TRANSFORMING GROWTH-FACTOR-BETA AND THE INITIATION OF CHONDROGENESIS AND OSTEOGENESIS IN THE RAT FEMUR

We have investigated the ability of exogenous transforming growth factor-β (TGF-β) to induce osteogenesis and chondrogenesis, critical events in both bone formation and fracture healing. Daily injections of TGF-β1 or 2 into the subperiosteal region of newborn rat femurs resulted in localized intramembranous bone formation and chondrogenesis. After cessation of the injections, endochondral ossification occurred, resulting in replacement of cartilage with bone. Gene expression of type II collagen and immunolocalization of types I and II collagen were detected within the TGF-β-induced cartilage and bone. Moreover, injection of TGF-β2 stimulated synthesis of TGF-β1 in chondrocytes and osteoblasts within the newly induced bone and cartilage, suggesting positive autoregulation of TGF-β. TGF-β2 was more active in vivo than TGF-β1, stimulating formation of a mass that was on the average 375% larger at a comparable dose (p < 0.001). With either TGF-β isoform, the dose of the growth factor determined which type of tissue formed, so that the ratio of cartilage formation to intramembranous bone formation decreased as the dose was lowered. For TGF-β1, reducing the daily dose from 200 to 20 ng decreased the cartilage/intramembranous bone formation ratio from 3.57 to zero (p < 0.001). With TGF-β2, the same dose change decreased the ratio from 3.71 to 0.28 (p < 0.001). These data demonstrate that mesenchymal precursor cells in the periosteum are stimulated by TGF-β to proliferate and differentiate, as occurs in embryologic bone formation and early fracture healing.