The fate decision of hematopoietic stem cells (HSCs), quiescence, proliferation or differentiation, is uniquely determined by functionally specialized microenvironments defined as the HSC niches. However, whether quiescence and proliferation of HSCs are regulated by spatially distinct niches is unclear. Although various candidate stromal cells have been identified as potential niche cells, the spatial localization of quiescent HSCs in the bone marrow remains controversial. In our recent study, we have established whole-mount confocal immunofluorescence techniques, which allow us to precisely assess the localization of HSCs and their relationships with stromal structures. Furthermore, we have assessed the significance of these associations using a computational simulation. These novel analyses have revealed that quiescent HSCs are specifically associated with small caliber arterioles, which are predominantly distributed in the endosteal bone marrow while the associations with sinusoidal vessels or osteoblasts are not significant. Physical ablation of the arteriolar niche causes the shift of HSC localization to sinusoidal niches, where HSCs are switched into non-quiescent status. This new imaging analyses together with previous studies suggest the presence of spatially distinct vascular niches for quiescent and non-quiescent (proliferating) HSCs in the bone marrow.
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