CHANGES IN THE SHAPE AND ORIENTATION OF PERIODONTAL-LIGAMENT FIBROBLASTS IN THE CONTINUOUSLY ERUPTING RAT INCISOR FOLLOWING REMOVAL OF THE OCCLUSAL LOAD

One of the main theories which attempts to explain the phenomenon of t ooth eruption suggests that periodontal ligament (PDL) fibroblasts mov e actively and pull the tooth with them out of its socket. To find fur ther support for this theory, we determined the changes in the shape a nd orientation of PDL fibroblasts induced by a transition from impeded to unimpeded eruption. We measured nuclear area, elongation (length-t o-width ratio), and orientation (angulation in relation to the eruptio n axis) of PDL fibroblasts in impeded (functionally loaded) and unimpe ded (hypoloaded) rat incisors. The mean cross-sectional nuclear area d id not differ between fibroblasts in the two groups. In contrast, unim peded eruption resulted in a marked increase in the mean nuclear elong ation (from about 2 to 2.56) and a significant increase in the mean nu clear orientation (from 25.6 to 14.0 degrees). Bivariate analysis sugg ested that these changes occurred in the same cells. Analysis of nucle ar elongation and orientation at various distances from the cementum t oward the alveolar bone revealed a profile of both parameters, such th at cells located 20 to 80 mu m away from the cemental surface were mor e elongated and more frequently oriented toward the eruption axis, whi le cells at 0 to 20 and 80 to 100 mu m were more round/oval and had a greater angulation with the eruption axis. These findings, together wi th Other observations of changes in cell number, number of microtubule s, and migration velocity which occur on the shift to unimpeded erupti on, support the theory of active movement of PDL fibroblasts as an imp ortant component of tooth eruption.