Cell size analysis of primary neurons innervating the cornea and tooth pulp of the rat

Primary neuronal cell bodies, whose peripheral axons comprised the cutaneous branch of the mylohyoid nerve or innervated the mandibular molar tooth pulp or the cornea, were labeled with HRP and their cross-sectional area was analyzed. Most of their cell bodies were smaller than 1000 μm² in cross-sectional area and the histogram of each showed a unimodal pattern. The modes of percentage distribution were 100-200 μm² (34.4%), 500-600 μm² (17.4%) and 300-400 μm² (35.1%) for the mylohyoid nerve, the tooth pulp and the cornea, respectively. A comparison of the 3 histograms indicated that there were at least 3 subpopulations of trigeminal primary neurons i.e., small, medium, and large cells. Electron microscopically, the large primary neurons innervating the tooth pulp had endoplasmic reticulum throughout the cytoplasm. The small primary neurons innervating the cornea showed a clear zonation of organelles and the endoplasmic reticulum was located in the periphery of the cytoplasm. The light microscopically identified small, medium and large cell groups may correspond to C-, Aδ- and Aβ-fibers. The tooth pulp and the cornea appear to receive mainly Aβ-fibers and Aδ-fibers, respectively. The cutaneous branch of the mylohyoid nerve appears to contain numerous C-fibers and progressively smaller proportions of Aδ- and Aδ-fibers.