Figure 3: Candidate cellular niches mediated in maintenance and regulation of HSCs in bone marrow; endosteal and vascular niches. HSCs are in contact with SNO cells, bone-lining osteoblasts, within endosteal niche. Osteblasts produce several signal molecules such as Notch ligands, angiopoietin-1 (Ang-1), CXCL12, and cathepsin X mediated in control HSC pool and maintenance. Most HSCs are found in sinusoids, particularly adherence to CAR cells that surround sinusoidal endothelial cells (reticular niche). Similarly, CAR cells produce CXCL12 in association with CXCR4 signaling essentially for HSC maintenance (modified from [12]).