['We investigated the effects of compressive mechanical stress on osteoclastogenesis of synovial cells to clarify the mechanism of osteoclast formation by those cells in temporomandibular joint (TMJ) disorders.', 'Synovial cells were isolated from rat knee joints and continuously compressed using a conventional method. The expression of receptor activator nuclear factor kappaB ligand (RANKL) mRNA and protein in synovial cells was analyzed by reverse transcriptase-polymerase chain reaction, immunoblotting, and immunofluorescence staining. Mouse bone marrow cells were cultured with synovial cells for 7 days to detect osteoclasts.', 'The expressions of RANKL mRNA and protein in synovial cells were increased with compressive force. When mouse bone marrow cells were cultured with continuously compressed synovial cells, tartrate-resistant acid phosphatase-positive multinucleated cells were formed. Osteoprotegerin completely inhibited osteoclast formation induced by culturing with compressed synovial cells.'] Does compressive mechanical stress promote osteoclast formation through RANKL expression on synovial cells?