Hematopoietic stem cells from different sources: biological and technical aspects

Hematopoietic stem cell (HSC) enumeration is crucial to predict the engraftment potential of a given HSC collection, and currently involves the surrogate count of nucleated cells, CFU or CD34(+) cells. However, there is raising evidence that CFU are HSC involved in shortterm but not in long-term reconstitution, and that only a small fraction of all CD34(+) cells have long term multilineage engraftment potential, In this regard, there is evidence that cord blood (CB), bone marrow (BM) and peripheral blood (PB) derived HSC are highly heterogeneous for a number of antigens useful for HSC enumeration by flow cytometry, Moreover, there is a raising evidence that a CD34(-) human HSC might exist, The CD34(-) HSC has been already described in animals and in human Hoechst 33342 negative HSC, This notwithstanding, clinical data have clearly demonstrated that purified allogeneic CD34(+) cells can reconstitute the myeloid and the lymphoid lineages in myeloablated recipients. In the lack of a suitable marker for CD34(-) HSC enumeration, it is hard to predict the role of CD34(-) HSC in hematopoietic reconstitution after transplantation, On the other hand, these cells might be a better target for HSC expansion and gene transfer.