Long-term bone marrow (BM) repopulating (LTR)-hematopoietic stem cells (HSCs) exist frequently in BM trabecular bone surfaces, and it was clarified that osteoblasts (OBs) are critical for sustaining HSCs. The interaction of HSCs with their particular microenvironments, known as stem cell niches, is critical for maintaining their stem cell properties, including self-renewal capacity and the ability to differentiate into single or multiple lineages. In the niche, the fates of HSCs are regulated by the signaling molecules, extracellular matrix, and cell adhesion molecules produced by osteoblasts. Interaction of HSCs with their stem cell niches is critical for cell cycle regulation of HSCs. Especially, the quiescence of the cell cycle is thought to be an indispensable property for the maintenance of HSCs. We demonstrate that HSCs expressing the receptor tyrosine kinase Tie2 are quiescent and anti-apoptotic, transplantable and comprise a side-population (SP) of HSCs, which contact closely with angiopoietin-1 (Ang-1), a ligand for Tie2, expressing osteoblasts in the BM niche. The interaction of Tie2 and Ang-1 regulates the functional criteria of HSCs in the BM niche, including quiescence, anti-apoptosis and cell adhesion.
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