Role of Msx2 as a promoting factor for Runx2 at the periodontal tension sides elicited by mechanical stress.

Early changes of Runx2 and Msx2 expressions were examined by immunohistochemistry in mouse periodontal ligament exposed to mechanical stress. 8-week-old ddY mouse was used as experimental animal. To provide a continuous mechanical stress on periodontal ligament, rubber dam sheet was placed between upper molars of the mouse. At 20 minutes, 1 hour, 3 hours, 9 hours and 24 hours after insertion of the sheet, relevant parts of the mouse tissues were excised and fixed in 4% paraformaldehyde/0.05M phosphate buffered fixative solution. Then serial paraffin sections were prepared and histopathological evaluation as well as examination of Runx2, Msx2 and alkaline phosphatase (ALP) expressions by immunohistochemistry were performed. Control animals were not subjected to mechanical stress. In the experimental group, strong expressions of Runx2 and Msx2 were seen in periodontal fibroblasts of the tension side at 20 minutes after mechanical stress. Expressions of Runx2 and Msx2 became stronger in parallel with time, and at 24 hours after mechanical stress, the periodontal fibroblasts, cementoblasts as well as osteoblasts showed strong expression. Moreover, ALP has also demonstrated similar strong expression. On the other hand, in the control group, although expressions of Runx2, Msx2 and ALP were detected at all the experiment times, the expressions were weak. All these results strongly suggested that Runx2 promoted differentiation of osteoblasts at early stage and Msx2 worked as an activator of Runx2 function.