SKELETAL UNLOADING IN RAT DECREASES PROLIFERATION OF RAT BONE AND MARROW-DERIVED OSTEOBLASTIC CELLS

The effects of skeletal unloading on osteoblastic cells were evaluated in tail-suspended rats. Hindlimb elevation for 14 days induced osteop enia, decreased histomorphometric indexes of bone formation in tibial metaphysis, and reduced plasma osteocalcin and alkaline phosphatase (A LP) levels compared with controls. The in vitro proliferation of osteo blastic cells isolated from the endosteal bone surface of suspended ti bias was decreased by 42 and 31% at 2 and 4 days of culture, respectiv ely, compared with controls, as shown by [H-3]thymidine labeling and c ell-number. The proliferation of ALP-positive marrow stromal cells was also decreased by 20-24% at 1 and 2 days of culture. However, ALP act ivity in bone-derived cells and marrow stromal cells was not different in unloaded and control rats, and the number of bone cells synthesizi ng osteocalcin, osteonectin, and type I or type III collagen was ident ical in the two groups. The results indicate that the inhibition of bo ne formation induced by skeletal unloading is related to a decreased p roliferation of putative osteoblast precursor cells present along the endosteal bone surface and in the marrow stroma.