In mammals, hematopoietic stem cells (HSCs), which give rise to all blood cells and their progenies, including
immune cells are controlled by special microenvironments, termed niches in the bone marrow during homeostasis and
infection. However, the identity, nature and function of these niches remain unclear. It has been reported that HSCs are in
contact with osteoblasts lining the bone surface and osteoblasts act as niches for HSCs (termed endosteal niche).
However, recent studies suggest that only a small number of HSCs reside in the endosteal niche. In contrast, many HSCs
are shown to be in contact with endothelial cells in the marrow. In addition, recent studies suggest that primitive
mesenchymal cells, including CXCL12-abundant reticular (CAR) cells and Nestin-expressing cells, which have the ability
to differentiate into adipocytes as well as osteoblasts act as niches for HSCs. Here we review candidate niches for HSCs in
the bone marrow controlling hematopoiesis and chronic inflammation.
Keywords: Bone marrow, chemokine, CXCL12, HSC, niche, SCF, stromal cell
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