Role of platelet-derived growth factor in osteoblast function and bone synthesis

In vitro studies have shown that platelet-derived growth factor binds to specific receptors on osteoblasts and induces proliferation. Some reports also indicate that platelet-derived growth factor may inhibit differentiation of bone cells. Such findings are difficult to interpret, however, because many of these studies used impure platelet-derived growth factor extracted from platelets (rather than pure, recombinant platelet-derived growth factor), used mixed cell populations, and added serum to the culture media (likely introducing additional growth factors). In vivo studies also have yielded conflicting results regarding the effects of platelet-derived growth factor on bone formation. Platelet-derived growth factor may act in concert with insulin-like growth factor I to induce bone formation in a canine periodontal system. Other investigators, however, have shown that platelet-derived growth factor inhibits the effects of osteoinductive proteins such as osteogenin. No evidence of bone formation has been detected in patients receiving platelet-derived growth factor treatment of soft tissue wounds overlying bony prominences in clinical trials. Thus, despite the observations that osteoblasts can specifically bind and proliferate in response to platelet-derived growth factor in tissue culture, platelet-derived growth factor alone has not yet been proved to be osteoinductive in vivo. These observations suggest that in the presence of the many cytokines and multiple cell types found in wounds and fractures, the direct effects of platelet-derived growth factor on osteoblasts are eclipsed.