Fig. 2

A heavy orthodontic force induces external root resorption. A–D Images of HE staining show the effects of different orthodontic forces on dental pulp and periodontal tissues assessed by light microscopy. The orange-colored rectangles indicate areas shown at higher magnification in (A-i–D-i). No inflammation cells or multinucleated cells were observed in coronal pulp tissues of the control or experimental groups. The blue-colored rectangles indicate areas shown at higher magnificent in (A-ii–D-ii). Multi-nucleated osteoclasts (OCs) were observed in the heavy force group (100 g) at the mesial side of the distal root of the left maxillary first molar. M: mesial, D: distal. E–H Light microscope images of TRAP staining showing the effects of different orthodontic forces on the expression of TRAP-positive cells in dental pulp and periodontal tissues after orthodontic treatment. The results show that the number of OCs on the pressure side of the distal root of the left maxillary first molar increased following the increase of orthodontic forces. The orange-colored rectangles indicate areas shown at higher magnificent in (E-i–H-i). No OCs were observed in dental pulp tissues of the control or experimental groups. The blue-colored rectangles indicate areas shown at higher magnificent in (E-ii–H-ii). I The number of OCs increased significantly in the heavy force group (100 g) compared to the control group (P < 0.05). M: mesial, D: distal, T: tooth, AB: alveolar bone, arrowheads indicate OCs. All results are representative of three repeated experiments. *p < 0.05. The scale bars in (A–H) are 100 μm, and are 50 μm in (A-i–H-i), and in (A-ii–H-ii)