Regulation of hematopoiesis and its interaction with stem cell niches.

Hematopoietic stem cells (HSCs) are responsible for blood cell production throughout the lifetime of an individual. Interaction of HSCs with their particular microenvironments, known as stem cell niches, is critical for maintaining stem cell properties, including self-renewal capability and ability for differentiation into single and multiple lineages. In the niche, the niche cells produce signaling molecules, extracellular matrix, and cell adhesion molecules and regulate stem cell fates. Long-term bone marrow (BM)-repopulating HSCs recently have been found frequently to exist in the BM trabecular bone surface, and it has been clarified that osteoblasts (OBs) are a critical component for sustaining HSCs. HSCs keep a balance between quiescence and cell division/proliferation in the osteoblastic niche. The specific properties of HSCs are controlled dynamically by signaling of receptor/ligand and cell adhesion molecules produced by OBs.