Abstract
The periodontal ligament has a rich sensory nerve supply which originates from the trigeminal ganglion and trigeminal mesencephalic nucleus. Although various types of mechanoreceptors have been reported in the periodontal ligament, the Ruffini ending is an essential one. It is unknown whether the distribution of periodontal nerve fibers in deciduous teeth is identical to that in permanent teeth or not. Moreover, morphological changes in the distribution of periodontal nerve fibers during resorption of deciduous teeth and eruption of successional permanent teeth in diphyodont animals have not been reported in detail. Therefore, in this study, we examined changes in the distribution of periodontal nerve fibers in the cat during changes in dentition (i.e., deciduous, mixed and permanent dentition) by immunohistochemistry of protein gene product 9.5. During deciduous dentition, periodontal nerve fibers were concentrated at the apical portion, and sparsely distributed in the periodontal ligament of deciduous molars. During mixed dentition, the periodontal nerve fibers of deciduous molars showed degenerative profiles during resorption. In permanent dentition, the periodontal nerve fibers of permanent premolars, the successors of deciduous molars, increased in number. Similar to permanent premolars, the periodontal nerve fibers of permanent molars, having no predecessors, increased in number, and were densely present in the apical portion. The present results indicate that the distribution of periodontal nerve fibers in deciduous dentition is almost identical to that in permanent dentition although the number of periodontal nerve fibers in deciduous dentition was low. The sparse distribution of periodontal nerve fibers in deciduous dentition agrees with clinical evidence that children are less sensitive to tooth stimulation than adults.
Highlights
The periodontal ligament receives dense sensory innervation by nociceptive-free nerve endings and mechanoreceptive specialized endings
We examined morphological changes in the periodontal nerve fibers during transition from deciduous dentition to permanent dentition, and found that, the periodontal nerve fibers in deciduous teeth was sparsely distributed, the distribution of the periodontal nerve fibers in deciduous teeth was almost identical to that in the succeeding permanent teeth
Previous studies of periodontal nerve fibers have mostly been performed in rat incisors, where they found that periodontal Ruffini endings of the rat incisor were extensively expanded, and were associated with terminal Schwann cells with kidney-shaped nuclei [1, 2, 3]
Summary
The periodontal ligament receives dense sensory innervation by nociceptive-free nerve endings and mechanoreceptive specialized endings. The morphology and histochemical properties of periodontal Ruffini endings have been extensively studied in the periodontal ligament of rat incisors under normal conditions, and those during development and regeneration after nerve injury. Periodontal Ruffini endings of the rat incisor are concentrated in the alveolar half of the lingual periodontal ligament, and display dendritic ramifications with expanded axon terminals through an association with specialized Schwann cells called terminal or lamellar Schwann cells [1, 2, 3]. The rodent incisors are the most popular experimental model for the examination of periodontal innervation, they have unique characteristics, i.e., they are rootless and continuously erupting. In this regard, the rat incisor is quite different from the molars. The Ruffini endings are mostly localized in the apical region, and have a similar morphology to the rat incisor [4, 5, 6]
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