Load related changes in bone cells

The mass and architecture of the skeleton, and thus its structural competence, are maintained as a result of continuing functional influence engendered by load bearing. In order to study this functional adaptation in bone, the following series of experiments were performed. 1) Bone cell enzyme activity was investigated using a quantitative cytochemical technique after a single short period of potentially osteogenic loading in vivo . Loading produced a rapid strain-related increase in the activity of glucose 6-phosphate dehydrogenase in both osteocytes and periosteal osteoblasts. There was no corresponding rise in the activity of glyceraldehyde 3-phosphate dehydrogenase or aldolase. These findings suggest that the increase in activity was directed towards synthetic activity. 2) The orientation of bone matrix proteoglycan molecules was investigated using quantitative polarised light microscopy after similar short periods of potentially osteogenic loading. Loading in vivo and in vitro was found to cause reorientation of the proteoglycans. The effect occurred rapidly, and was found to persist in vivo for over 24 hours. Critical electrolyte concentration staining and enzyme digestion revealed that the molecules involved contained predominantly chrondroitin sulphate glycosaminoglycans. Electron microscopy allowed direct visualisation of the reorientation effect. Reorientation of proteoglycans was observed in response to loading in bones from birds, rats and dogs, and in cortical and cancellous bone. This phenomenon could provide the means to `lq capture and `lq average the effect of transient strains and so provide a lasting influence on the populations of cells responsible for the control of remodelling. 3) The effect of short periods of daily loading in vivo on sections of canine fibula protected from functional strains was investigated by measuring the changes in cross-sectional area. Functional isolation alone resulted in bone loss. Loading not only prevented this, but also engendered significant new bone deposition, confirming the earlier studies in birds. 4) The effect of pulsed electromagnetic fields (PEMFs) on bone was investigated using the same canine fibula preparation. Functionally isolated canine fibulae were exposed in vivo to PEMFs for a short period each day. After twelve weeks, these bones had experienced only 50% of the bone loss previously shown to be associated with the preparation.