The use of in vitro techniques to investigate cellular responses to mechanical load in bone

The skeleton is able to adapt its architecture in response to changes in its mechanical environment by altering the balance between bone formation nd bone resorption in a site-specific manner. To study the cellular mechanisms underlying this phenomenon, two artificially loadable bone organ culture systems were established, one in adult and one in embryonic bone. These models were validated by demonstrating strain-related cellular responses which were similar to those previously demonstrated in vivo.

Using explants of adult canine cncellous bone in organ culture, it was shown that a short period of mechanical loading, of physiological magnitude, stimulated the incorporation of ³H-uridine into RNA, measured over a six hour period following loading. Using autoradiography the response was localized within osteocytes and lining cells. The effect could be blocked by addition of indomethacin to the culture medium duringloading, indicating the involvement of prostanoids in the regulation of the response.

To follow load-related responses over a longer time scale and to estbalish whether embryonic bone in organ culture responded similarly to adult bone, an artificially loadable bone organ culture system was developed using embryonic chick tibiotarsi. It was shown that mechanical loading caused a strain-magnitude related increase in the incorporation of ³H-uridine into RNA, detectable 8 hours following loading, with a more pronounced effect at 24 hours. Loading also produced a strain magnitude-related increase in the activity of the enzyme glucose 6-phosphate dehydrogenase in both osteocytes and periosteal osteoblasts, detactable immediately following loading. Both the RNA and G6PD responses could be blocked by indomethacin treatment, suggesting that they may form part of a strain-related sequence of cellular activation and showing that the involvement of prostanoids was probably at an early stage in this pathway.