Light and electron microscopic analysis of the somata and parent axons innervating the rat upper molar and lower incisor pulp

The morphology of intradental nerve fibers of permanent teeth and of continuously growing rodent incisors has been studied in detail but little information is available on the parent axons that give rise to these fibers. Here we examined the axons and somata of trigeminal neurons that innervate the rat upper molar and lower incisor pulp using tracing with horseradish peroxidase and light and electron microscopic analysis. The majority (∼80%) of the parent axons in the proximal root of the trigeminal ganglion that innervated either molar or incisor pulp were small myelinated fibers (<20 μm2 cross-sectional area). The remaining ∼20% of the fibers were almost exclusively large myelinated for the molar pulp and unmyelinated for the incisor pulp. The majority of neuronal somata in the trigeminal ganglion that innervated either molar (48%) or incisor pulp (62%) were medium in size (300–600 μm2 cross-sectional area). Large somata (>600 μm2) constituted 34% and 20% of the trigeminal neurons innervating molar and incisor pulp, respectively, while small somata (<300 μm2) constituted 17% of the molar and 18% of the incisor neurons. The present study revealed that the morphology of parent axons of dental primary sensory neurons may differ from that of their intradental branches, and also suggests that the nerve fiber function may be carried out differently in the molar and incisor pulp in the rat.