A quantitative comparison of the ultrastructure of the periodontal ligaments of impeded and unimpeded rat incisors.

The periodontal ligaments of impeded and unimpeded rat mandibular incisors were examined to find structural correlates for the known functional differences between the tissues. The structures quantified were fibroblasts (area and membrane length, endoplasmic reticulum, mitochondria, microtubules, lysosomes, intracellular collagen profiles, intercellular contacts), oxytalan fibres, collagen fibrils and ground substance. The only changes seen on rendering a tooth unimpeded were an increase in the number of microtubules within the fibroblasts, an increase in the number of simplified desmosomes between the fibroblasts and a decreased amount of ground substance within the extracellular matrix. The results show that it is possible for a connective tissue to undergo marked changes in function, turnover and biomechanical properties without major structural changes.