Non-side-population hematopoietic stem cells in mouse bone marrow

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Abstract
Most hematopoietic stem cells (HSCs) are assumed to reside in the so-called side population (SP) in adult mouse bone marrow (BM). We report the coexistence of non-SP HSCs that do not significantly differ from SP HSCs in numbers, capacities, and cell-cycle states. When stained with Hoechst 33342 dye, the CD34-/low c-Kit+Sca-1+lineage marker- (CD34-KSL) cell population, highly enriched in mouse HSCs, was almost equally divided into the SP and the main population (MP) that represents non-SP cells. Competitive repopulation assays with single or 30 SP- or MP-CD34-KSL cells found similar degrees of repopulating activity and frequencies of repopulating cells for these populations. Secondary transplantation detected self-renewal capacity in both populations. SP analysis of BM cells from primary recipient mice suggested that the SP and MP phenotypes are interconvertible. Cell-cycle analyses revealed that CD34-KSL cells were in a quiescent state and showed uniform cell-cycle kinetics, regardless of whether they were in the SP or MP. Bcrp-1 expression was similarly detected in SP- and MP-CD34-KSL cells, suggesting that the SP phenotype is regulated not only by Bcrp-1, but also by other factors. The SP phenotype does not specify all HSCs; its identity with stem cell function thus is unlikely.