Bone marrow niches for hematopoietic stem cells and immune cells

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.