An Ultrastructural Study of the Relationship between Sensory Trigeminal Nerves and Odontoblasts in Rat Dentin/Pulp as Demonstrated by the Anterograde Transport of Wheat Germ Agglutinin-Horseradish Peroxidase (WGA-HRP)

Abstract

Because the ultrastructure of the trigeminal sensory nerves in dentin, especially in relation to odontoblasts, remains to be clarified, we investigated the relationship between the trigeminal sensory nerves and the odontoblast processes using the anterograde axonal transport technique by injecting wheat germ agglutinin-horseradish peroxidase (WGA-HRP) into the rat trigeminal ganglion. Light microscopically, the nerves labeled with WGA-HRP were mainly concentrated at the pulpal horn, forming a nerve plexus at the subodontoblastic region and penetrating the predentin/dentin about 50 to 70 microns. Ultrastructurally, HRP reaction products were observed intra-axonally in the myelinated (A delta) and unmyelinated (C) axons in the subodontoblastic region. Most nerves lost the Schwann sheath and were naked in the predentin/dentin. The labeled varicosities were close to the odontoblast processes in the dentinal tubules. No synaptic structures could be detected between the varicosities and the odontoblasts, but a gap about 20 nm wide was found between them. One type of varicosity was a rich mitochondria-containing varicosity, while the other was a rich vesicle-containing (large dense core vesicles and small clear vesicles) one. The reaction products were also found in the extracellular spaces surrounding the axons. Sometimes the reaction products were seen in the coated pits or the endocytotic vesicles of the odontoblast processes. The present study demonstrated that nerve endings (varicosities) derived from the trigeminal ganglion were present in the dentinal tubules, and that WGA-HRP extracellularly extruded from the sensory nerves in the odontoblastic layer or predentin/dentin. These findings thus suggest that sensory nerves may have some (e.g., trophic) effect on either odontoblasts or the environment around the sensory nerves in the dentin/pulp.

Characterization and Gene Expression of High Conductance Calcium-activated Potassium Channels Displaying Mechanosensitivity in Human Odontoblasts

“…Additionally, changes in osmotic gradient or probing dentin were shown to increase the discharge rate of pulp cells and evoke action potentials in their primary afferents. Afferent nerve terminals are known to coil around odontoblasts, and the close association of odontoblast processes and nerve endings (30,31) has presupposed an interaction between these cells as the earliest step of tooth pain transmission. Stretch-activated K Ca channels could be involved in this process.…”

Voltage-gated Sodium Channels Confer Excitability to Human Odontoblasts

“…␤ subunits modulate channel activity but also function as cell adhesion molecules and participate in cell adhesion (25). ␤2 subunits were suggested to preferentially associate with ␣2 (26), leading us to believe that these Na ϩ channel proteins present in in vivo and in vitro odontoblast cell membrane may participate in the intimate nerve-odontoblast relationship described previously (1)(2)(3). In co-culture, ␣2 and ␤2 subunits co-localized at the sites of axon-odontoblast contact, indicating that a clustering of Na ϩ channels occurred.…”

“…Each cell has an extension running into the dentinal tubule and bathed in the dentinal fluid. Sensory unmyelinated nerve fibers belonging to the trigeminal ganglion enter the inner dentin and coil around the monopolar processes of odontoblasts (1,2). A hydrodynamic concept, based on the spatial situation of odontoblasts, nerve endings, and fluid movements in dentinal tubules, postulated that nociceptive responses may result from an increase in intradentinal pressure, which in turn might activate nerve endings.…”

Dental Injury Models: Experimental Tools for Understanding Neuroinflammatory Interactions and Polymodal Nociceptor Functions

“…For example, SP, neurokinin A, and CGRP affect pulpal fibroblast proliferation in vitro (Bongenhielm et al, 1995). Anterogradely transported wheat germ agglutinin conjugated to horseradish peroxidase has been shown to transfer into the odontoblasts (Ibuki et al, 1996). The forms of the odontoblasts and their processes are specialized in the innervated regions and differ at reparative dentin (Byers et al, 1992a;Byers and Sugaya, 1995) (Figs.…”