Developmental growth and degeneration of pulpal axons in feline primary incisors

Trigeminal Sensory Neurons and Pulp Regeneration

The pulp-dentin complex is innervated by a high density of trigeminal neurons free nerve endings. These neuronal fibers are highly specialized to sense noxious stimuli such as thermal, mechanical, chemical, and biological cues. This robust alert system provides immediate feedback of potential or actual injury triggering reflex responses that protect the teeth from further injury. In the case of patients, pain is the most important experience that leads them to seek oral health care. The adequate removal of the etiology, such as caries, provides ample opportunity for the robust reparative and regenerative potential of the pulp-dentin complex to restore homeostasis. In addition to this elaborated surveillance system, evidence has accumulated that sensory neuronal fibers can potentially modulate various steps of the reparative and regenerative process through cellular communication processes. These include modulation of immunologic, angiogenic, and mineralization responses. Despite these orchestrated cellular events, the defense of the pulp-dentin complex may be overwhelmed, resulting in pulp necrosis and apical periodontitis. Regenerative endodontic procedures have evolved to restore the once lost function of the pulp-dentin complex. After these procedures, a large subset of successful cases demonstrates a positive response to sensitivity testing, suggesting reinnervation of the canal space. This process is likely mediated through cellular and noncellular release of neurotrophic factors such as brain-derived nerve growth factor. In addition, these newly recruited nerve fibers appear equipped to sense thermal stimuli through nonhydrodynamic mechanisms. Collectively, the significance of innervation in the normal physiology of the pulp-dentin complex and its role in regeneration need to be better appreciated to promote further research in this area that could potentially bring new therapeutic opportunities.

Molecular, cellular and behavioral changes associated with pathological pain signaling occur after dental pulp injury

Persistent pain can occur after routine dental treatments in which the dental pulp is injured. To better understand pain chronicity after pulp injury, we assessed whether dental pulp injury in mice causes changes to the sensory nervous system associated with pathological pain. In some experiments, we compared findings after dental pulp injury to a model of orofacial neuropathic pain, in which the mental nerve is injured. After unilateral dental pulp injury, we observed increased expression of activating transcription factor 3 (ATF3) and neuropeptide Y (NPY) mRNA and decreased tachykinin precursor 1 gene expression, in the ipsilateral trigeminal ganglion. We also observed an ipsilateral increase in the number of trigeminal neurons expressing immunoreactivity for ATF3, a decrease in substance P (SP) immunoreactive cells, and no change in the number of cells labeled with IB4. Mice with dental pulp injury transiently exhibit hindpaw mechanical allodynia, out to 12 days, while mice with mental nerve injury have persistent hindpaw allodynia. Mice with dental pulp injury increased spontaneous consumption of a sucrose solution for 17 days while mental nerve injury mice did not. Finally, after dental pulp injury, an increase in expression of the glial markers Iba1 and glial fibrillary acidic protein occurs in the transition zone between nucleus caudalis and interpolaris, ipsilateral to the injury. Collectively these studies suggest that dental pulp injury is associated with significant neuroplasticity that could contribute to persistent pain after of dental pulp injury.

Stem cells of the apical papilla regulate trigeminal neurite outgrowth and targeting through a BDNF-dependent mechanism

Regenerative endodontic procedures have become a valuable alternative for the treatment of immature teeth with pulp necrosis. In addition to resolution of periradicular pathosis and promotion of continued root development, positive vitality testing has been observed in some regenerative clinical cases. Importantly, the positive response to electric stimulation of the regenerated tissue requires targeting of periradicular axons into the previously empty root canal space. However, the mechanism by which this process occurs is largely unknown. Since stem cells of the apical papilla (SCAP) have been proposed to populate the root canal following regenerative endodontic procedures, we hypothesized that SCAP regulate neurite outgrowth and axonal targeting. To test this hypothesis, we established primary co-cultures of human SCAP and rat trigeminal neurons, and performed neurite outgrowth assays using ELISA and confocal microscopy to determine the effect of increasing concentration of SCAP on the total neurite outgrowth and axonal targeting. In addition, we evaluated whether SCAP evoked axonal targeting in vivo using a matrigel subcutaneous implant assay. Data were analyzed by ANOVA with Bonferroni's post hoc test, and significance was set at p<0.05. The results demonstrated that SCAP release a soluble factor that regulates neurite outgrowth from cultured trigeminal neurons. Next, we demonstrated that this effect is completely abolished by pretreatment with a neutralizing antibody to brain-derived neurotrophic factor (BDNF), but not by antibodies to other neurotrophins. Further, SCAP release BDNF in a concentration-dependent manner as detected by ELISA, and trigger directed axonal targeting both in vitro and in vivo as demonstrated by microfluidic and matrigel implant experiments, respectively. Collectively, these results suggest that SCAP may be responsible for the chemical signal driving axons to target regenerated tissue via a BDNF-dependent mechanism.

Sensory experiences in relation to pulpal nerve activation of human teeth in different age groups

There are no data on the correlation of intradental nerve activity and sensation from intact human teeth. We used microneurography to examine this relation and to determine whether it changes with age. Fifteen informed and healthy male volunteers were divided into three age groups: group A (18.6+/-1.82 years, mean+/-standard deviation (S.D.)), group B (38.4+/-2.70 years) and group C (64.0+/-4.06 years). Ratings of perceived pain intensity to thermal stimulation were obtained using a visual analogue scale (VAS). In addition, each subject chose one or two words from the short-form McGill Pain Questionnaire to describe perceived pain. A total of 90 single pulpal axons were studied with microneurography at the same time as the sensory experiences were recorded. Mean conduction velocities and variance estimates correlated closely with age. With advancing age, first, the percentage of teeth from which the subjects did not perceive any sensations to thermal stimulation increased, second, units responding to heat stimulus decreased, and third, latencies of sensation induced by thermal stimulation increased. In addition, a burst of afterdischarges following thermal stimulation and neural discharges evoked by thermal stimulation produced no sensation only in some of group B and C units. In contrast, no significant difference was found among three groups in VAS scores and words to describe the perceived pain to thermal stimulation. These results suggest that pulpal afferents were activated by the same mechanism(s); the hydrodynamic mechanism works immediately after thermal stimulation and is possibly followed by direct activation of some nerves, especially slow conducting fibres. In older tooth pulps, the decrease in the number of fast conducting afferents and mineral apposition of dentinal tubules impaired the nerve activation, especially by heat, as per the hydrodynamic mechanism. Spike discharges without sensation in older individuals were suggested to be due to insufficient spatial and temporal summation and may be involved with abnormal uncomfortable sensations.

Development of innervation in primary incisors in the foetal period

Sections from the frontal part of the mandible of 43 human foetuses from 9 to 39 weeks of prenatal age, which contained two, three and sometimes four lower incisors were immunohistochemically examined using protein gene product and neuron specific enolase (NSE) antibodies in order to establish the time of appearance of nerve fibres in the developing tooth germ and to define their topography. Nerve fibres were first detected in the dental follicle in the 11th week of intrauterine life. Their presence in the dental papilla was confirmed in the 18th week when the first layers of dentine and enamel were deposited. In the 24th week of intrauterine life, the nerve fibres first reached the subodontoblastic region. In the subsequent weeks, an increase in the number of nerve fibres accompanying blood vessels in the central portion of the dental papilla resulted in the formation of neuro-vascular bundles. Moreover, the progressive deposition of enamel and dentine was accompanied by branching of papillary nerves, which thereby formed a fan-pattern. In the foetal period, no evidence was found for the formation of a subodontoblastic plexus. However, we did observe single nerve fibres in close proximity to the odontoblast layer at the end of intrauterine life. Nerve fibres were not detected in either predentine or dentine throughout foetal life.

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.

Molecular signaling and pulpal nerve development

The purpose of this review is to discuss molecular factors influencing nerve growth to teeth. The establishment of a sensory pulpal innervation occurs concurrently with tooth development. Epithelial/mesenchymal interactions initiate the tooth primordium and change it into a complex organ. The initial events seem to be controlled by the epithelium, and subsequently, the mesenchyme acquires odontogenic properties. As yet, no single initiating epithelial or mesenchymal factor has been identified. Axons reach the jaws before tooth formation and form terminals near odontogenic sites. In some species, local axons have an initiating function in odontogenesis, but it is not known if this is also the case with mammals. In diphyodont mammals, the primary dentition is replaced by a permanent dentition, which involves a profound remodeling of terminal pulpal axons. The molecular signals underlying this remodeling remain unknown. Due to the senescent deterioration of the dentition, the target area of tooth nerves shrinks with age, and these nerves show marked pathological-like changes. Nerve growth factor and possibly also brain-derived neurotrophic factor seem to be important in the formation of a sensory pulpal innervation. Neurotrophin-3 and -4/5 are probably not involved. In addition, glial cell line-derived neurotrophic factor, but not neurturin, seems to be involved in the control of pulpal axon growth. A variety of other growth factors may also influence developing tooth nerves. Many major extracellular matrix molecules, which can influence growing axons, are present in developing teeth. It is likely that these molecules influence the growing pulpal axons.

Innervation of human tooth pulp in relation to caries and dentition type

The neural status of carious teeth, particularly those associated with a painful pulpitis, is largely unknown. This study sought to determine differences in the innervation density of human primary and permanent teeth and whether caries or painful pulpitis was associated with anatomical changes in pulpal innervation. Coronal pulps were removed from 120 primary and permanent molars with a known pain history. Teeth were categorized as intact, moderately carious, or grossly carious. Using indirect immunofluorescence, we labeled sections for the general neuronal marker, protein gene product 9.5. Using image analysis, we found permanent teeth to be significantly more densely innervated than primary teeth. While there was no significant correlation with reported pain experience, neural density in both dentitions increased significantly with caries. Analysis of these data suggests that caries-induced changes in neural density may be functionally more important in the regulation of pulpal inflammation and healing than in the processing and perception of dental pain.

Dental injury models: experimental tools for understanding neuroinflammatory interactions and polymodal nociceptor functions

Recent research has shown that peripheral mechanisms of pain are much more complex than previously thought, and they differ for acutely injured normal tissues compared with chronic inflammation or neuropathic (nerve injury) pain. The purpose of the present review is to describe uses of dental injury models as experimental tools for understanding the normal functions of polymodal nociceptive nerves in healthy tissues, their neuroinflammatory interactions, and their roles in healing. A brief review of normal dental innervation and its interactions with healthy pulp tissue will be presented first, as a framework for understanding the changes that occur after injury. Then, the different types of dental injury that allow gradation of the extent of tissue damage will be described, along with the degree and duration of inflammation, the types of reactions in the trigeminal ganglion and brainstem, and the type of healing. The dental injury models have some unique features compared with neuroinflammation paradigms that affect other peripheral tissues such as skin, viscera, and joints. Peripheral inflammation models can all be contrasted to nerve injury studies that produce a different kind of neuroplasticity and neuropathic pain. Each of these models provides different insights about the normal and pathologic functions of peripheral nerve fibers and their effects on tissue homeostasis, inflammation, and wound healing. The physical confinement of dental pulp and its innervation within the tooth, the high incidence of polymodal A-delta and C-fibers in pulp and dentin, and the somatotopic organization of the trigeminal ganglion provide some special advantages for experimental design when dental injury models are used for the study of neuroinflammatory interactions.

Course and composition of the nerves that supply the mandibular teeth of the rat

The rodent dentition has become an important model for investigations of interactions between dental tissues and peripheral neurons. Although experimental nerve injury has been widely used for such studies, there is uncertainty about the courses of nerve fibers supplying the mandibular teeth. In order to clarify this, we used a mixture of monoclonal antibodies against neurofilament proteins to enhance demonstration of nerve fibers so that small nerves could be readily traced in serial frozen sections of mandibles of Sprague Dawley rats ranging in age from embryonic day (E) 18 to postnatal day (P) 90. The 1st molar and anterior portion of the 2nd molar were innervated by small nerves that emerged as distinct branches of the IAN trunk at or near the mandibular foramen. In contrast, the nerve supply to the 3rd molar and posterior part of the 2nd molar was a branch of the lingual nerve that bypassed the mandibular canal altogether. The IAN trunk split into the mental nerve and a large branch to the incisor about 2 mm anterior to the mandibular foramen. Thick branches of the incisor nerve descended into the incisor socket to form a dense plexus of nerve fiber bundles extending along the length of the incisor periodontium. The sparse pulpal innervation of the incisor was provided by a few thin fascicles that emerged from the caudal portion of the periodontal plexus to enter the incisor apex. The dental branches of the IAN and lingual nerve seen in the adult were well established and readily identifiable at age E18 even though their targets were limited to the follicles of the developing teeth. These studies show that the trigeminal branches that supply the mandibular teeth can be identified at a wide range of ages as distinct nerves at a considerable distance proximal to their targets. This detailed information on the courses taken by the dental nerves can provide an anatomical basis for increased precision in characterization and perturbation of neural pathways from the molars and incisor.

On the pulpal nerve supply in primary human teeth: evidence for the innervation of primary dentine

The presence of nerves in human tooth pulp has been recognized for over a hundred years, and the innervation of dentine for about 40 years. These observations have been made in permanent teeth. Very few studies have reported on the innervation of the primary pulp and dentine. The purpose of this study was to describe the innervation of the primary tooth pulp-dentine complex. Ten mature primary teeth (one incisor, six canines and three molars) were used. Immediately following extraction they were divided into three sections using a diamond disc and saline coolant. They were then immersion fixed in a solution of formaldehyde and picric acid dissolved in a phosphate buffer pH 7.4). The teeth were then demineralized for 1-3 weeks in formic acid. Following complete demineralization, 30 microns sections were cut on a freezing microtome. Neural tissue was stained using a specific antibody to calcitonin gene related peptide (CGRP). Sections were mounted on glass slides and examined using light microscopy. No individual nerve fibres were seen in the control sections, suggesting that the method used was specific for CGRP-containing nerve fibres. The primary teeth appeared to be well innervated. Myelinated and unmyelinated nerves were seen. There was a dense but variable subodontoblastic plexus of nerves (plexus of Raschkow) and nerve fibres were seen to leave this to travel towards the odontoblast layer. Most terminated here, but a few penetrated the odontoblast layer to enter predentine and the dentine tubules. The maximum penetration was 125 microns but most terminated within 30 microns of the dentinopulpal junction. The coronal region was more densely innervated than the root. Within the crown the cervical third was the most densely innervated region, followed by the pulp horn and the middle third. In conclusion, this study has demonstrated that mature primary tooth contains a pulp which is well innervated and has many nerve endings terminating in or near the odontoblast layer, with a small number penetrating into dentine.

Immunohistochemical and electron microscopic demonstration of nerve fibres in relation to gingiva, tooth germs and functional teeth in the lower jaw of the cichlid Tilapia mariae

Immunohistochemistry revealed the presence of numerous neurofilament (NF)-like immunoreactive axons in relation to gingiva and dental follicles surrounding mineralizing tooth germs. The gingival nerve fibres frequently approached the prospective papilla of early tooth primordia. Electron microscopic (EM) analysis revealed the presence of bundles of unmyelinated axons immediately below the epithelial-proprial junction of the gingiva. Bundles of nerve fibres were also present in the border zone between the prospective papilla of bud-stage tooth germs and surrounding mesenchyme and in close proximity to blood vessels of the follicles surrounding older tooth germs, but no axons were observed within the emerging dental papilla. In the individual functional tooth, a bundle of NF-like immunoreactive nerve fibres entered the apical part of the pulp forming a subodontoblastic plexus at mid-pulpal levels. EM analysis showed that the apical bundle consisted of many unmyelinated and a few myelinated axons invested by Schwann cell processes. The subodontoblastic plexus contained unmyelinated axons only. Thin, axon-like profiles were also seen in predentinal tubules. Nerve fibres were not observed at pulpal horn levels and in the ligamentous attachment. It is concluded that both immature and mature parts of the lower-jaw dentition of the cichlid T. mariae are innervated and that the microscopic anatomy of this innervation is partly similar to the pattern seen in developing and adult mammals.

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.

Dynamic plasticity of dental sensory nerve structure and cytochemistry

Hypersensitive dentine responds to normal changes in touch or temperature with abnormal pain sensations. This paper reviews studies that have shown dynamic changes in sensory nerve structure, cytochemistry and location after tooth injury, suggesting that those changes contribute to dentine hypersensitivity. Nerve fibres containing calcitonin gene-related peptide (CGRP) are the main type of sensory fibre to innervate dentine. Evidence that many of those dentinal nerve endings originate from small myelinated fibres is presented here. The location of CGRP nerve terminals correlates with the pulpal gradients of nerve growth factor that have been demonstrated in normal teeth by in situ hybridization histochemistry. When shallow cavities are drilled into the outer dentine of rat molars a five-to-eight-fold increase in pulpal nerve growth factor precedes the extensive structural changes in the sensory nerve reactions eventually subside if healing occurs, but both continue if inflammation continues. Evidence correlating pulpal inflammation with long-term changes in central trigeminal pain pathways is reviewed. There can be extensive neuroplasticity after tooth injury, both within dental pain fibres and in central pain pathways. The timing of those alterations of nerve structure, location, and cytochemistry is consistent with their involvement in mechanisms of dentine hypersensitivity.

An immunocytochemical study of the innervation of developing human fetal teeth using protein gene product 9.5 (PGP 9.5)

Developing teeth of 32 human fetuses (crown-rump length 11-205 mm) were examined immunohistochemically by antisera to protein gene product 9.5 (PGP 9.5) in an attempt to shed light upon the possible role of innervation in odontogenesis. As a control for the specificity of PGP 9.5 as a neuronal marker, the results were verified by immunocytochemical co-localization in peripheral nerves of neurone-specific enolase, neurofilaments and S-100 protein. The dental follicle received the first nerve fibres in the early cap stage. At this stage, fibroblasts differentiated in the presence of nerve fibres and formed the dental follicle surrounding the developing tooth. In the dental papilla, however, no fibres were demonstrated until the dentine and enamel matrices had formed, about half of the present height of the tooth germ. Most nerve fibres were localized in the basal part of the papilla until the last stage examined and usually followed the blood vessels of the papilla. Thus the effect of innervation on tooth development may be associated with the development of the dental follicle. A novel finding was that functional odontoblasts were not only positive for S-100 but also for PGP 9.5, indicating their neural crest origin.

Analysis of low affinity nerve growth factor receptor during pulpal healing and regeneration of myelinated and unmyelinated axons in replanted teeth

Nerve regeneration was examined in rat molars that were briefly extracted and then replanted in the socket for 1-90 days. Immunocytochemistry was used to evaluate neural and nonneural immunoreactivity (IR) for low affinity nerve growth factor receptor (p75-NGFR) and for laminin and calcitonin gene-related peptide (CGRP). Three different types of pulpal response to replantation were found. Type I: Some replanted teeth had mild injury and still contained coronal odontoblasts and associated fibroblasts that retained p75-NGFR-IR; they continued regular dentin formation and had excellent reinnervation. Type II: Teeth with intermediate injury lost most or all of the coronal pulp tissue, but they regenerated odontoblast-like cells that formed irregular dentin, they had numerous dispersed p75-NGFR-IR fibroblasts in crown pulp during early regeneration, and they had excellent reinnervation. Type III: Severely injured teeth lost their original pulp; they filled with dense connective tissue and bone and had poor reinnervation. After Type I or II injury the Schwann cells around degenerating myelinated and unmyelinated axons had increased expression of p75-NGFR by 1-3 days. By 7-10 days those Schwann cells had formed hollow tubes (bands of Bungner) along the degenerating axon tracks. They maintained their increased p75-NGFR-IR during and after regeneration of unmyelinated axons, whereas Schwann cells involved in remyelination lost p75-NGFR-IR at that stage. The number of CGRP-IR axons in the regenerating pulp increased from 7 to 90 days. Laminin-IR increased in all replanted teeth at 3-10 days and only returned to normal patterns in teeth with Type I or Type II response at 20-90 days. The special p75-NGFR-IR of pulpal fibroblasts of adult rat molars did not usually persist in regenerated, reinnervated pulp. The extensive depletion of fibroblast p75-NGFR-IR and the continuing enhanced p75-NGFR-IR in unmyelinated nerve fibers at 90 days show that altered growth factor conditions characterize regenerated pulp of replanted teeth.

Effect of ageing on responses of nerve fibres to pulpal inflammation in rat molars analysed by quantitative immunocytochemistry

The response of sensory nerve fibres to inflammation in young adult rat molars has recently been shown to include increases in nerve sprouting and neuropeptide content. The objective was to evaluate neural responses to class V dental preparations in molars of old (1-2 yr) as compared with young adult rats (3-4 months). Tissues were investigated immunocytochemically 4 days post-injury for the sensory neuropeptides calcitonin gene-related peptide (CGRP) and substance P. Quantitative image analysis of the material demonstrated that more immunoreactivity was present for CGRP than for substance P in intact control teeth for each age group. Four days after injury, both immunoreactivities were increased in pulp adjacent to the injury in both young and old teeth. The increase depended on at least three factors: (1) enhanced immunoreactivity of the nerve fibres; (2) increased terminal nerve sprouts near the injury and (3) elevated peptide content of the pulp tissue. Although the incidence of CGRP- and substance P-immunoreactive nerve fibres had decreased in older teeth, the proportional increases in both neuropeptides near the injury were greater in old than in young teeth, owing to a reduction in pulpal volume during ageing. Pulpal tissue was also immunostained for the low-affinity nerve growth factor receptor (p75-NGFR) as an index of pulpal ageing; and an extensive decrease was found in the old adult as compared to young adult rats. These results indicate that old rats maintain the capacity for nerve sprouting despite the decreases in p75-NGFR labelling of pulp cells, pulp volume and nerve fibre numbers that occur as part of dental ageing.

Anatomy and developmental chronology of the rat inferior alveolar nerve

This report describes the anatomy of the inferior alveolar neurovascular bundle in the adult rat and provides a quantitative analysis of the developing inferior alveolar nerve (IAN). Soon after its entrance in the mandibular canal, the IAN splits into a mental nerve (MN) and an inferior dental nerve (IDN), which course in separate bony compartments. The MN passes unbranched through the mandibular canal. The IDN sends branches to the incisor, the first molar, and the second molar. The third molar (M3) is supplied by a separate IAN branch. The adult rat IAN contains 8,000-10,000 axons, 70% of which are myelinated. The MN accounts for 70% of all IAN axons, the IDN 26%, and 4% form the M3 branch. The proportion of large myelinated axons is lower in the MN than in the IDN. Following chemical sympathectomy, the IAN axon number does not change in a statistically significant way. The total number of IAN axons, which is high prenatally and neonatally, has decreased to the adult level about 1 week after birth. De novo myelination commences at birth and is complete 3-4 weeks later. The size spectrum of the myelinated fibres is narrow and unimodal during the first postnatal weeks. By 1 month, the largest fibres reach diameters of approximately 6 microns, and a bimodal pattern is emerging. From 3 months and on, the size range reaches up to 10-12 microns, and the distribution is bimodal. These data provide a basis for further studies on developmental tooth-nerve interactions.

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.

Central representation of dental structures in the kitten, including projections to the mesencephalic trigeminal nucleus

Horseradish peroxidase (HRP) was injected into either a single maxillary or a single mandibular primary (deciduous) cuspid tooth of 8- to 10-week-old kittens. The large apex of the primary cuspid allowed for some leakage of the HRP from the pulpal chamber to the periodontal ligament (PDL). Thus, the injection procedure resulted in the application of HRP to the PDL as well as to the pulpal tissues. The transganglionic transport of HRP resulted in discrete terminal fields within the spinal trigeminal nucleus (STN) and the main sensory nucleus (MSN). These projections were clearly somatotopically organized within the STN, but less so within MSN. Within pars oralis (PO) and pars interpolaris (PI), mandibular cuspid dental structures (MdCDS) were represented in a dorsal position relative to the maxillary cuspid dental structures (MxCDS), whereas within pars caudalis (PC) and the adjacent reticular formation the somatotopic representation was not dorsoventral, but rather mediolateral, with the MdCDS represented more medially than the MxCDS. Areas of overlap between MxCDS and MdCDS were found within MSN and to a lesser degree within the superficial laminae of PC. In addition, the fiber pathway leading to labeled somata in the mesencephalic trigeminal (Mes V) nucleus was clearly identified. The majority of the fibers traced to the Mes V nucleus exited the spinal trigeminal tract at the level of the transition from PO to the MSN and traversed the nuclear region in a position dorsal to and separate from the trigeminal motor tract. As in STN, fibers within the caudal Mes V tract appeared to be somatotopically organized, with the fibers from the MdCDS generally more dorsal than the ones from the MxCDS. Labeled fibers, some with terminal arbors, were also identified in close association with the trigeminal motor tract. The findings show a complex pattern of central representation in the immature feline central nervous system for deciduous dental structures.

Innervation of the tooth pulp by the mesencephalic trigeminal nucleus in the cat: a retrograde horseradish peroxidase study

HRP was applied to the tooth pulp of 8 cats. Six were subjected to postoperative administration of the anti-inflammatory drug, prednisolone, whereas the remaining two were not. In all prednisolone-treated cats, labeled neurons were found in both the mesencephalic trigeminal nucleus and trigeminal ganglion, ipsilaterally. On the other hand, no labeled neurons were observed in the mesencephalic nucleus in cats receiving no steroid.

A comparison of pulpal and tactile detection threshold levels in young adults

Pulpal and tactile sensory detection threshold (SDT) values of the maxillary and mandibular incisor and canine teeth were determined and recorded for young adult subjects at three test sessions. A commercially available monopolar pulp-testing device was used to determine pulpal SDT values, and von Frey hairs were used to determine the tactile SDT values. Statistical analysis of the data indicated that the pulpal and tactile test procedures were sufficiently reliable in identifying what is defined as the true SDT value for both parameters. The study confirmed the constancy of these SDT values over days and the independence of the values for jaw, side, and sex. SDT values were influenced, however, by tooth type, with canine teeth displaying higher tactile and pulpal values than the central and lateral incisor teeth. These data should provide a suitable baseline for a longitudinal study to identify the SDT fluctuations known to occur in tooth pulp and dental supporting tissues in a growing human population.

Combined retrograde tracing and enzyme/immunohistochemistry of trigeminal ganglion cell bodies innervating tooth pulps in the rat

Rat trigeminal neurons innervating tooth pulps were retrogradely labelled with fluorogold and analysed enzyme- and immunohistochemically for their content of substance P, calcitonin gene-related peptide, fluoride-resistant acid phosphatase, GM 1 ganglioside, carbonic anhydrase and neurofilament protein. The data showed that both small, medium-sized and large trigeminal neurons were labelled after fluorogold deposition in maxillary molar pulps, with a majority of the cells being medium-sized and large. Less than 2% of the pulpal neurons showed substance P-like immunoreactivity. Fifty-six per cent of the pulpal nerve cells were calcitonin gene-related peptide-positive. These cells were small, medium-sized and large. Only 1% of the fluorogold-labelled cells contained fluoride-resistant acid phosphatase enzyme activity. This paralleled the finding that the pulpal neurons were unstained by Griffonia simplicifolia isolectin I-B4, a plant lectin which preferentially binds to fluoride-resistant acid phosphatase-positive cells. Choleragenoid-like immunoreactivity, which identifies cells with the GM 1 ganglioside receptor, was found in 70% of the fluorogold-labelled pulpal neurons. Approximately 80% of the fluorogold-labelled cells showed RT 97-immunoreactivity. RT 97 labels neurofilament protein and is present in large light primary sensory neurons. No pulpal neurons appeared to contain carbonic anhydrase, as judged from both enzyme- and immunocytochemical observations. The findings suggest that, in the rat, trigeminal tooth pulp neurons, which according to the classical view are nociceptive, form a heterogeneous group of neurons with a minority of small cells which may contain calcitonin gene-related peptide but rarely either substance P or fluoride-resistant acid phosphatase. However, the vast majority of pulpal nerve cells appear to have sizes and cytochemical characteristics which are not generally associated with nociceptive primary sensory neurons.

Proportion of unmyelinated axons in rat molar and incisor tooth pulps following neonatal capsaicin treatment and/or sympathectomy

The occurrence of unmyelinated axons was examined ultrastructurally in rat molar and incisor root pulps of normal rats, of neonatally capsaicin-treated rats, of rats subjected to neonatal capsaicin treatment followed by resection of the superior cervical ganglion and of sympathectomized but otherwise normal rats. Following capsaicin treatment the occurrence of unmyelinated pulpal axons was slightly subnormal. Sympathectomy was largely without effect on the population of unmyelinated axons in tooth pulps of capsaicin-treated rats. In normal rats the proportion of unmyelinated axons in molar pulps was not altered by sympathectomy but it caused a slight decrease in the number of unmyelinated incisor pulpal axons. These findings support the view that most of the unmyelinated axons in rat molar and incisor pulps are sensory, that the parent neurons of these axons differ from nociceptive neurons at other sites by being largely resistant to neonatal capsaicin treatment and that very few unmyelinated tooth pulp axons represent postganglionic efferents.

Trigeminal alveolar nerve of the lower jaw in the cichlid Tilapia mariae: evidence for continual axon generation and presence of exceptionally small myelinated axons

Cross sections from the trigeminal alveolar nerve of the lower jaw in the cichlid Tilapia mariae were examined by electron microscopy. The nerve fibers are arranged in groups with a core of unmyelinated and small myelinated axons, surrounded by myelinated axons of varying sizes. The core contains large bundles of unmyelinated axons collectively ensheathed by circumferentially located Schwann cells, as well as smaller bundles of unmyelinated axons partly separated from each other by Schwann cell processes. Among the unmyelinated axons, occasional scattered profiles resembling growth cones are seen. The total number of axons in this tooth-related nerve increases from approximately 1,500 to 5,000, as the animals grow in length from 4.5 to 21.5 cm. Some 24-49% of the axons are unmyelinated. The myelinated axons have maximum diameters of 1.0-3.0 micron, depending on body size. Most myelinated axons have diameters less than 1.0 micron and the smallest ones reach down to 0.3 micron. These results show that there is a continual addition of axons to the alveolar nerve of the lower jaw in Tilapia mariae and that the critical diameter for myelination in this peripheral nerve is similar to that typically found in the mammalian CNS.

Periapical innervation of the ferret canine and the local retrograde neural changes after pulpectomy

The amputation of the dental pulp severs a population of axons that are predominantly in the A delta and C fiber size range and are principally involved in nociception. Local periapical neuromas, if they are formed after pulpectomy, may be the sites of spontaneous nervous activity that may, in some circumstances, be involved in the genesis of chronic pain. The periapical tissues from the mandibular canines of four ferrets were examined 3 months after pulpectomies. Silver-stained paraffin sections were examined in three dimensions at the light microscope level. Ultrathin sections were examined at the electron microscope level. Compared with contralateral and independent controls, the principal changes were the loss of the periodontal plexus around the root apex, the extension of damage well below the apical foramen, and the persistence of inflammation 12 weeks postoperatively. While a somewhat disorderly mass of nerve fibers develops subapically, the arrangement has only some of the features usually associated with neuromas.

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.

Metamorphic changes within the lateral-line system of Anura

The metamorphic loss of lateral-line organs, lateral-line nerves and second order lateral-line neurons was examined in two Anuran species. At the onset of metamorphic climax, terminals within the lateral-line neuropil showed accumulation of glycogen-like granules. Neither the lateral-line nerve nor the organs or the nerve terminals inside the organs displayed any sign of degeneration at this stage. A few second order neurons exhibited accumulations of chromatin into conspicuous masses. These cells were partially or completely engulfed by phagocytes. At mid-metamorphosis all lateral-line organs were lost. The proximal parts of the lateral-line nerve fibers entering the rhombencephalic alar plate showed signs of degeneration. Within the lateral-line neuropil, pre- and some postsynaptic elements exhibited the flocculent type of degeneration or, to a lesser extent, the dark type of degeneration. Second order lateral-line neurons underwent an electron-dense or electron-lucent type of degeneration and were taken up by phagocytes. At the end of metamorphic climax the distal parts of the lateral-line nerves showed numerous dark degenerating fibers inside an intact myelin sheath. Within the lateral-line neuropil, numerous dark degenerating presynaptic elements were found next to some elements showing flocculent degeneration. Fewer degenerating second order neurons were found in the alar plate. They showed predominantly the dark type of degeneration. In contrast to earlier reports, our data suggest that the degenerative metamorphic changes observed in the present study are initiated in all parts of the lateral-line system simultaneously, and lead to the complete loss of all lateral-line organs and nerves and presumably all second order lateral-line neurons as well.

Sympathetic neuronotrophic activity in the pulp of the cat canine tooth

Extracts of these dental pulps from adult cats contained a non-dialysable agent or agents which could support neurone survival and neurite development for at least three days in neurone-enriched cultures of sympathetic ganglion cells from 11-day chick embryos. The neurone survival-promoting activity differed from nerve growth factor (NGF) in that: (1) anti-mouse NGF serum did not inhibit it; (2) nearly all ganglionic neurones survived in optimum concentrations of pulp extract, whereas only about 35 per cent were supported by NGF; and (3) cell bodies of NGF-supported neurones were markedly larger than in neurones supported by pulp extracts. The neuronotrophic activity in individual dental pulps was highly variable among different cats, but similar between mandibular canines from the same animal. Smaller pulps had higher concentrations of trophic activity than larger ones. Gingival tissue and the anterior belly of the disgastric muscle contained little neuronotrophic activity.

Innervation of the dental pulp during tooth succession in the cat

The possibility that axons branch to supply the pulps of both the upper deciduous canine tooth and its permanent successor has been investigated by stimulating the pulp of one tooth and recording from the pulp of the other. In cats less than about 14 weeks of age, the permanent canine was too poorly developed to allow electrodes to be applied to it satisfactorily. In 5 of 14 preparations in cats aged 14-23 weeks, compound action potentials were recorded in one canine during stimulation of the other. These responses were not abolished by sectioning the infraorbital nerve or its canine branch in the floor of the orbit or by paralysing the animal, but they were abolished by sectioning the pulp of the permanent canine, indicating that they were due to branched axons. In preparations in which there was no tooth-to-tooth response, there was usually evidence that the pulp of one or other of the teeth did not have a functional innervation. The results indicate that at least some of the nerves which supply the pulp of a deciduous tooth are retained to supply its permanent successor.

On nerves and teeth in the lower jaw of the cichlid Tilapia mariae

The anatomy of the teeth and tooth-related nerves in the lower jaw was examined in the cichlid Tilapia mariae. This was done in order to establish a basis for studies on dental neuroplasticity in a polyphyodont vertebrate. The region of interest was explored in specimens fixed by glutaraldehyde perfusion, and by using X-ray photography, maceration, scanning electron microscopy, gross dissection, and light microscopic examination of serial sections. The results show that the lower jaw carries some 60-65 functional teeth. In addition, numerous replacement teeth and tooth germs in various stages of development are located in a cavity in the dentary bone. Numerous nerve bundles are present in immediate relation to the dental follicles of tooth germs. Unerupted teeth do not contain light-microscopically discernible pulpal axons, but the pulps of functional teeth contain myelinated axons. Both perifollicular and pulpal nerve bundles derive from a nerve plexus, which is formed by branches from r. mandibularis trigemini. This nerve is easily accessible to experimental manipulation, where it courses through the adductor mandibulae muscular complex. Thus, the lower jaw of T. mariae seems to represent a suitable system for the study of tooth-nerve interactions in a polyphyodont species.

Innervation of teeth: qualitative, quantitative, and developmental assessment

Anatomical characteristics of tooth innervation provide insights into functional capabilities as well as limitations of this organ. In this review, innervation will be discussed from two major viewpoints. The first part will present distribution of nerve fibers in the tooth; nerve pathways, both autonomic and sensory, will be discussed mostly from a descriptive standpoint. In the second part, quantitation of neural units along key points of the pathways will be presented at milestones in tooth and organism development and aging.

Structural development of the feline mental nerve

The qualitative and quantitative structural development of the feline mental nerve (MN), a branch of the inferior alveolar nerve (IAN), was studied by electron microscopy from 40 days postconception (dpc) (about 2 weeks before birth) to 11 years after birth. Myelination was initiated at 40-45 dpc. At 2 months after birth de novo myelination was completed, and the larger myelinated axons had achieved a fully differentiated nodal-paranodal morphology. Size growth of myelinated axons continued until 6 months, when a bimodal size distribution between 1 and 12 months was established. When compared to the IAN, the MN contained a higher proportion of unmyelinated axons. Age-related signs of axon degeneration, which previously were recorded in the IAN, were lacking in the MN. This suggests that senescent IAN axon degeneration is related to dental rather than to cutaneous MN branches.

Short internodal lengths of canine tooth pulp axons in the young adult cat

Light-microscopical measurements have been made on teased pulpal nerve fibers from young adult cat canine teeth. In the root canal portion (apical to first branching point) the mean internodal length was approximately 250 micron. In the pulpal chamber portion (coronal to the first branching point) the pulpal nerve fibers had a mean internodal length of approximately 125 micron. A local decrease in internodal length was evident at bifurcations and in preterminal regions. These internodal lengths are shorter than in similarly sized stem axons. The possible functional significance of this difference is discussed.

Changes with age in canine tooth pulp-nerve fibres of the cat

Light microscopical observations have been made on teased pulpal nerve fibres from mandibular canine teeth of cats aged 3 years or more. Both internodal lengths and external fibre diameters appeared to be reduced compared to the young adult. Qualitative myelin sheath changes were commonly observed. These consisted of extremely short, smooth or distorted intercalated internodes, myelin wrinkling, nodal lengthening and formation of myelin ovoids. Features suggesting de-myelination were also present. These findings should be considered when functional aspects of pulpal axons in the ageing tooth are discussed.

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)

Proportion of unmyelinated axons in the rat inferior alveolar nerve and mandibular molar pulps after neonatal administration of capsaicin

Neonatal rats were given capsaicin subcutaneously. Controls received vehicle only. Six months later the nerve fiber populations in the dorsal root L4, in the inferior alveolar nerve and in mandibular molar pulps were examined by electron microscopy. The proportion of unmyelinated axons was found to be markedly decreased in root and nerve specimens from capsaicin-treated rats. However, the pulpal nerves in tooth specimens from these rats showed no obvious alterations.

The number and size of axons at the apex of the cat's canine tooth

Using electron microscopy and morphometric analysis the number and size of axons entering the apex of the cat's mandibular canine tooth have been measured. The total number of axons varied from 761 to 1,903 between different animals but the maximum difference between right and left sides of the same animal was 353. From 56 to 79.6% of the axons were nonmyelinated; the difference in proportion between right and left sides never exceeded 6.4%. The mean circumference of myelinated axons ranged from 10.2 to 18.3 micrometers but again the right and left variation was much less and never exceeded 2 micrometers. In one tooth 38.8% of the myelinated axons were larger than 19 micrometers in circumference and thus outside the A delta range. The proportion was much smaller in other teeth but some "large" fibers were always present. Of all the nonmyelinated axons 19.7% showed some degree of axonal exposure to the extracellular space and 1.7% showed ax-oaxonal apposition. A small proportion of nonmyelinated axons showed evidence of apparent degeneration. Comparison of these data with those from studies at more coronal levels suggests that there is considerable branching and narrowing of fibers during their course through the dental pulp and that the degree of axonal exposure and apposition increases considerably. Some of the pulpal fibers are derived from larger axons than are normally associated with pain. The animal to animal variation in the parameters measured is considerable but right and left sides are similar.

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

Regeneration of the inferior alveolar nerve and mandibular incisor pulpal reinnervation was qualitatively and quantitatively examined by electron microscopy 2 days--11 months after intramandibular neurotomy in young adult cats. Fifteen millimeters central to the proximal stump moderate atrophic alterations of myelinated axons were observed 1--2 months after surgery. By 4--11 months a principally normal picture had been restored. The proportion of unmyelinated axons was increased 2--4 months after operation but had normalized by 11 months. In the distal stump the first regenerating axons were observed at 2 weeks. The regenerated myelinated axons failed to re-establish the previous fibre size range and normal axo-glial relations did not appear. A seemingly stable morphological pattern was reached 4--11 months postoperatively. In the late survival period the proportion of unmyelinated axons was subnormal. In the incisor pulps virtually all axons disappeared after surgery. By two weeks pulpal reinnervation had begun. From two months on, a structurally largely normal pulpal axon population was present except for some persisting unmyelinated axon degeneration. The findings are consistent with previous physiological data and suggest that structural normalization at proximal and preterminal levels follows upon re-establishment of peripheral contacts.

Myelin sheath thickness and internodal length of nerve fibres in the developing feline inferior alveolar nerve

The relation between the number of myelin lamellae (nl) and axon size (d) was examined in the developing and adult feline inferior alveolar nerve (IAN). The internodal lengths (L) and total diameters (D) were measured on teased IAN specimens from kittens and cats. The results show that relations nl/d and L/D principally similar to those in young adult cats had been established 2 months after birth. This coincides with the maturation of the primary dentition. During the first 3 postnatal weeks signs of a developmental demyelination were common. Comparisons between the internodal elongation of early myelinating axons and the longitudinal growth of the IAN in the mandibular canal indicated that some 50% of all prospective large internodes must be removed. Between 2 months and the young adult stage, when the permanent dentition is established, the relations nl/d and L/D were essentially unaltered, but the ranges extended towards larger sizes. In the young adult the average g-value was 0.67. In the old adult cat the relation nl/d was less uniform than in young adults, and the average g-value had decreased to 0.59. In addition, successive short (100-150 micrometer) internodes were found, indicating a senescent de- and remyelination in the IAN. These alterations may be related to the age-dependent deterioration of the mandibular dentition.

Axon number and size distribution in the developing feline inferior alveolar nerve

The number and size distribution of axons in the developing feline inferior alveolar nerve (IAN) were examined by electron microscopy. Seven cat fetuses and thirty kittens and cats aged from 25 days post conception (dpc) to 11 years were used. The total number of IAN axons increased from 4,400 to 16-17,000 between 25 and 40 dpc, and then decreased to about 13,000 before birth. This level was maintained up to at least 11 years. Myelinated axons first appeared by 45 dpc and constituted 28% at birth. The young adult proportion of about 45% myelinated axons was established at 2 months. In the old adult (11 years) 55% of the IAN axons were myelinated. Size measurements showed that unmyelinated axons had diameters of 0.1-0.5 micrometer at 25 dpc. From 55 dpc and on the size range extended from 0.1 micrometer to 1 micrometer. The size range of myelinated axons was 1-4.5 micrometers at birth and 1-8 micrometers 2 months postnatally. A bimodal size distribution first appeared by 2 months, and the range was 1-13 micrometers from 6 months and on. The findings were compared with age-related changes in the primary and permanent dentitions.