Bone and Aletered Mechanical Loading: Basic Science Approaches
The appearance of osteoblasts following a brief episode of increased mechanical loading has been documented, but the proliferation of osteoblast precursors in the cellular periosteum has not yet been investigated. In this study, a single bending load (36 cycles, 2 Hz, 2000 μ strain) was applied to the right tibia of skeletally mature female rats and the time course of pre-osteoblast proliferation in the periosteum was examined. A single injection of bromodeoxyuridine (40 mg/kg) was administered 1 h prior to collection times at 12, 24, 30, 36, 42, 48, 60, and 72 h post-load. Endpoints included surface type (based on the morphology of surface cells), pre-osteoblast labeling index, and the distance of labeled and unlabeled cells to the bone surface. Loaded bones showed a significant increase in preosteoblast proliferation from 36–72 h as compared to non-loaded bones. Osteoblast surface was increased during the same time span, indicating that at least some of the early osteoblasts were derived from non-proliferative sources. The distance of labeled and unlabeled cells increased with time, but the distance of the two cell populations did not differ, indicating that there was no partitioning of the proliferative cells. Collectively, these results indicate that a single load application is sufficient to initiate an osteogenic response that includes the proliferation of osteoblast precursors. At earlier post-load time-points, the primary response may be the conversion of surface cells to active osteoblasts; however, at later time points, it seems likely that the proliferating pre-osteoblats are contributing new osteoblasts to active osteogenic sites.