Model of HSC-OB adhesion-ligand pairs. The homing and tethering of HSCs to the osteoblast–stem cell niche is a rapid and reversible process because HSCs can be released through changes in receptor activation/expression, shedding, or enzymatic cleavage. The mechanisms regulating this 2-way movement of HSCs are beginning to be elucidated. Molecules involved in this regulation include cell-associated growth factors and adhesion molecules. Cell-associated growth factors are known to induce the quiescence or survival, proliferation, and niche localization of HSCs. Similarly, adhesion molecules may be functionally divided into those that localize HSCs to their niches and those that, once engaged, regulate G₀ quiescence and survival.