Effects of Strain Magnitude on Mechanical Responses of Three-Dimensional Gel-Embedded Osteocytes Studied with a Novel 10-Well Elastic Chamber

Osteocytes inside bone matrix play an important role in detecting the local mechanical environment. In order to study their mechanical responses, we have developed a mechanical loading device that can apply physiological and supraphysiological strains to osteocytes embedded in a three dimensional (3-D) gel. The newly designed elastic chamber with ten separated culture wells can simultaneously apply five different strain magnitudes for the mechanical stretching of the cells. When the gel-embedded MLO-Y4 cells were prepared in the wells, they were subjected to mechanical stretching of physiological and supraphysiological strain levels for 24 h. The cell viability assay indicated that significant dead cells were observed for strain values greater than 8890 με. Beyond this threshold, the number of dead cells linearly increased with the strain magnitude. The supernatant of MLO-Y4 cells, which was exposed to strain levels beyond the threshold, showed a significant increase in tartrate-resistant acid phosphatase (TRACP) activity in the bone marrow culture. These experimental findings indicate that the local death of osteocytes provides an important mechanism to initiate bone resorption.