Migration and Differentiation of Osteoclast Precursors under Gradient Fluid Shear Stress

The skeleton is able to adapt to mechanical loading through bone remodeling, i.e. bone resorption followed by bone formation. The osteoclasts close to microdamages are believed to initiate bone resorption, but whether local mechanical loading such as fluid flow regulates recruitment and differentiation of osteoclast precursors at the site of bone resorption has yet to be investigated. In the present study, finite element analysis first revealed that there exists low fluid shear stress (FSS) field inside microdamage. Basing on a custom-made device of cone-and-plate fluid chamber, finite element analysis and particle image velocimetry measurement were performed to verify the formation of gradient FSS flow field. Furthermore, the effects of gradient FSS on the migration, aggregation, and fusion of osteoclast precursors were observed. Results showed that osteoclast precursor RAW264.7 cells migrate along radial direction toward the region with lower FSS during exposure to gradient FSS stimulation for 40 min, obviously deviating from the direction of actual fluid flow indicated by fluorescent particles. When inhibiting calcium signaling pathway with gadolinium and thapsigargin, cell migration toward low-FSS region was significantly reduced. For other cell lines, MC3T3-E1, PDLF, rMSC and MDCK, gradient FSS stimulation did not lead to the low-FSS-inclined migration. After being cultured under gradient FSS stimulation for 6 days, the density of RAW264.7 cells and the ratio of TRAP-positive multinucleated osteoclasts in low-FSS region were significantly higher than those in high-FSS region. Therefore, osteoclast precursor cells may have special ability to sense FSS gradient and tend to actively migrate toward low-FSS region, which is regulated by calcium signaling pathway.