Delayed expression of calbindin D28k during regeneration of the periodontal Ruffini endings of the rat incisor following injury to the inferior alveolar nerve

Modulation of Trigeminal Spinal Subnucleus Caudalis Neuronal Activity Following Regeneration of Transected Inferior Alveolar Nerve in Rats

Modulation of trigeminal spinal subnucleus caudalis neuronal activity following regeneration of transected inferior alveolar nerve in rats. To clarify the neuronal mechanisms of abnormal pain in the face innervated by the regenerated inferior alveolar nerve (IAN), nocifensive behavior, trigeminal ganglion neuronal labeling following Fluorogold (FG) injection into the mental skin, and trigeminal spinal subnucleus caudalis (Vc) neuronal properties were examined in rats with IAN transection. The mechanical escape threshold was significantly higher at 3 days and lower at 14 days after IAN transection, whereas head withdrawal latency to heat was significantly longer at 3, 14, and 60 days after IAN transection. The number of FG-labeled ganglion neurons was significantly reduced at 3 days after IAN transection but increased at 14 and 60 days. The number of wide dynamic range (WDR) neurons with background (BG) activity was significantly higher at 14 and 60 days after IAN transection compared with naïve rats, and the number of WDR and low-threshold mechanoreceptive (LTM) neurons with irregularly bursting BG activity was increased at these two time points. Mechanically evoked responses were significantly larger in WDR and LTM neurons 14 days after IAN transection compared with naïve rats. Heat- and cold-evoked responses in WDR neurons were significantly lower at 14 days after transection compared with naïve rats. Mechanoreceptive fields were also significantly larger in WDR and LTM neurons at 14 and 60 days after IAN transection. These findings suggest that these alterations may be involved in the development of mechanical allodynia in the cutaneous region innervated by the regenerated IAN.

Regeneration of periodontal Ruffini endings of rat lower incisors following nerve cross-anastomosis with mental nerve

The present study utilized protein gene product 9.5 (PGP 9.5) and S-100 protein immunohistochemistry to examine if Ruffini endings, the primary mechanoreceptors in periodontal ligaments, can regenerate following nerve cross-anastomosis with an inappropriate nerve. Normally, axon terminals of periodontal Ruffini endings are extensively ramified, and terminal Schwann cells, identified by their S-100 immunoreactivity, are associated with axon terminals. Schwann cells are restricted to the alveolus-related part (ARP), but not tooth-related part (TRP) or the shear zone at the border between the ARP and the TRP of the lingual periodontal ligament of the lower incisor. When the central portion of the mental nerve (MN) was connected with the peripheral portion of the inferior alveolar nerve (IAN), regenerating MN fibers invaded the IAN around postoperative day 5 (PO 5). During the postoperative period, numerous S-100-immunoreactive (IR) cells, presumably terminal Schwann cells, began to migrate to the shear zone and the TRP. PGP 9.5-IR elements reappeared at PO 7 and gradually increased in number. Around PO 28, the terminal portion of the regenerating Ruffini endings appeared dendritic, but less expanded, and the rearrangement of terminal Schwann cells was noted. Regenerated periodontal Ruffini endings were slightly smaller in number. The number of trigeminal ganglion neurons sending peripheral processes beyond the site of injury was smaller compared to those of normal MN, but their cross-sectional areas were almost comparable. Expressions of calbindin D28k and calretinin, normally localized in axonal elements in Ruffini endings, were first detected around PO 56. The present results show that parts of periodontal Ruffini endings can regenerate following nerve cross-anastomosis with mental nerve.

Regeneration of periodontal Ruffini endings in adults and neonates

We reviewed the regeneration of periodontal Ruffini endings, primary mechanoreceptors in the periodontal ligament, following injury to the inferior alveolar nerve (IAN) in adult and neonatal rats. Morphologically, mature Ruffini endings are characterized by an extensive arborization of axonal terminals and association with specialized Schwann cells, called lamellar or terminal Schwann cells. Following injury to IAN in the adult, the periodontal Ruffini endings of the rat lower incisor ligament regenerate more rapidly than Ruffini endings in other tissues. During regeneration, terminal Schwann cells migrate into regions where they are never found under normal conditions. The development of periodontal Ruffini endings of the rat incisor is closely associated with the eruption of the teeth; the morphology and distribution of the terminal Schwann cells became almost identical to those in adults during postnatal days 15-18 (PN 15-18d) when the first molars appear in the oral cavity, while the axonal elements showed extensive ramification around PN 28d when the functional occlusion commences. When the IAN was injured in neonates, the regeneration of periodontal Ruffini endings was delayed compared with the adults. The migration of terminal Schwann cells is also observed following IAN injury, after which the distribution of terminal Schwann cells became almost identical to that of the adults, i.e., PN 14d. Since the interaction between axon and Schwann cell is important during regeneration and development, further studies are required to elucidate its molecular mechanism during the regeneration as well as the development of the periodontal Ruffini endings.

Morphological and cytochemical characteristics of periodontal Ruffini ending under normal and regeneration processes

Current knowledge on the Ruffini endings, primary mechanoreceptors in the periodontal ligament is reviewed with special reference to their cytochemical features and regeneration process. Morphologically, they are characterized by extensive ramifications of expanded axonal terminals and an association with specialized Schwann cells, called lamellar or terminal Schwann cells, which are categorized, based on their histochemical properties, as non-myelin-forming Schwann cells. Following nerve injury, the periodontal Ruffini endings of the rat incisor ligament can regenerate more rapidly than Ruffini endings in other tissues. During regeneration, terminal Schwann cells associated with the periodontal Ruffini endings migrate into regions where they are never found under normal conditions. Also during regeneration, alterations in the expression level of various bioactive substances occur in both axonal and Schwann cell elements in the periodontal Ruffini endings. Neuropeptide Y, which is not detected in intact periodontal Ruffini endings, is transiently expressed in their regenerating axons. Growth-associated protein-43 (GAP-43) is expressed transiently in both axonal and Schwann cell elements during regeneration, while this protein is localized in the Schwann sheath of periodontal Ruffini endings under normal conditions. The expression of calbindin D28k and calretinin, both belonging to the buffering type of calcium-binding proteins, was delayed in periodontal Ruffini endings, compared to their morphological regeneration. As the importance of axon-Schwann cell interactions has been proposed, further investigations are needed to elucidate their molecular mechanism particularly the contribution of growth factors during the regeneration as well as development of the periodontal Ruffini endings.