The stem cell niches in bone
J. Clin. Invest. Tong Yin, et al. 116:1195 doi:10.1172/JCI28568 [Go to this article.]

Figure 2
The osteoblastic and vascular niches in bone. Under normal physiological conditions, HSCs reside in either the osteoblastic or vascular niche. A portion of HSC daughter cells, in response to changes in levels of SDF-1 in the BM, will leave the niche and begin to mobilize and circulate. HSC homing is the reverse of mobilization, occurring in response to higher levels of SDF-1 in the BM. The osteoblastic niche may provide a quiescent microenvironment for HSC maintenance. In contrast, the vascular niche facilitates HSC transendothelial migration during mobilization or homing and may favor HSC proliferation and further differentiation. The process of recruiting HPCs to the vascular niche may depend on endothelium-derived FGF-4 and SDF-1. Higher FGF-4 and oxygen concentration gradients as the cells progress from the osteoblastic niche to the vascular niche might play a role in recruitment, proliferation, and differentiation of HSCs/HPCs. Under stress such as thrombocytopenia, SDF-1 and VEGF activate MMP-9, which converts membrane-associated Kit ligand into soluble Kit ligand (sKitL) and in turn promotes HSCs entry into the cell cycle, mobilization to the vascular niche, and differentiation (34).