Functional characterization of hematopoietic stem cells in the spleen

Hematopoietic stem cells (HSCs) reside in both bone marrow (BM) and spleen in adult mice. However, whether BM and spleen HSCs are functionally similar is not known. Spleen HSCs were compared with BM HSCs by various assays.Whole BM and spleen cells were quantitatively analyzed by competitive repopulation. Single-cell transplantation was performed with HSCs purified from BM and spleen. A parabiosis model was used to distinguish organ-specific HSCs from circulating HSCs. The cell cycle was analyzed with pyronin Y staining and bromodeoxyuridine uptake.Repopulating and self-renewal potentials were similar on a clonal basis between BM and spleen HSCs, whereas the HSC frequency in the spleen was significantly lower than that in the BM. Analysis of parabiotic mice suggested that most HSCs are long-term residents in each organ. Cell-cycle analysis revealed that spleen HSCs cycle twice as frequently as do BM HSCs, suggesting that G(0) phase length is longer in BM HSCs than in spleen HSCs. The cycling difference between BM and spleen HSCs was also observed in mice that had been reconstituted with BM or spleen cells, suggesting that HSC quiescence is regulated in an organ-specific manner.Spleen HSCs and BM HSCs are functionally similar, but their cycling behaviors differ.