The effects of preventing movement of the rat incisor on the structure of its periodontal ligament.

A quantitative, ultrastructural study was made on the periodontal ligaments of rat mandibular incisors immobilized for 18 days. Fibroblasts and their organelles (i.e. endoplasmic reticulum, mitochondria, microtubules, microfilament bundles, lysosomes, intracellular collagen profiles and intercellular contacts), oxytalan fibres, collagen fibrils and ground substance were quantified. There was re-orientation of tissue at the lateral borders of the ligament and an apparent increase in size of epithelial cell rests. Within the matrix, the mean collagen-fibril diameter decreased and the amount of ground substance increased. No change occurred in oxytalan fibres. Within the cells, effects were seen only in cell contacts and in the relative volume of intracellular collagen, lending no support to the view that fibroblast activity generates the force responsible for tooth eruption.