobering possibility that stem cells are not capable of true transdifferentiation across tissue types. "Although the frequency of spontaneous fusion resulting in [blood marrow-derived hepatocytes] is very low, it should nonetheless be noted that these cells can have therapeutic use when present in sufficient quantity. At present, this requires a strong selective growth advantage, but it is conceivable that induced cell fusion may achieve the efficiency necessary for the treatment of genetic diseases," write Wang and colleagues.-Hakon Heimer.

References:
Wang X, Willenbring H, Akkari Y, Torimaru Y, Foster M, Al-Dhalimy M, Lagasse E, Finegold M, Olson S, Grompe M. Cell fusion is the principal source of bone-marrow-derived hepatocytes. Nature. 2003 Mar 30. Abstract

Vassilopoulos G, Wang P-R, Russell DW. Transplanted bone marrow regenerates liver by cell fusion. Nature. 2003 Mar 30. Abstract