Runx2 Enhances Endochondral Bone Formation in Condyle upon Mandibular Advancement

Objectives: Runx2 is a transcription factor prerequisite for osteoblast differentiation and chondrocyte maturation. The purpose of this study is to evaluate the expression pattern of Runx2 and correlate it to chondrocyte maturation and endochondral ossification in condyle during mandibular advancement. Methods: One hundred female Sprague-Dawley rats at 35 days of age were randomly divided into five control and five experimental groups. The experimental animals were fitted with functional appliances that positioned the mandible forward. Rats were labeled with bromodeoxyuridine and sacrificed on day 3, 7, 14, 21 and 30. Alcian blue-PAS staining was used to identify the calcified extracellular matrix in condylar cartilage and the new bone formation in subchondral bone. In situ hybridization and immunohistochemical staining were used to evaluate the expression of Runx2, VEGF and type X collagen. Results: Mandibular advancement elicited Runx2 expression in the hypertrophic chondrocytes of condylar cartilage. The increases were 152% and 58% on day 14 and day 21 respectively. The elevation of the expression of type X collagen followed that of Runx2 and corresponded with the increase in the deposition of calcified extracellular matrix. The domain of Runx2 immunostaining overlapped with that of VEGF in the chondrocytes. Mandibular advancement also led to higher levels of Runx2 expression in osteoblasts and chondroclasts. Concomitantly, an increase in the recruitment of these cells into the erosive front was observed, which preceded the increase of new bone formation in the condyle. Conclusions: Mandibular advancement solicited Runx2 expression in condyle. Runx2 enhanced chondrocyte terminal maturation and the pace of endochondral ossification, thus leading to more new bone formation in the condyle.