Decreased mitogenic and osteogenic responsiveness of calvarial osteoblasts isolated from aged rats to basic fibroblast growth factor.

Osteoporosis seen in aged individuals is represented by the reduced bone mass most likely resulting from decreased bone formation by osteoblasts. To examine whether aging causes a decrease in osteoblast activity, calvarial osteoblasts were isolated from aged rats (AOB) and studied for the capacity of the cells to form mineralized bone-like nodules in comparison with that of fetal calvarial osteoblasts (FOB). There were no significant differences in basal mineralized bone-like nodule formation determined by quantifying the size of the nodules which were formed in the cultures of AOB and FOB. However, the responsiveness of AOB to growth factors was profoundly reduced. AOB showed only marginal increase in mineralized bone-like nodule formation and growth in response to basic fibroblast growth factor (bFGF). On the other hand, bFGF markedly promoted mineralized bone-like nodule formation and proliferation in FOB. These results suggest that decreased responsiveness to local osteotropic growth factors such as bFGF might account for the reduced bone formation by aged osteoblasts, which in turn leads to the loss of bone mass characteristic for senile osteoporosis.