Unique distribution of Ruffini and free nerve endings in the periodontal tissue of continuously erupting guinea pig teeth

Type and distribution of nerve elements in the periodontal tissues (PDT) vary among species and types of tooth even in the same species and hence exploring their functional significance according to the anatomical location is intriguing. In continuously erupting incisor teeth of rodents, all kinds of nerve elements are reported to be confined to the alveolar bone-related part of PDT, including the Ruffini endings (RE), which are only seen in the lingual aspect of incisors, where regular periodontal ligament is developed. Objective: Here we report on the presence of mechanoreceptors in the enamel-related PDT of continuously erupting guinea pig teeth, and assess their possible roles in relation to masticatory function. Methods: Fixed and decalcified frozen sections of the guinea pigs jaws were processed for immunohistochemistry for detection of nerve elements using antibodies against PGP 9.5 and glia specific S-100 protein, by mean of ABC method. Results: In the incisors, RE were condensed in the bone-related part of lingual PDT like in many other rodents. However, some RE like endings showing dendritic ramification and rounded terminal Schwann cells were found in the labial PDT of incisors. Well-developed RE like elements were also located in the enamel-related part of molars in relation to thick fiber bundles inserting in the cementum pearls on the enamel surface. In addition, we observed a few free nerve elements in the tooth-related part of PDT in both molars and incisors despite continuous eruption of the respective teeth. Conclusion: Our data provide the first morphological evidence of the presence of RE in the enamel-related PDT and also the presence of nerve elements in the tooth-related part of periodontal ligament in continuously erupting teeth. This study was supported by Grants-in aid (No. 12557151) for scientific research from the Ministry of Education, Science, Sports &Culture of Japan.