Requirement of occlusal force for maintenance of the terminal morphology of the periodontal Ruffini endings

The present study examined whether mechanical stimulation is required for morphological maintenance of the Ruffini endings - primary mechanoreceptors in the periodontal ligament of the rat incisors, using a hypofunctional model by immunohistochemistry for protein gene product 9.5. The periodontal Ruffini endings of adult rats were observed to be restricted to the alveolar half of the lingual ligament where they displayed a dendritic arborization of expanded axon terminals with threadlike microprojections. In the experimental group, the tips of the upper and lower incisors were unilaterally ground to reduce mechanical stimulation of the ligament, i.e. occlusal force. A reduction in the occlusal force induced morphological changes in the terminal morphology of the periodontal Ruffini endings: they became smooth, unlike the irregular profiles exclusively observed in the control group. Quantitative analysis demonstrated significantly lower percentages of immunoreactive areas in the restricted portion on the ground sides than in normal animals. When incisor occlusion was re-established, the terminal portions of the Ruffini endings returned to their normal appearance, and the percentages of immunoreactive areas also recovered. The present results confirm the reduced size and number of axon terminals of periodontal Ruffini endings following reduced occlusal force and restoration of the morphological alteration after the re-establishment of incisor occlusion, indicating that proper mechanical stimulation is an important factor for maintaining the morphology of mechanoreceptors.