Proliferation of epithelial rests of Malassez during experimental tooth movement

The epithelial rests of Malassez (ERM), remnants of Hertwig's epithelial root sheath, are found near the root surface in the periodontal ligament. The functional significance of the ERM is still unknown. The purpose of this study was to examine the behavior of the ERM during experimental tooth movement. Tooth movement was achieved in 12 male Sprague-Dawley rats (each, 120-200 g) by placing elastic bands between the maxillary right first and second molars. The left molars served as controls. The rats were killed after 6, 12, 18, 24, 60, and 72 hours. The mitotic activity of the ERM was assessed by injecting the animals with 5-bromo-2'-deoxyuridine (BrdU) 2 to 3 hours before killing by intracardial perfusion with 4% paraformaldehyde. The molar-bearing segments were dissected and processed for histological examination. The incorporated BrdU was detected by immunohistochemistry. The number of cells in each ERM cluster was counted in all groups. In the 18-, 24-, 60-, and 72-hour experimental groups, the cell numbers were significantly higher than in the controls. The surface areas of the ERM clusters were also measured in all groups, but only in the 18-, 24-, 60-, and 72-hour specimens were the areas significantly higher in the experimental than in the control groups. The ERM cells in the experimental specimens were labeled with anti-BrdU, while those in the controls were not. It was concluded that experimental tooth movement stimulates ERM cells to proliferate and increase in size. These increased activities of the ERM are consistent with a putative role for these cells in collagen turnover in the periodontal ligament that is accelerated during tooth movement.