Hard Tissue Pathology Unit, Department of Hard Tissue Research, Matsumoto Dental University, Graduate School of Oral Medicine, 1780 Hirooka-Gobara, Shiojiri, Japan. tshimizu@po.mdu.ac.jp
OBJECTIVE: The purpose of this study was to investigate the expression pattern of Runx2 in mandibular condylar cartilage, a type of secondary cartilage. METHODS: Mandibular condyle of ddY mice were fixed from embryonic day 14 (E14) through just after birth (equivalent to E19). Samples were cut into 4 micro m serial sections through the central area of the mandibular condyle at the sagittal plane. Serial sections were examined using histological, immunohistochemical (IHC) and in situ hybridization (ISH) techniques. RESULTS: There are no developmental features of mandibular condyle. At the distal upper portion of developmental mandibular bone, mesenchymal cell proliferation and condensation without metacholomatic reaction to Toluidine blue (TB) were seen at E14. At E15, mandibular condylar cartilage was clearly evident, as TB metacholomasia. In IHC specimens at E14, expression of Runx2 peptide was observed in the nuclei and the cytoplasms cells of coagulating mesenchymal cells, both in the cytoplasm and nucleus. After E17, Runx2 appeared in the cells of the condylar cartilage sheath. In ISH examination at E14 and E15, expressions of Runx2 mRNA appeared in the cytoplasms of proliferating chondrocytes. From E16 to E18, Runx2 mRNA was detected throughout almost all cytoplasm in all layers. CONCLUSION: These IHC and ISH results suggest that Runx2 plays an essential role for mandibular condylar cartilage development, especially that Runx2 is essential for the onset of secondary cartilage differentiation.