Schematic Representation Illustrating the Major Cellular Constituents of the Bone Marrow HSC Niche. A variety of cells, including osteoblasts, Cxcl12-abundant reticular (CAR) cells, Nestin+ mesenchymal stem cells (MSC), Leptin receptor (Lepr)-expressing perivascular cells, and endothelial cells, have been reported as possible components of the niche. Schwann cells, wrapping sympathetic nerve fibers, promote HSC quiescence. In total, the structural makeup of these various cells provides a specialized microenvironment regulating HSC self-renewal and differentiation, either through soluble factors such as CXCL12, Kit ligand (also known as SCF), angiopoietin-1, and VCAM-1, or through contact-dependent signals.Modified from original figure design by Daniel Lucas, PhD, and Sandra Pinho, PhD, both from Albert Einstein College of Medicine.