A Quantitative Electron-Microscopic Analysis of the Axons at the Apex of the Canine Tooth Pulp in the Dog

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Abstract
The morphology of the dog intradental nerves has not been studied in detail, although dogs have been increasingly used in electrophysiological experiments on pulp nerve function. In this investigation electron microscopy and morphometric analysis were used to study the number and dimensions of the axons at the apex of the dog canine tooth. Two upper and two lower canines, each taken from a different animal, were used. The average number of axons entering a tooth was 2,089 (range: 1,241–3,034), 74.3% (range: 62.2–77.9%) of which were unmyelinated. The mean circumference of the myelinated and unmyelinated axons ranged from 11.1 to 13.9 µm and from 1.3 to 1.7 µm, respectively. Of the myelinated axons 13.7% had a circumference over 19 µm, which is considered to be the upper limit of the Aδ-class. Of the unmyelinated axons 13.8% showed apposition to each other and 20% were partly exposed to the extracellular space; these features could, in part, offer the morphological basis for the extreme pain sensitivity of the tooth. The findings of the present study were considered in general to be comparable to the results of earlier histological and electrophysiological studies on pulp nerves of different species. Thus, it seems that the dog tooth is an adequate model for studying the pulp nerve function and morphology.