[Bone and marrow niches for hematopoiesis].

Hematopoietic stem cells (HSCs) and their progeny are thought to be regulated by special microenvironments, termed niches in the bone marrow during homeostasis. However, the identity and function of these hematopoietic niches remains unclear. It has been reported that HSCs are in contact with osteoblasts lining the bone surface and osteoblasts act as niches for HSCs (endosteal niche) ; however, other studies suggest that few HSCs reside in the endosteal niche. In contrast, most HSCs are shown to be in contact with endothelial cells (vascular niches) and/or primitive mesenchymal cells, including CXCL12-abundant reticular (CAR) cells or Nestin-expressing cells, which have ability to differentiate into adipocytes as well as osteoblasts. Recent in vivo studies revealed that CAR cells acted as niches for hematopoietic stem and progenitor cells (HSPCs) and that endothelial cells and Nestin-expressing cells acted as niches for HSCs. In addition, marrow nonmyelinating schwann cells might be involved in the maintenance of HSCs. Here we review candidate niches for HSCs and hematopoiesis in the marrow.