Innervation of the Periodontal Ligament in the Dog with Special Reference to the Morphology of Ruffini Endings

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The distribution and terminal formation of nerves in the periodontal ligament of dog incisors and canines were investigated by immunohistochemistry for neurofilament protein (NFP) and by electron microscopy. The NFP-immunoreactive nerve fibers were found to be densely distributed in the apical third of the periodontal ligament, while they were sparse in the coronal two thirds. Most of the nerve endings in the periodontal ligament showed a tree-like appearance and resembled those nerve endings demonstrated in the periodontal ligament of human and monkey under the category of free nerve endings. Presumable axon terminals of these were slightly thicker than preterminal portions, running along periodontal collagen fibers and tapering within them. In light microscopic images, at least, they differed from the Ruffini endings which are commonly seen in rodents, displaying a glove-like configuration with extremely expanded tips. Under the electron microscope, however, the tree-like endings of the dog appeared similar to the Ruffini endings of rodents: their terminals were filled with mitochondria, covered with a cytoplasmic process of a Schwann cell, and surrounded by collagen fibers. These ultrastructural findings, combined with the results of previous physiological studies suggest that the nerve endings demonstrated in the present study can be identified as Ruffini endings. It is even stressed that the dog-type of Ruffini ending can be regarded as a representative of the sensory receptors in the mammalian periodontal ligament. In addition to these endings, knobbed endings, corpuscular (lamellated and glomerular) endings, and free nerve endings were rarely encountered in the periodontal ligament of incisors and canines of the dog.