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CHANGES IN OSTEOPROGENITOR PROLIFERATION IN THE RAT SKELETON DUE TO MECHANICAL UNLOADING.   N. Basso, Y. Jia, C.G. Bellows and J.N.M. Heersche. Faculty of Dentistry, University of Toronto, Toronto.

A lack of mechanical stimulation associated with space flight or tail suspension in rats results in a reduction in bone matrix production and bone formation. This may be due toa  disruption/delay of the development of cells along the osteoblast lineage pathway. Our experiments address specifically the effects of in vivo unloading conditions on the proliferative capacity of osteoprogenitors. Using the NASA model of tail suspension, hind limbs of three-month-old female Sprague-Dawley rats were unloaded for 14 days. Normally loaded control rats were pair-fed. Explants were prepared from the proximal femur, calvarium, and proximal humerus and outgrowth cells were collected after 13 days and used to initiate experiments. Cells were plated at a density 3000 cells per dish and grown in medium either unsupplemented or supplemented with 10 nM dexamethasone (dex) or 3 μM progesterone (prog). Preliminary data revealed no significant difference in the number of fibroblastic colony forming units (CFU-F, a measure of total number of progenitors) between cell populations derived from suspended and control rats in any of the cell populations examined. We observed a decreased number of alkaline phosphatase positive progenitors (CFU-AP) to CFU-F observed in cells isolated from femur (dex treated) and calvarium (all treatment groups), but not in humerus, of suspended animals. The data suggests that hind limb unloading results in a decreased proportion of CFU-AP and that dex-dependent progenitors are affected by unloading to a greater extent than prog-dependent progenitors in femur and calvarium. Evaluation of bone nodule numbers, indicative of CFU-osteoblast (O), and of bone histomorphometry to evaluate bone formation parameters is in progress.