Niches for hematopoietic stem cells and immune cell progenitors.

The special microenvironments, termed niches, with which hematopoietic stem cells (HSCs) are in contact, have been thought to be required for the maintenance of HSCs and the generation of immune cells in bone marrow. Although the identity of the HSC niche has been a subject of long-standing debate, recent findings demonstrate that a population of mesenchymal stem cells, termed CXC chemokine ligand (CXCL)12-abundant reticular (CAR) cells or leptin receptor-expressing (LepR+) cells, are the major cellular components of niches for HSCs and lymphoid progenitors, which express specific transcription factors, including Foxc1 and Ebf3, and cytokines, including CXCL12 and stem cell factor (SCF), essential for their niche functions. The identity and functions of other types of cells, including osteoblasts, sinusoidal endothelial cells, periarteriolar cells, megakaryocytes and a population of macrophages in HSC maintenance, have also been shown.