Inferior alveolar nerve regeneration and incisor pulpal reinnervation following intramandibular neurotomy in the cat

Neural Regulations in Tooth Development and Tooth-Periodontium Complex Homeostasis: A Literature Review

The tooth-periodontium complex and its nerves have active reciprocal regulation during development and homeostasis. These effects are predominantly mediated by a range of molecules secreted from either the nervous system or the tooth-periodontium complex. Different strategies mimicking tooth development or physiological reparation have been applied to tooth regeneration studies, where the application of these nerve- or tooth-derived molecules has been proven effective. However, to date, basic studies in this field leave many vacancies to be filled. This literature review summarizes the recent advances in the basic studies on neural responses and regulation during tooth-periodontium development and homeostasis and points out some research gaps to instruct future studies. Deepening our understanding of the underlying mechanisms of tooth development and diseases will provide more clues for tooth regeneration.

Integration of tooth morphogenesis and innervation by local tissue interactions, signaling networks, and semaphorin 3A

The tooth, like many other organs, develops from both epithelial and mesenchymal tissues, and has proven to be a valuable tool with which to investigate organ formation and peripheral innervation. Tooth formation is regulated by local epithelial-mesenchymal tissue interactions, and is closely integrated with stereotypic dental nerve navigation and patterning. Recent analyses of the function and regulation of semaphorin 3A (SEMA3A) have shed light on the regulatory mechanisms that coordinate organogenesis and innervation at the tissue and molecular levels. In the tooth, SEM3A acts as a developmentally regulated secretory chemo-repellent, that controls tooth innervation during embryonic and postnatal development. The tooth germ governs its own innervation by a combination of local tissue interactions and SEMA3A expression. SEMA3A signaling, in turn, is controlled by a number of conserved signaling effectors, including TGF-β superfamily members, FGF, and WNT; all function in embryo and organ development, and are essential for tooth histo-morphogenesis. Thus, SEMA3A driven axon guidance is integrated into key odontogenic signaling networks, establishing this protein as a critical molecular tether between 2 distinct developmental processes (morphogenesis and sensory innervation), both of which are required to obtain a functional tooth.

Coordination of tooth morphogenesis and neuronal development through tissue interactions: lessons from mouse models

In addition to being an advantageous model to investigate general molecular mechanisms of organ formation, the tooth is a distinct target organ for peripheral nerve innervation. These nerves are required for the function and protection of the teeth and, as shown in fish, also for their regeneration. This review focuses on recent findings of the local tissue interactions and molecular signaling mechanisms that regulate the early nerve arrival and patterning of mouse mandibular molar tooth sensory innervation. Dental sensory nerve growth and patterning is a stepwise process that is intimately linked to advancing tooth morphogenesis. In particular, nerve growth factor and semaphorin 3A serve as essential functions during and are iteratively used at different stages of tooth innervation. The tooth germ controls development of its own nerve supply, and similar to the development of the tooth organ proper, tissue interactions between dental epithelial and mesenchymal tissues control the establishment of tooth innervation. Tgf-β, Wnt, and Fgf signaling, which regulate tooth formation, are implicated to mediate these interactions. Therefore, tissue interactions mediated by conserved signal families may constitute key mechanism for the integration of tooth organogenesis and development of its peripheral nerve supply.

Dental neuroplasticity, neuro-pulpal interactions, and nerve regeneration

This review covers current information about the ability of dental nerves to regenerate and the role of tooth pulp in recruitment of regenerating nerve fibers. In addition, the participation of dental nerves in pulpal injury responses and healing is discussed, especially concerning pulp regeneration and reinnervation after tooth replantation. The complex innervation of teeth is highly asymmetric and guided towards specific microenvironments along blood vessels or in the crown pulp and dentin. Pulpal products such as nerve growth factor are distributed in the same asymmetric gradients as the dentinal sensory innervation, suggesting regulation and recruitment of those nerve fibers by those specific factors. The nerve fibers have important effects on pulpal blood flow and inflammation, while their sprouting and cytochemical changes after tooth injury are in response to altered pulpal cytochemistry. Thus, their pattern and neuropeptide intensity are indicators of pulp status, while their local actions continually affect that status. When denervated teeth are injured, either by pulp exposure on the occlusal surface or by replantation, they have more pulpal necrosis than occurs for innervated teeth. However, small pulp exposures on the side of denervated crowns or larger lesions in germ-free animals can heal well, showing the value of postoperative protection from occlusal trauma or from infection. Current ideas about dental neuroplasticity, neuro-pulpal interactions, and nerve regeneration are related to the overall topics of tooth biomimetics and pulp/dentin regeneration.

Neuropeptide expression in the ferret trigeminal ganglion following ligation of the inferior alveolar nerve

Previous studies have found changes in neuropeptide expression in trigeminal ganglion cells after inferior alveolar nerve (IAN) section. These changes may play a part in the persistent sensory abnormalities that can be experienced after trigeminal nerve injuries. Here, neuropeptide expression after IAN ligation was studied, as this type of injury is thought to be more likely to result in sensory disturbances. The neuropeptides investigated were substance P, calcitonin gene-related peptide, enkephalin (ENK), galanin (GAL), neuropeptide Y (NPY) and vasoactive intestinal polypeptide. In anaesthetised adult female ferrets the left IAN was sectioned and the central stump tightly ligated. Recovery was allowed for 3 days, 3 or 12 weeks before perfusion-fixation. In a second procedure, 1 week before perfusion, the IAN was exposed and an injection made central to the injury site, using a mixture of 4% Fluorogold and 4% Isolectin B4 conjugated to horseradish peroxidase, to identify cell bodies with axons in the inferior alveolar nerve and cells with unmyelinated axons within this population, respectively. Control experiments involved tracer injection alone. After harvesting the tissue, sagittal sections were taken from both the right and left ganglia and immunohistochemical staining used to reveal the presence of peptides and Isolectin B4 tracer. The results showed a significant decrease in GAL expression after injury and an increase in ENK and NPY expression. No significant differences were seen in the expression of the other peptides or in the proportion of lectin-positive cells at any time after injury. When compared with previous data, significant differences were found between peptide expression following nerve ligation and nerve section. These results reveal that the changes in neuropeptide expression in the trigeminal ganglion that follow IAN injury are dependent upon the type of injury. The extent to which changes in the central neuropeptide levels contribute to the development of sensory disorders remains to be established.

Changes in neuropeptide expression in the trigeminal ganglion following inferior alveolar nerve section in the ferret

Changes in neuropeptide expression in afferent nerve fibres may play a role in the persistent sensory abnormalities that can be experienced following trigeminal nerve injuries. We have therefore studied changes in the expression of the neuropeptides substance P, calcitonin gene-related peptide, enkephalin, galanin, neuropeptide Y and vasoactive intestinal polypeptide in the trigeminal ganglion following peripheral nerve injury. In anaesthetised adult female ferrets, the left inferior alveolar nerve was sectioned and recovery allowed for three days, three weeks or 12 weeks prior to perfusion-fixation. During a second procedure, one week prior to perfusion, the inferior alveolar nerve was exposed and an injection made central to the injury site using a mixture of 4 % Fluorogold and 4 % isolectin B4 conjugated to horseradish peroxidase to identify cell bodies with axons in the inferior alveolar nerve and cells with unmyelinated axons within this population, respectively. Control animals received tracer injection alone. After harvesting the tissue, sagittal sections were taken from both the right and left ganglia and immunohistochemical staining was used to reveal the presence of peptides and isolectin B4-horseradish peroxidase tracer. Within the Fluorogold-labelled population, cell counts revealed a significant reduction in the proportion of substance P-containing cells at three days (P = 0.0025), three weeks (P = 0.0094) and three months (P = 0.0149) after nerve section, and a significant reduction in the proportion of calcitonin gene-related peptide-containing cells at three days (P = 0.0003) and three weeks (P = 0.007). No significant changes were seen in the expression of the other peptides, or at other time periods. A significant reduction in the number of isolectin B4-horseradish peroxidase-positive cells (with unmyelinated axons) was seen at three days (P = 0.0025), three weeks (P = 0.0074) and three months after the injury (P = 0.0133). These results demonstrate a significant reduction in the expression of some neuropeptides in the early stages after inferior alveolar nerve section. Some of the results differ markedly from those reported previously in other systems, and may be related to the specific nerve studied, species variations or differences between spinal and trigeminal nerves.

Displacement of the contents of dentinal tubules and sensory transduction in intradental nerves of the cat

Experiments were performed on anaesthetized cats to test the hypothesis that fluid flow through dentinal tubules is part of the mechanism involved in the transduction of pain-producing stimuli in teeth. In 11 animals, fluid flow through dentine and single- and multi-unit activity in intradental nerves were recorded simultaneously during the application of changes in hydrostatic pressure (-500 to +500 mm Hg) to exposed dentine. Seventeen A-fibres (conduction velocity (CV), 10.6-55.1 m s(-1)) were isolated that were pressure sensitive. The thresholds of these units in terms of dentinal fluid flow were in the range 0.3-2.1 nl s(-1) mm(-2) during outward flow from the pulp and 2.0-3.5 nl s(-1) mm(-2) during inward flow. All the units were more sensitive to outward than inward flow. Twenty-eight units (CV, 0.6-48.8 m s-1) were not pressure sensitive, and 12 of these had conduction velocities in the C-fibre range (< 2.5 m s(-1)). The velocities of the tubular contents were calculated by estimating the number and diameters of dentinal tubules exposed. At the threshold of single-fibre responses these velocities were in the range 31.7-222.9 microm s(-1) during outward flow 211.4-369.6 microm s-1 during inward flow. Repetitive pressure stimulation of dentine resulted in a progressive reduction in the evoked discharge, which was probably due to pulp damage. In seven animals, 10 single intradental nerve fibres were selected that responded to hydrostatic pressure stimuli and their responses to the application of hot, cold, osmotic, mechanical and drying stimuli to exposed dentine were investigated. With these stimuli dentinal fluid flow could not be recorded in vivo for technical reasons and was therefore recorded in vitro after completion of the electrophysiological recordings. With each form of stimulus, the discharge evoked in vivo was closely related to the flow predicted from the in vitro measurements. The results were therefore consistent with the hypothesis that the stimuli act through a common transduction mechanism that involves fluid flow through dentine.

Afferent activity from myelinated inferior alveolar nerve fibers in ferrets after constriction or section and regeneration

To investigate possible peripheral mechanisms for post-injury sensory disorders in the trigeminal system, we have made electrophysiological recordings from myelinated fibres in the inferior alveolar nerve (IAN) which have previously sustained an injury. In earlier experiments we have shown that axons in ligature-induced neuromas of the IAN develop spontaneous activity and mechanical sensitivity. The present study has investigated these responses after two different types of injury. In 24 anaesthetised adult male ferrets the left IAN was either chronically constricted by four loose chromic gut ligatures (12 animals) or sectioned and regeneration permitted (12 animals). After recovery periods of 3 days, 1, 3, 6, 12 or 24 weeks, single unit recordings were made from the nerve proximal to the injury site. The proportion of units which were spontaneously active ranged from 0% to 19% after constriction injury and from 0% to 10% after nerve section and regeneration. Both groups revealed a marked variability between individual animals at similar time periods. Mechanical sensitivity was found in 0-42% of units after constriction and 0-25% of units after nerve section; both groups showed a significant negative correlation between mechanical sensitivity and recovery period. None of the fibres which had regained peripheral receptive fields was either spontaneously active or mechanically sensitive. There was no significant difference between the levels of spontaneous activity or mechanical sensitivity in the two groups or that previously found in ligature-induced neuromas. Thus we conclude that widely differing types of peripheral nerve injury are capable of initiating similar raised levels of afferent activity in myelinated inferior alveolar nerve fibres.

Involvement of nitric oxide in re-innvervation of rat molar tooth pulp following transection of the inferior alveolar nerve

The aim of this study was to elucidate whether nitric oxide (NO) is involved in re-innervation of rat molar tooth pulp following transection of the inferior alveolar nerve. The inferior alveolar nerves (IAN) of rats were transected unilaterally under anesthesia with chloral hydrate. The animals received horseradish peroxidase (HRP) application to mandibular molar tooth pulps on both sides and were fixed by transvascular perfusion. The average number of labeled cells on each side of the trigeminal ganglion was not significantly different [101 +/- 11 (mean +/- S.E.M.; n = 6, left) and 89 +/- 11 (n = 6, right)]. With HRP application on postoperative day 3, the ratio of the number of labeled neurons in the transected vs. non-transected (contralateral) sides was 31.5 +/- 5.8% (n = 11). The i.p. administration of N(omega)-nitro-L-arginine methyl ester (L-NAME; 100 mg/kg, once a day for a period of 4 days), but not D-NAME, significantly decreased the ratio of the number of labeled neurons (10.1 +/- 7.0%, n = 10). L-Arginine (300 mg/kg, i.p., once a day for a period of 4 days) slightly increased the number of labeled neurons on the transected side. Clonidine (25 microg/kg, i.p., once a day for a period of 4 days) failed to exhibit any significant effect on nerve regeneration. In the trigeminal ganglion ipsilateral to the transected IAN on postoperative day 4, NADPH-diaphorase (NADPH-d)-positive neurons had significantly increased. On the other hand, no changes in NADPH-d were observed in the superficial layers of the subnucleus caudalis of the spinal trigeminal nucleus from where primary neurons innervating the mammalian tooth pulp project. These results suggest that NO is involved in several mechanisms related to neuronal regeneration.

Experimental trigeminal nerve injury

The successful reinnervation of peripheral targets after injury varies with the axonal population of the nerve that is injured and the extent of the dislocation of its central component from the peripheral endoneurial tube. Larger-diameter axons such as those supplying mechanoreceptors recover more readily than narrower axons such as those supplying taste. A complex, bi-directional interaction between lingual epithelium and sprouting nerve results in the redifferentiation of taste buds after denervation. Dentin and the dental pulp provide a strong attraction to sprouting nerves and will become reinnervated from collateral sources if recovery of the original innervation is blocked. The most effective repair technique for transected lingual nerves is one which brings the cut ends together rather than one that provides a temporary bridge. Injuries can result in cell death in the trigeminal ganglion but only if the injury is severe and recovery is prevented. Lesser damage results in chromatolysis and the increased expression of neuropeptides. All nerve injuries bring about changes in the trigeminal nucleus. These occur as changes in receptive field and the incidence of spontaneously active neurons, effects which are consistent with the unmasking of existing afferents. These functional changes are short-lived and reversible. Morphologically, nerve injury results in terminal degeneration in the nuclei and an increased expression of the c-Fos gene and some neuropeptides. Only a chronic constriction injury induces behavioral changes. The adult trigeminal system retains considerable plasticity that permits it to respond successfully to nerve injury. Much remains to be learned about this response, particularly of the trophic factors that control peripheral recovery and the central response to more severe injuries.

Recovery of infraorbital nerve function after zygomaticomaxillary cheek pedicled flap

The zygomaticomaxillary cheek pedicled flap (ZMCF) involves the intentional section of the infraorbital nerve to reflect the flap laterally in order to give access to the rhinopharynx, clivus and upper cervical spine. The aim of this trial was to examine the recovery of sensation of the infraorbital nerve, both quantitatively (touch sensation, localisation test, two-point discrimination) and qualitatively (sharp/blunt test, temperature sensation, pain sensitivity, dental sensitivity) in 7 patients, at least 12 months after surgery. In each patient, four cutaneous areas (lower eyelid, nose ala, upper lip, cheek) and the upper vestibulum were tested. Results of each test in all the examined areas were evaluated and compared with the data obtained on the nonoperated side (control side). Results of neurosensory tests indicated good recovery of sensation with little difference in comparison with the control side, showing that the functional consequence of ZMCF should actually be considered only as a transitory event.

Haemodynamic and immunohistochemical studies of rat incisor pulp after denervation and subsequent re-innervation

The effects of injury to the inferior alveolar nerve on the distribution of neuropeptides and neurogenic blood-flow reactions were studied in rat mandibular dental pulp. In normal incisor pulps, calcitonin gene-related peptide (CGRP)-like immunoreactivity was common, while substance P- and neurokinin (NKA)-positive nerve fibres were much less abundant. There were no signs of vasoactive intestinal peptide-like, neuropeptide Y-like or 5-hydroxytryptamine-like immunoreactivity. In normal pulps, electrical stimulation (100 microA, 5 ms, 15 Hz for 30 s) of the tooth crown resulted in transient vasoconstriction followed by vasodilation, which was enhanced after alpha-adrenoceptor blockade. At 3 days-4 weeks after unilateral nerve section there were no signs of CGRP-, substance P- and NKA-immunoreactivity, and there was no vasodilation in response to tooth stimulation. The vasoconstrictor response was also absent during this period but at 4 weeks postoperatively a weak response was obtained and after 7 weeks the vasoconstrictor response had regained normal amplitude. At 7 weeks postoperatively, a large number of CGRP-positive fibres had reappeared and at 11 weeks the pattern of CGRP-immunoreactivity was normal. However, substance P- and NKA-immunoreactivity were not found at 7 or 11 weeks after surgery. Vasodilator responses appeared at 7 weeks, and showed normal amplitude at 11 weeks after the creation of the nerve lesion. The results show that during nerve regeneration, sympathetic vasoconstriction was regained earlier than neurogenic vasodilation in rat incisor teeth. The reappearance of neurogenic vasodilation after nerve injury was temporarily associated with the presence of CGRP-immunoreactivity in regenerating trigeminal afferent nerves.

Teeth and tooth nerves

(1) Although our knowledge on teeth and tooth nerves has increased substantially during the past 25 years, several important issues remain to be fully elucidated. As a result of the work now going on at many laboratories over the world, we can expect exciting new findings and major break-throughs in these and other areas in a near future. (2) Dentin-like and enamel-like hard tissues evolved as components of the exoskeletal bony armor of early vertebrates, 500 million years ago, long before the first appearance of teeth. It is possible that teeth developed from tubercles (odontodes) in the bony armor. The presence of a canal system in the bony plates, of tubular dentin, of external pores in the enamel layer and of a link to the lateral line system promoted hypotheses that the bony plates and tooth precursors may have had a sensory function. The evolution of an efficient brain, of a head with paired sense organs and of toothed jaws concurred with a shift from a sessile filter-feeding life to active prey hunting. (3) The wide spectrum of feeding behaviors exhibited by modern vertebrates is reflected by a variety of dentition types. While the teeth are continuously renewed in toothed non-mammalian vertebrates, tooth turnover is highly restricted in mammals. As a rule, one set of primary teeth is replaced by one set of permanent teeth. Since teeth are richly innervated, the turnover necessitates a local neural plasticity. Another factor calling for a local plasticity is the relatively frequent occurrence of age-related and pathological dental changes. (4) Tooth development is initiated through interactions between the oral epithelium and underlying neural crest-derived mesenchymal cells. The interactions are mediated by cell surface molecules, extracellular matrix molecules and soluble molecules. The possibility that the initiating events might involve a neural component has been much discussed. With respect to mammals, the experimental evidence available does not support this hypothesis. In the teleost Tilapia mariae, on the other hand, tooth germ formation is interrupted, and tooth turnover ceases after local denervation. (5) Prospective dental nerves enter the jaws well before onset of tooth development. When a dental lamina has formed, a plexus of nerve branches is seen in the subepithelial mesenchyme. Shortly thereafter, specific branches to individual tooth primordia can be distinguished. In bud stage tooth germs, axon terminals surround the condensed mesenchyme and in cap stage primordia axons grow into the dental follicle.(ABSTRACT TRUNCATED AT 400 WORDS)

Substance P-, calcitonin gene-related peptide, growth-associated protein-43, and neurotrophin receptor-like immunoreactivity associated with unmyelinated axons in feline ventral roots and pia mater

The spinal pia mater receives a rich innervation of small sensory axons via the ventral roots. Other sensory axons enter the ventral roots but end blindly or turn abruptly in hairpin loop-like formations and continue in a distal direction. In the present study, the content of substance P (SP)-, calcitonin gene-related peptide (CGRP)-, growth-associated protein (GAP-43)-, and low-affinity neurotrophin receptor protein (p75NGFr)-like immunoreactivity (-LI) associated with these different types of sensory axons was assessed with light and electron microscopic immunohistochemical techniques. In addition, the binding of antibodies against synthetic peptides representing unique sequences of residues in the products of the trk and trkB protooncogenes was analyzed. These genes encode membrane spanning proteins, which have been shown to constitute specific high affinity binding sites for several members of the nerve growth factor family of neurotrophic factors. The results of the present study imply that the ventral root afferents comprise several different types of sensory axons, which all contain SP-, CGRP-, GAP-43-, and p75NGFr-like immunoreactivities. In addition, at least some of the presumed sensory fiber bundles in ventral roots and the pia mater were immunoreactive for the trkB gene product. Moreover, leptomeningeal cells and nonneuronal cells of the ventral roots were shown to bind antibodies to both the trk and trkB gene products. The ventral root afferents seem to share their immunohistochemical pattern with pain-transducing axons at some other locations, such as the tooth pulp. The contents of SP- and CGRP-LI in sensory axons that reach the central nervous system (CNS) through the ventral root indicate that ventral root afferents may be involved in sensory mechanisms, such as the ventral root pain reaction, as well as in the control of the pial blood vessels. The demonstration of GAP-43 and neurotrophin receptor-immunoreactivities associated with unmyelinated fibers in ventral roots and the pia mater is discussed in relation to previous reports on postnatal plasticity in these axonal populations.

Cat dental pulp after denervation and subsequent re-innervation: changes in blood-flow regulation and distribution of neuropeptide-, GAP-43- and low-affinity neurotrophin receptor-like immunoreactivity

The effects of unilateral extramandibular inferior alveolar nerve injury on pulpal blood-flow responses to electrical stimulation and i.v. injections of substance P (SP) in cat mandibular canine teeth with a dentinal lesion were investigated with laser Doppler flowmetry. After blood-flow recordings, the teeth were fixed and the pulps were examined with light and electron microscopy. The distribution of pulpal SP, neurokinin A (NKA), calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), growth-associated protein (GAP-43) and low-affinity neurotrophin receptor (LANR)-like immunoreactivity was examined with immunohistochemical fluorescence microscopy. Blood-flow recordings, performed at 10 days and 1 month postoperatively, showed that vasodilation, occurring in control teeth after bipolar electrical stimulation of the tooth crown, was absent in the denervated pulps, whereas at 3 months, five of six teeth had regained responsiveness, although at a low level. There was enhanced vasodilation (by 370%) to SP injections (400 fmol i.v.) at 10 days in denervated pulps. Such supersensitivity was reduced at 1 month despite the apparent lack of nerve fibers, and the response fell further towards the level in control teeth at 3 months when pulpal axons reappeared. At 10 days and 1 month postoperatively, light and electron microscopy demonstrated that surgery had resulted in total pulpal denervation. At 3 and 6 months, a large number of regenerated pulpal axons reappeared, in accordance with previous findings. At 10 days and 1 month after nerve transection immunohistochemistry showed a complete loss of pulpal immunoreactivity to all the neuropeptides that were studied. At 3 and 6 months, neuropeptide immunoreactivity reappeared but far fewer number of pulpal nerve fibers were SP-, NKA- and CGRP-immunoreactive than under normal conditions, as demonstrated by double-labeling experiments with GAP-43- or LANR-antiserum. The results indicate that pulpal hemoregulatory functions, which are lost after denervation, do not return to normal levels after nerve regeneration. This malfunction may be caused by inadequate target re-innervation and/or a deficiency of neuropeptides in the re-innervated pulp.

Neural changes in periapical lesions after systemic steroids in the ferret

This study was intended to clarify the relationship between the neural changes which occur around the apex of the ferret canine after pulpectomy and the inflammatory process induced by the procedure. In 12 young adult ferrets, under general anesthesia, the pulps in the mandibular canine teeth were removed and replaced with gutta percha and Grossman's sealer. Six of the animals were treated with dexamethasone to reduce the inflammatory response. Three months later, the animals, again under general anesthesia, were perfused with a fixative mixture. Three unoperated animals that had not been treated with dexamethasone were also perfused. The mandibular canine teeth and their supporting tissues were removed, processed, and serially sectioned. Three-dimensional reconstructions of the periapical lesions in each animal were assembled and their volumes measured. The density of innervation in the periapical region was estimated. The mean lesion volume in the pulpectomized animals not treated with dexamethasone was 3.54 (+/- 2.27) mm3 and in the dexamethasone-treated animals 1.33 (+/- 1.31) mm3. The differences were statistically significant when tested by the Mann-Whitney U test (p < 0.01). Bacteria were not seen within any of the lesions. The innervation density beneath the canines in the pulpectomized animals not treated with dexamethasone was 164 units per mm2 (+/- 80) and in the steroid-treated animals 151 +/- 68 units per mm2. In the control, untreated animals, the innervation density was 22 +/- 10 units per mm2. The difference between the steroid-treated pulpectomized animals and the untreated pulpectomized animals was not statistically significant (p > 0.5).(ABSTRACT TRUNCATED AT 250 WORDS)

Immuno-electron-microscopic localization of laminin and collagen type IV in normal and denervated tooth pulp of the cat

The distribution of laminin-like immunoreactivity in adult normal and denervated cat mandibular tooth pulps was studied by the use of fluorescence microscopy and pre-embedding immunogold electron microscopy. Immunoreactivity to collagen IV was also assessed in order to distinguish basement membranes. In normal pulps, light-microscope laminin-like immunoreactivity was strong along blood vessels and Schwann cell sheaths, and a faint immunoreactivity was seen also in the odontoblast layer. Electron microscopy confirmed the laminin-like immunoreactivity of endothelial and Schwann cell basement membranes at all pulpal levels. In the odontoblast layer and the predentine, nerve-like structures lacking basement membranes but possessing strong membrane laminin-like immunoreactivity were encountered. In addition, a clear-cut laminin-like immunoreactivity of plasma membranes of the somata and processes of odontoblasts was seen. Observations on denervated pulps as well as pulps in which nerve regeneration had taken place did not reveal any changes in the pattern of laminin-immunoreactivity in basement membranes or odontoblasts. Distribution of collagen IV-like immunoreactivity was very similar to laminin-like immunoreactivity in basement membranes of blood vessels and Schwann cells, and appeared unaffected by denervation. The odontoblasts and nerve-like profiles in the odontoblast layer were devoid of collagen IV-like immunoreactivity. We propose that odontoblast-associated laminin could be of significance as guidance for regenerating terminal pulpal nerve fibers to appropriate targets.

Growth-associated protein (GAP-43)-like immunoreactivity in primary and permanent tooth pulp nerve fibers of the cat

GAP-43-like immunoreactivity in developing and mature incisor and canine tooth pulp nerve fibers in the cat was examined with fluorescence immunohistochemistry and pre-embedding immunogold electron microscopy. As expected, pulpal and periodontal nerve fibers in primary teeth aged 2-3 weeks showed strong immunoreactivity. Double-labeling experiments demonstrated that 50-70% of primary pulpal GAP-43-positive nerve fibers showed CGRP-like immunoreactivity. However, in adult permanent teeth the vast majority of pulpal nerve fibers also displayed intense GAP-43-like immunoreactivity both when surrounding pulpal blood vessels and in the subodontoblast/odontoblast region. There was a high degree (90-95%) of simultaneous expression of GAP-43-like immunoreactivity and CGRP-like immunoreactivity in adult permanent pulps. Immunogold GAP-43 labeling was mainly associated with the cytoplasmic side of axonal membranes. However, occasional examples of immunolabeled Schwann cells were also found. High levels of GAP-43 in normal mature permanent pulpal nerves may facilitate neural plasticity after dental wear or injury.

Anterograde horseradish peroxidase tracing and immunohistochemistry of trigeminal ganglion tooth pulp neurons after dental nerve lesions in the rat

The peripheral reorganization of pulpal nerves after tooth injury was studied, in the rat, with anterograde horseradish peroxidase tracing techniques, and combined retrograde Fluorogold tracing and immunohistochemistry was employed to examine the effects of inferior alveolar nerve lesions or tooth injury on some cytochemical characteristics of pulpal trigeminal ganglion nerve cells, namely content of substance P, calcitonin gene-related peptide and the ganglioside GM1 (binding subunit of cholera toxin), as well as affinity to RT 97 (antibody to neurofilament protein) and the lectin Griffonia simplicifolia isolectin I-B4. Anterograde horseradish peroxidase tracing demonstrated that pulpal nerves either disappear or reinnervate novel targets after loss of pulpal tissue. There were no obvious signs of neuroma formation. Retrograde Fluorogold labelling with immunohistochemistry showed that after inferior alveolar nerve lesions with subsequent regeneration, a much higher proportion of Fluorogold cells (15%) were substance P-positive compared to normal (2%). In addition, 3% of the cells were Griffonia simplicifolia isolectin I-B4-positive. Such cells were very rare in controls. Proportions of calcitonin gene-related peptide-, GM1- and RT-97-positive cells were normal. After tooth lesions, the proportions of Fluorogold-positive substance P-, Griffonia simplicifolia isolectin I-B4-, GM1- and RT 97-labelled cells were similar to controls, while the proportion of calcitonin gene-related peptide-positive neurons was reduced. The results show that pulpal deafferentation may change the long-term cytochemical characteristics of affected trigeminal ganglion neurons.

Re-innervation of rat molar tooth pulp following transection of the inferior alveolar nerve

The inferior alveolar nerves (IAN) of young male Wistar rats (b.wt. greater than or equal to 200 g) were transected unilaterally, slightly proximal to the mandibular foramen under anesthesia with chloral hydrate (0.4 g/kg, i.p.). After various postoperative periods of time, the animals received horseradish peroxidase (HRP) applications to 3 mandibular molar tooth pulps on both sides and were fixed by transvascular perfusion 24 h later. Horizontal 60 microns sections of the trigeminal ganglion were incubated with tetramethylbenzidine hydrochloride and the cross-sectional areas of all the labeled neuronal cell bodies were measured. The average number of labeled cells on the untransected (control) side was 148 (n = 26), with cross-sectional areas ranging between 131.9 and 2129.6 microns 2. Of these, 42.5% fell between 300 and 600 microns 2. About 13.5% (n = 7) of the primary neurons innervating the tooth pulps escaped the ipsilateral neurotomy and were labeled by HRP application on postoperative day 0. With HRP application on postoperative day 3, the number of labeled neurons recovered to 56.8% (n = 7) that of the control and maintained this level up to postoperative day 75. From postoperative days 3 through 75, the cell size spectrum of labeled neurons on the transection side was similar to that of the control and no consistent tendency of alteration was observed; i.e. they were distributed between 134.4 and 2214.3 microns 2, with the mode being 41.5% in the range between 300 and 600 microns 2 (n = 19).

Morphological and physiological properties of neurons after long-term axonal regeneration: observations on chronic and delayed sequelae of peripheral nerve injury

Axonal regeneration has been the focus of extensive investigation of mechanisms which mediate structural and functional recovery after injury to mammalian peripheral nerves and has proven to be a valuable model for development and plasticity in the nervous system. Although details of the acute morphological and physiological responses to nerve injury are well-described, less information is available to nerve injury are well-described, less information is available about long-term alterations which persist or develop after regenerated axons have established connections with their targets. The present paper briefly discusses the mammalian neuron's initial response to peripheral nerve injury and subsequent events which occur during regeneration. Morphological and physiological alterations observed in neurons after long-term axonal regeneration are described and are considered in the context of their potential implications for clinical recovery after nerve injury, as well as their potential contribution to the appearance of delayed neurological dysfunction. Selective responses to neuronal injury during development and in different fiber populations are discussed.

Observations on the recovery of sensation following inferior alveolar nerve injuries

The recovery of sensation to the lower lip and chin following unilateral inferior alveolar or mental nerve injuries has been examined in 21 adult patients for periods of up to 45 months. Sensory testing was performed using light touch, pin prick and thermal stimuli as well as testing for sharp/blunt differentiation, localisation and two-point discrimination. The area of anaesthesia to light touch stimuli had disappeared by 4 months after nerve compression and by 3.5-8 months after nerve section, but persisted after nerve section if nerve regeneration was impeded. In this latter group there was a significant reduction (mean 65%) in the area of anaesthesia by 1 year post injury and this would be consistent with the development of a collateral reinnervation. Nerve section injuries and 46% of the nerve compression injuries resulted in a persistent sensory abnormality. The tests most likely to reveal this abnormality were localisation, pin prick threshold and two-point discrimination.

Nerve growth to tooth buds after homotopic or heterotopic autotransplantation

Feline permanent incisor tooth buds (bell stage) were autotransplanted to mandibular alveolar sockets (homotopic site) or to the submandibular subcutis or the leg (heterotopic sites). This was done in 34 kittens aged 1-2 months. After survival times of 3-8 months the animals were fixed by glutaraldehyde perfusion. A total of 56 mineralized teeth, which had developed at the recipient sites, were removed, demineralized and processed for light microscopic (LM) general evaluation. Fourty-four teeth, which were judged to be grossly normal in the LM, were selected for electron microscopic (EM) analysis with respect to the occurrence of pulpal nerve fibres. The highest proportion of normal teeth (16 of 16) was obtained from the alveolar site, followed by the submandibular (11 of 14) and hindlimb (17 of 26) sites. Most of the grossly normal grafts possessed pulpal axons (37 of 44). The alveolar grafts were all innervated and exhibited a largely normal appearance qualitatively and in terms of percentage of myelinated fibres. The proportion of innervated pulps was lower among the heterotopic mandibular (10 of 11) and hindlimb (11 of 17) grafts. In addition, signs of nerve fibre degeneration appeared more frequently at the heterotopic sites. On the basis of these findings, and in view of the results of other workers, we conclude that tooth germs are attractive targets for all divisions of the trigeminal nerve and for cutaneous nerves outside the trigeminal system. However, the morphological picture tends to become increasingly abnormal with increasing distance from the normal locus.

Reinnervation of teeth after segmental osteotomy

The reinnervation of 8 canine teeth denervated by segmental osteotomy has been investigated in cats by using electron microscopy. 12 weeks after osteotomy, the mean total number of axons at the canine apices was 36% of that found in normal animals. The myelinated axons were smaller than normal with thinner myelin sheaths. In non-myelinated fibres, there were fewer axons per fibre (Remak bundle) and more fibres containing only one axon. The proportions of myelinated and non-myelinated fibres were normal.

Reinnervation of the canine tooth pulp after section of the inferior alveolar nerve in the cat

The apices of lower canine teeth have been examined 9 weeks after unilateral section of the inferior alveolar nerve. The teeth were extensively reinnervated although there was considerable variation in the number of axons present. The myelinated axons were smaller than normal with thinner myelin sheaths. There were fewer axons per non-myelinated fibre (Remak bundle). The proportions of myelinated and non-myelinated fibres were normal.

Evidence for the persistence of axons at the apex of the cat's lower canine tooth after section of the inferior alveolar nerve

The aim of this investigation was to establish the degree of denervation produced by inferior alveolar nerve section and to provide histological evidence for the presence of pulpal nerve fibres supplying the teeth which do not travel with the inferior alveolar nerve. Four adult cats were used. Each stage of the experiment was carried out under general anesthesia. The left inferior alveolar nerve was exposed and sectioned near the mandibular foramen. After 56 hours and 7 days, respectively, the jaw opening reflex to electrical stimulation of each lower canine was tested. Recordings were made from the left canine during electrical stimulation of the ipsilateral inferior alveolar nerve central and peripheral to the site of section as well as from the ipsilateral and contralateral inferior alveolar nerve during electrical stimulation of the left canine. Recordings were also made from the lingual nerve. After the recordings were completed two animals were perfused 56 hours after inferior alveolar nerve section, two more 7 days after section. Ultrathin sections of the apices of the lower canine teeth were examined in the electron microscope and each nerve fibre photographed. Each axon was examined to determine whether it was degenerating or normal. A jaw-opening reflex could not be elicited by stimulation of the left canine either 2 or 7 days after nerve section, whereas a normal response was evoked by stimulation of the right, control canine. At 2 days small responses could be recorded from the left canine teeth during stimulation of the left inferior alveolar nerve peripheral to the point of section. In one 2-day animal, responses could be recorded in the lingual nerve during stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Dental sensory receptors

Teeth are innervated by unmyelinated sympathetic axons, and by unmyelinated and small myelinated sensory axons. Some sensory axons in teeth are terminal branches of larger parent axons, so that conduction from teeth to CNS in trigeminal nerves includes C-fiber, A-delta, and A-beta velocities. Sensory dental axons contain acetylcholine or substance P-like immunoreactivity. The sympathetic axons contain noradrenalin. Other neuropeptides may also be present, such as vasoactive intestinal peptide and serotonin. Dental axons of mature teeth of many species (man, monkey, cat, rodents, fish) are essentially the same, but continuously erupting teeth have smaller and fewer axons. Free sensory nerve endings in mature teeth are found in the peripheral plexus of Raschkow, the odontoblastic layer, the predentin, and the dentin. Free nerve endings are most numerous in those regions near the tip of the pulp horn, where more than 40% of the dentinal tubules can be innervated. Many dentinal tubules contain more than one free nerve ending. Intradentinal axons can extend as far as 0.2 mm into dentin but usually end less than 0.1 mm from the pulp. Some sensory endings also occur along pulpal blood vessels. In continuously erupting teeth nerve endings do not enter the dentin but remain within the pulp. Nerve endings in dentin are labeled by axonal transport. They are therefore as viable and active as the nerve endings in pulp. The axoplasm of the free nerve endings contains organelles typical of other somatosensory receptors. These organelles are most common in the successive beaded regions along the free nerve endings and include mitochondria, clear and dense-core vesicles, multivesicular bodies, profiles of smooth endoplasmic reticulum, and relatively few microtubules and neurofilaments. The beads can vary in size from about 0.2 to 2.0 microns and can have varying amounts of receptor organelles. The interbead axonal regions are thin and contain mainly microtubules and neurofilaments. Nerve endings are associated with companion cells after they leave the coronal nerve bundles; these companion cells include Schwann cells, fibroblasts, and odontoblasts. There is no good evidence of gap junctions or synapses between nerve endings and odontoblasts. Instead, the two cell types form appositions that have a 20-40 nm extracellular cleft and parallel apposed plasmalemmas but no unusual membrane-associated material. No special organelles occur in the odontoblastic cytoplasm at these sites.(ABSTRACT TRUNCATED AT 400 WORDS)