Carbonic anhydrase isozyme II immunoreactivity in the mechanoreceptive Ruffini endings of the periodontal ligament in rat incisor

Morphological and cytochemical characteristics of periodontal Ruffini ending under normal and regeneration processes

Current knowledge on the Ruffini endings, primary mechanoreceptors in the periodontal ligament is reviewed with special reference to their cytochemical features and regeneration process. Morphologically, they are characterized by extensive ramifications of expanded axonal terminals and an association with specialized Schwann cells, called lamellar or terminal Schwann cells, which are categorized, based on their histochemical properties, as non-myelin-forming Schwann cells. Following nerve injury, the periodontal Ruffini endings of the rat incisor ligament can regenerate more rapidly than Ruffini endings in other tissues. During regeneration, terminal Schwann cells associated with the periodontal Ruffini endings migrate into regions where they are never found under normal conditions. Also during regeneration, alterations in the expression level of various bioactive substances occur in both axonal and Schwann cell elements in the periodontal Ruffini endings. Neuropeptide Y, which is not detected in intact periodontal Ruffini endings, is transiently expressed in their regenerating axons. Growth-associated protein-43 (GAP-43) is expressed transiently in both axonal and Schwann cell elements during regeneration, while this protein is localized in the Schwann sheath of periodontal Ruffini endings under normal conditions. The expression of calbindin D28k and calretinin, both belonging to the buffering type of calcium-binding proteins, was delayed in periodontal Ruffini endings, compared to their morphological regeneration. As the importance of axon-Schwann cell interactions has been proposed, further investigations are needed to elucidate their molecular mechanism particularly the contribution of growth factors during the regeneration as well as development of the periodontal Ruffini endings.

The Ruffini ending as the primary mechanoreceptor in the periodontal ligament: its morphology, cytochemical features, regeneration, and development

The periodontal ligament receives a rich sensory nerve supply and contains many nociceptors and mechanoreceptors. Although its various kinds of mechanoreceptors have been reported in the past, only recently have studies revealed that the Ruffini endings--categorized as low-threshold, slowly adapting, type II mechanoreceptors--are the primary mechanoreceptors in the periodontal ligament. The periodontal Ruffini endings display dendritic ramifications with expanded terminal buttons and, furthermore, are ultrastructurally characterized by expanded axon terminals filled with many mitochondria and by an association with terminal or lamellar Schwann cells. The axon terminals of the periodontal Ruffini endings have finger-like projections called axonal spines or microspikes, which extend into the surrounding tissue to detect the deformation of collagen fibers. The functional basis of the periodontal Ruffini endings has been analyzed by histochemical techniques. Histochemically, the axon terminals are reactive for cytochrome oxidase activity, and the terminal Schwann cells have both non-specific cholinesterase and acid phosphatase activity. On the other hand, many investigations have suggested that the Ruffini endings have a high potential for neuroplasticity. For example, immunoreactivity for p75-NGFR (low-affinity nerve growth factor receptor) and GAP-43 (growth-associated protein-43), both of which play important roles in nerve regeneration/development processes, have been reported in the periodontal Ruffini endings, even in adult animals (though these proteins are usually repressed or down-regulated in mature neurons). Furthermore, in experimental studies on nerve injury to the inferior alveolar nerve, the degeneration of Ruffini endings takes place immediately after nerve injury, with regeneration beginning from 3 to 5 days later, and the distribution and terminal morphology returning to almost normal at around 14 days. During regeneration, some regenerating Ruffini endings expressed neuropeptide Y, which is rarely observed in normal animals. On the other hand, the periodontal Ruffini endings show stage-specific configurations which are closely related to tooth eruption and the addition of occlusal forces to the tooth during postnatal development, suggesting that mechanical stimuli due to tooth eruption and occlusion are a prerequisite for the differentiation and maturation of the periodontal Ruffini endings. Further investigations are needed to clarify the involvement of growth factors in the molecular mechanisms of the development and regeneration processes of the Ruffini endings.