Objective This study aimed to investigate the expression of transformation growth factor (TGF)-β/Smad signal pathway-related proteins in the epithelial rests of Malassez (ERM) and the functional change in ERM during orthodontic tooth movement in rats to explore the mechanism by which the TGF-β/Smad signal pathway regulates ERM that is involved in tooth movement. Methods Tooth movement was achieved in 30 healthy male Sprague-Dawley rats (weighing 200-250 g each) by placing NiTi coil springs between the maxillary right first molar and incisor with the force of 50 g. The left side served as controls. Rats were killed at 0, 1, 4, 7, 10 and 14 days. Bilateral masses, including the maxillary first molars and the surrounding alveolar bone, were removed. Immunohistochemical staining was used to analyse the sections with the Image-Pro Plus Image Analysis System. The integrated optical density (IOD) of the stained positive signal was measured under the same light intensity. The IOD values of TGF-β1, Smad2, Smad3 and proliferative cell nuclear antigen (PCNA) were measured for all groups. The number of ERM was counted, and the surface areas of the ERM cluster were measured in the cervical and furcational regions of the first molar for all groups. The results were evaluated using the t-test and Kruskal-Wallis test with SPSS 17.0 software. Results The TGF-β1, Smad2, Smad3 and PCNA expression was weak in control rats. The immunofluorescence staining of TGF-β1, Smad2 and Smad3 increased after the 1st day, peaked on the 7th day and then decreased. On the 1st, 4th, 7th, 10th and 14th day, the IOD values of TGF-β1, Smad2 and Smad3 were statistically significant compared with those of the control group. After 4 days, PCNA immunohistochemistry positive staining increased. After 7 days, the PCNA expression level significantly increased, peaked on the 10th day and then decreased afterwards. On the 4th, 7th, 10th and 14th day, the IOD values of PCNA were statistically significant compared with those of the control group. The number of ERM and the surface area of the ERM cluster were statistically significant compared with those of the control group on the 4th, 7th, 10th and 14th day. Conclusion The number of ERM and the surface area of the ERM cluster both increased under a mechanical force. The TGF-β/Smad signalling pathway-related proteins (namely, TGF-β1, Smad2 and Smad3) were expressed in ERM and showed a trend change with the increase in force time. This result indicated that ERM can regulate its function in tooth movement through the TGF-β/Smad signalling pathway under a mechanical force.
