Stem cells in the bone marrow. HSCs are assumed to reside in endosteal and perivascular niches. Osteoblasts mainly constitute the endosteal niche. MSCs, perivascular stromal cells, CAR cells, and sympathetic neurons together with Schwann cells constitute the perivascular niche surrounding endothelial cells. Macrophages and adipocytes may lie between these niches and influence HSC/niche interaction.