Proving plasticity

A number of recent reviews have outlined the current controversies in stem cell plasticity (12,17-19). Problems with initial studies in this field are numerous. First, plasticity has primarily been inferred from the behavior of undefined mixtures of cells. It is therefore unclear which cells in these mixtures produce the cells that give rise to the original and new phenotypes and whether separate cell lineages arise from the same cell. Second, cell populations have been transplanted following time in tissue culture, and it is unclear whether the stem cells as originally isolated had the ability to produce heterologous tissue or whether epigenetic modification occurred because of the culture period. Third, most studies have not demonstrated the ability of transdifferentiating stem cells to self-renew. Finally, most studies have not demonstrated functionality of the progeny of transdifferentiated stem cells.

These criteria are the measure by which all further studies that claim to demonstrate stem cell plasticity should be evaluated. Hematopoietic stem cell biologists have developed a number of approaches to identify and characterize the behavior of putative stem cells, and by using modifications of these approaches, many of the controversial questions in this field will be resolved. In summary, the science that has been performed to date suggesting stem cell plasticity has not clearly established that adult stem cells are plastic.

Technical limitations to each approach used to measure the presence of donor-derived cells in recipient tissue following transplantation can also lead to misleading results. For instance, two recent studies have demonstrated that embryonic stem cells will fuse with hematopoietic cells or with NSCs when cultured together, and that these chimeric cells will display the pheno-type of both original cell types (20,21). The possibility of such fusion events would call into question plastic behavior observed following a culture period, especially when different cell types were mixed. When using the Y chromosome or P-galactosidase (P-Gal) staining to detect donor-derived cells, controls that measure background staining within specific tissue types are crucial (18). Autofluorescence can be mistaken for the fluorescence of green fluorescent protein. When evaluating studies that argue that stem cells are plastic, a careful review of the model system, discovery whether prior culturing of the cells was performed, and a review of experimental controls are essential.

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