HEMATOPOIESIS AND STEM CELLS Positioning of bone marrow hematopoietic and stromal cells relative to blood flow in vivo: serially reconstituting hematopoietic stem cells reside in distinct nonperfused niches

Hematopoietic stem cell (HSC) niches have been reported at the endosteum or adjacent to bone marrow (BM) vasculature. To investigate functional attributes of these niches, mice were perfused with Hoechst 33342 (Ho) in vivo before BM cell collection in presence of pump inhibitors and antibody stained. We report that the position of phenotypic HSCs, multipotent and myeloid progenitors relative to blood flow, follows a hierarchy reflecting differentiation stage, whereas mesenchymal stromal cells are perivascular. Furthermore, during granulocyte colony-stimulating factor– induced mobilization, HSCs migrated closer to blood flow, whereas stromal cells did not. Interestingly, phenotypic Lin Sca1 KIT CD41 CD48 CD150 HSCs segregated into 2 groups (Honeg or Homed), based on degree of blood/Ho perfusion of their niche. HSCs capable of serial transplantation and longterm bromodeoxyuridine label retention were enriched in Honeg HSCs, whereas Homed HSCs cycled more frequently and only reconstituted a single host. This suggests that the most potent HSC niches are enriched in locally secreted factors and low oxygen tension due to negligible blood flow. Importantly, blood perfusion of niches correlates better with HSC function than absolute distance from vasculature. This technique enables prospective isolation of serially reconstitutingHSCsdistinct fromother lesspotent HSCs of the same phenotype, based on the in vivo niche in which they reside. (Blood. 2010;116(3):375-385)