Runx2, Osterix, Sox9 Expression in Developing Mouse Mandibular Condylar Cartilage

Objectives: Osteogenesis and chondrogenesis are regulated by transcription factors including Runx2, Osterix and Sox9, but little is known on their involvement in secondary cartilage formation. The purpose of this study is to investigate the expression of these molecules in developing mouse mandibular condylar cartilage, a main secondary cartilage, using in situ hybridization. Methods: ICR mice, E 14 - 16 were used for this study. The heads of each mouse were fixed with paraformaldehyde and routinely embedded in paraffin. Five μm sections were cut longitudinally to the mandibular condylar process. In situ hybridization for Runx2, Osterix, Sox9 was applied for serial sections using ³⁵SUTP-labeled RNA probes. Toluidine blue staining and mRNA expression for collagen types II and X were accepted as markers of chondrocyte differentiation. Results: At E 14 the anlage of mandibular condylar process consisted of mesenchymal cell condensation and mRNAs for all these three transcription factors were expressed in this condensation. At E 15 cartilage tissue was first detected and mRNA expression for collagen types II and X was simultaneously detected in newly formed cartilage. Expression for Sox9 mRNA was still maintained but Runx2 mRNA expression was slightly reduced and Osterix mRNA expression was remarkably reduced in it. At E 16, cartilage was extended in length especially the hypertrophic cell zone. Sox9 mRNA was expressed throughout the cartilage except the lowest part of the hypertophic cell zone. meanwhile, Runx2 mRNA was strongly expressed in the upper part of cartilage but weakly in the hypertrophic cell zone. Osterix mRNA showed similar staining pattern to Runx2 mRNA, but its expression was remarkably reduced in the hypertrophic cell zone. Conclusions: These findings indicated that these three transcription factors seem to be involved in condylar cartilage formation, especially reduced expression of Osterix may be important for this phenomenon.