Quantitative analysis of connective tissue resorption in the supra-alveolar region of the mouse incisor ligament

During eruption of the mouse incisor part of the periodontal ligament moves along with the tooth in the occlusal direction and is degraded in the supra‐alveolar region. In the present study stereologic analysis at the ultrastructural level has shown that significant reduction of the amount of extracellular collagen is confined to the most occlusal region of the ligament, just apically to the functional epithelium. In connection herewith a relatively high amount of homogeneous material of moderate electron density was found in the intercellular space, suggesting a process of extracellular collagen breakdown near the epithelium‐connective tissue interface. With respect to the distribution of collagen phagocytosis by fibroblasts, it appeared that the volume density of cytoplasmic vacuoles containing collagen fibrils was considerably higher in the supra‐alveolar region of the ligament than in its subcrestal part, particularly in the zone along the tooth surface. This phenomenon is interpreted as being indicative of increased phagocytic activity of fibroblasts leading to the detachment of the periodontal ligament from the cemeatum before it enters the region of final degradation near the apical termination of the junctional epithelium. In an attempt to identify the cells involved in the degradation of (fragments of) periodontal fibroblasts in the supra‐alveolar region, periodontal ligament cells were labelled with tritiated thymidine at their site of origin. By means of both autoradiography and electron microscopy, it was shown that, following the arrival of Jabelled fibroblasts in the supra‐alveolar region, at least part of the degradation of cellular constituents is accomplished via a process of heterophagocytosis among fibroblasts.