| Objective: To approach the role of OPG/RANKL/RANK system in the root resorption of orthodontic tooth. Methods: Thirty twelve-month-old male SD rats were studied. Use the method of own control, right side was the experimental group, left side was the control group. 100g force was loaded on the right first molar, rats were killed after 0 day,1 day,2 days,4 days,8 days and 12 days'force loaded. The root resorption of mesial root of first molar, the quantity of osteoclasts and the expression of OPGmRNA and RANKLmRNA were observed by histological section and scanning electron micrograph, TRAP staining and in-situ hybridization. Results: 1.A serious resorption appeared at the pressure side of the mesial root at day 8, which reached shallow of the dentin. 2. From the scanning electron micrograph, at day 4, small area of root resorption appeared. At days 8 and 12, large area of resorption can be seen. 3. The peak quantity of the osteoclasts, which located in the alveolar resorption lacunaes, appeared at day 4, and at days 8 and 12, the quatity of these osteoclasts went down. The osteoclasts in the dentin resorption lacunae appeared at day 4, and at days 8 and 12 increased continuously. 4. The expression of OPGmRNA was significantly increased in the tension side. At day 4, it reached the peak. OPGmRNA was expressed in fibroblasts,odontoblasts and cementoblasts. 5. The expression of RANKLmRNA was significantly increased in the pressure side. At day 4, it reached the peak. RANKLmRNA was expressed in osteoclasts, cementoclasts, fibroblasts and cementoblasts. 6. The ratio of RANKLmRNA and OPGmRNA was significantly increased in pressure side. Conclusions: The increase of RANKLmRNA and the ratio of the RANKLmRNA and OPGmRNA followed the increase of the extent of root resorption and the quantity of osteoclasts and cementoclasts, so OPG/RANKL system played an important role in root resorption.
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