MORPHOLOGICAL-STUDY OF RECOMBINANT HUMAN TRANSFORMING GROWTH-FACTOR BETA-1-INDUCED INTRAMEMBRANOUS OSSIFICATION IN NEONATAL RAT PARIETAL BONE

Recombinant human transforming growth factor beta1 (rhu TGFbeta1) was injected singly or repeatedly for 3-12 days into the periosteum of the right side parietal bone of neonatal rats under the period of bone growth, and the time course of histological changes of the bone was observed by light and electron microscopy and by enzyme histochemistry. The repeated injections of rhu TGFbeta1 at 200 ng/day increased the thickness of the bone tissue on the treated side, which was about twice the nontreated side value after 12-day injections. On the dermal side, preosteoblasts in the periosteum increased in an early stage of treatment, and thereafter, differentiation into osteoblasts, increase of bone matrix, bone marrow cavity formation, and increase of osteoclasts within the bone marrow cavities were observed. Activation of osteoblasts on the dura mater side was also seen. The single injection of rhu TGFbeta1 at 200 ng resulted only in increased osteoprogenitor cell layers and bone matrix formation in an early stage, and the thickness of the osteoprogenitor cell layers and bone tissue at 12 days after single injection was comparable to the values on the nontreated side. At 1 mug, however, the osteoblasts were activated, and the osteoprogenitor cell layers and bone matrix formation were markedly increased. At 12 days, the bone tissue thickness on the treated side was about twice the nontreated side value, as in the repeated treatment groups. As described above, the 12-day repeated or single treatment of rhu TGFbeta1 to the neonatal rat parietal bone (membrane bone) induced marked proliferation of preosteoblasts and promoted bone formation. No cartilage formation was seen in any group.