ABSTRACT
Lineage fidelity of a stem cell may not be as strictly governed as was once thought. This lineage plasticity is most easily studied with hematopoietic stem cells because, even in humans, they are readily obtainable from blood and bone marrow, and the assays to define them are well-characterized, having been the subject of studies for over 30 years. Because of the ease with which they can be obtained from humans, hematopoietic stem cells (HSCs) are an ideal source of cells, which, if truly plastic, could be exploited for the therapeutic repair of damaged and diseased tissues of any type. However, to harness this potential, these cells must be easily identified and isolated from different sources, effectively expanded ex vivo, and evaluated functionally by assays that test the desired potential. In order to study and eventually utilize HSCs for the treatment of human diseases, these cells must be expanded ex vivo without change in the identifiable and functional phenotype. The largest impediment to the successful development of these expansion methods has been the difficulty in accurate identification and enumeration of HSCs.
