[The Hematopoietic Stem Cell Niche]

Somatic stem cells self-renew to maintain tissue homeostasis for the lifetime of organisms through tightly controlled proliferation and differentiation. Hematopoietic stem cells (HSCs) are essentially required for hematopoietic homeostasis. Therefore, they not only ensure lifelong replenishment of all blood lineages, but also maintain a constant pool. Cell cycle quiescence is a critical feature contributing to stem cell maintenance. Recent studies have highlighted the importance of bone marrow (BM) microenvironments that regulate HSC functions (HSC niches). In the HSC field, there has been considerable interest and debate regarding whether or not quiescence and proliferation of HSCs is regulated by distinct niches. Previous reports suggest that quiescent HSCs reside near osteoblasts in the BM whereas actively cycling HSCs are found near sinusoids. However, this popular concept has not been supported by rigorous analyses. To gain more insight into the spatial localization of HSCs, we have developed a whole-mount staining technique that allows precise measurements of 3D distances of HSCs from structures and allows computational simulation to define the significance of these interactions. This novel approach has allowed us to uncover two distinct types of vessels associated with quiescent and proliferating HSCs and to underscore the importance of arteriolar vessels for stem cell quiescence. We will discuss the crosstalk between the two hematopoietic and mesenchymal stem cells with a review of the recent literature.