Trigeminal nerve endings in gingiva, junctional epithelium and periodontal ligament of rat molars as demonstrated by autoradiography

A Case of Respiratory Epithelium-Lined Cyst with Enriched Nociceptor Innervation

Apical lesions of endodontic origin can be classified as either granulomas or cysts. In rare cases, respiratory epithelium can proliferate and encapsulate a lesion, forming a cyst. Moreover, the innervation of apical lesions has only been previously reported in animal models of apical periodontitis. This report demonstrates an unusual case in which tooth #15 was initially treated with nonsurgical root canal therapy. Still, the patient remained in moderate to severe pain for several days following the procedure. Next, an intentional replantation was performed in which a periapical cyst was curetted from the alveolus. The patient experienced immediate pain relief following the procedure. Histological analysis revealed that the periapical cyst was lined entirely with respiratory epithelium, and immunohistochemical analysis showed it to be densely innervated. In addition, these nerve fibers expressed the LPS receptor, TLR4. This is the first demonstration of the innervation pattern of a periapical cyst. Further studies are warranted to evaluate innervation in apical lesions and its correlation with pre- and intra-operative symptoms and their participation in the pathogenesis of apical periodontitis.

The junctional epithelium: from health to disease

The junctional epithelium is located at a strategically important interface between the gingival sulcus, populated with bacteria, and the periodontal soft and mineralized connective tissues that need protection from becoming exposed to bacteria and their products. Its unique structural and functional adaptation enables the junctional epithelium to control the constant microbiological challenge. The antimicrobial defense mechanisms of the junctional epithelium, however, do not preclude the development of gingival and periodontal lesions. The conversion of the junctional to pocket epithelium, which is regarded as a hallmark in disease initiation, has been the focus of intense research in recent years. Research has shown that the junctional epithelial cells may play a much more active role in the innate defense mechanisms than previously assumed. They synthesize a variety of molecules directly involved in the combat against bacteria and their products. In addition, they express molecules that mediate the migration of polymorphonuclear leukocytes toward the bottom of the gingival sulcus. Periodontopathogens-such as Actinobacillus actinomycetemcomitans or, in particular, Porphyromonas gingivalis-have developed sophisticated methods to perturb the structural and functional integrity of the junctional epithelium. Research has focused on the direct effects of gingipains, cysteine proteinases produced by Porphyromonas gingivalis, on junctional epithelial cells. These virulence factors may specifically degrade components of the cell-to-cell contacts. This review will focus on the unique structural organization of the junctional epithelium, on the nature and functions of the various molecules expressed by its cells, and on how gingipains may attenuate the junctional epithelium's structural and functional integrity.

Regeneration of nerve fibres in the peri-implant epithelium incident to implantation in the rat maxilla as demonstrated by immunocytochemistry for protein gene product 9.5 (PGP9.5) and calcitonin gene-related peptide (CGRP)

The response of nerve fibres in the peri-implant epithelium to titanium implantation was investigated with an experimental model using rat maxilla and immunohistochemical techniques. The latter employed antibodies to protein gene product 9.5 (PGP9.5), and to calcitonin gene-related peptide (CGRP). In control rats without an implantation, a dense innervation of PGP9.5- and CGRP-positive nerve fibres was recognized throughout the junctional epithelium, as has been previously reported. A titanium-implantation induced a remarkable inflammatory reaction, as well as the destruction of covering epithelial cells. By 3-5 days post-implantation, inflammatory reaction showed a tendency to disappear, and the peri-implant epithelium showed proliferation and down-growth along the implant. At this stage, no nerve fibres were found around the peri-implant epithelium. At 10 days, a few nerve fibres reached the basal cell layers of the peri-implant epithelium, and entered it 15 days after implantation when the peri-implant epithelial cells showed morphological features roughly resembling those of normal junctional epithelial cells. At the complete osseointegration stage (days 20-30), the PGP9.5- and CGRP-positive nerve fibres, thin and beaded in appearance, were found distributed in the peri-implant epithelium. After 20 days, the numerical density of the intraepithelial nerves in the peri-implant epithelium appeared the same as, or less than, that in the normal junctional epithelium. These findings indicate that the peri-implant epithelium shows the same innervation as that in normal junctional epithelium, and that the intraepithelial nerve fibres in the peri-implant epithelium might have diverse functions, which have been suggested in the literature.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Effect of inferior alveolar nerve axotomy on periodontal and pulpal blood flow subsequent to experimental tooth movement in rats

The aims of this study were to evaluate the effect of inferior alveolar nerve (IAN) axotomy on periodontal (PDL) and pulpal blood flow incident to experimental tooth movement and to investigate whether nerve fiber regeneration coincides with blood flow changes. The first right mandibular molar was moved mesially for 3, 7, and 14 days after ipsilateral IAN axotomy in 29 rats. Four rats served as unoperated controls. At the end of each experimental period fluorescent microspheres (FM) were injected into the left ventricle and thereafter counted in serial sections in the PDL and pulp of the right and left first mandibular molars. The number of FM per tissue volume was taken as a measure of blood flow. Re-innervation of nerve fibers was mapped immunohistochemically 7, 14, and 21 days after IAN axotomy in 9 rats that had no orthodontic appliance. The statistical analysis showed no significant differences in the number of FM/mm3 PDL between the denervated and the contralateral side at 3 and 7 days. At 14 days the PDL on the denervated side showed a significant increase in the number of FM/mm3, coinciding with the initial periodontal nerve fiber re-innervation. In the pulp no significant differences were found between the denervated and the contralateral, innervated side in any experimental period. It can be concluded that IAN axotomy postpones an increase in periodontal blood flow until a sensory tissue re-innervation is established, thus indicating that neurogenic mechanisms play an important role in the development of the inflammatory reaction induced by experimental tooth movement.

Peptidergic innervation of the mesencephalic trigeminal nucleus in the cat

BACKGROUND

The trigeminal processing of proprioceptive information is unique and very little is known about the neurochemical organization of trigeminal primary afferent neurons which mediate the sensory aspects of proprioception. In studies using immunocytochemicalretrograde tracing techniques, some classical neurotramsitters mediating the afferent modulation of the mesencephalic trigeminal nucleus (MTN) have been investigated. This paper summarizes our current understanding of the peptidergic innervation of the cat MTN.

METHODS

The distribution of immunoreactive substances was studied using specific antisera against 11 major neuropeptides. Light and electron microscopic peroxidase-antiperoxidase immunocytochemical staining techniques in colchicine-treated animals were used to clarify the distribution of peptide-identified fibers related to the MTN.

RESULTS

Immunoreactivity to any of the tested neuropeptides could not be detected in the MTN cell bodies. Numerous fibers containing various peptides such as substance P, bombesin, enkephalins, cholecystokinin, vasoactive intestinal polypeptide, vasopressin, and neuropeptide Y were present in the nucleus, however. These thin positive fibers covered the neuronal surface of the MTN cell bodies and some of the immunoreactive varicosities appeared to be in close proximity to profiles of MTN neurons. Electron microscopic observations revealed that perisomatic fibers were in direct apposition to perikarya of unstained large cells and some of them made synaptic contacts with their cell bodies and dendrites.

CONCLUSIONS

The present results demonstrate that the MTN neurons receive dense basket-like innervation from peptidergic neurons on somata and processes and have supported earlier evidence that the MTN of the cat is under influence of peptidergic input. Results of this study provide further evidence that the neuropeptides examined may play an important role in the integration and transmission of trigeminal proprioceptive information. Most likely they may co-exist with a classical but hitherto unknown neurotransmitter(s), that is unique for this region and whose release can be modulated by peptides.

Evidence for slowly conducting afferent fibres innervating both tooth pulp and periodontal ligament in the cat

The presence of afferent nerve fibres branching to innervate both the dental pulp and periodontal ligament was studied in pentobarbitone-anaesthetised cats. Extracellular single nerve-fibre recordings were made from fine filaments split from the proximally cut end of the inferior alveolar nerve. Nerve fibres were identified by bipolar constant-current stimulus pulses applied to the periodontal space via platinum wire electrodes. In each case activation of the nerve fibres was also attempted by monopolar electrical stimulation of the dental pulp via a platinum wire electrode inserted into the dentine. Eleven of 142 C fibres and 4 of 97 A delta fibres identified by electrical stimulation of the periodontal ligament also could be activated by electrical stimulation of the dental pulp. Fourteen of the 142 C fibres identified by electrical stimulation of the periodontal ligament exhibited discrete latency jumps at different suprathreshold stimulus strengths. Eight of them also could be activated by electrical stimulation of the dental pulp. Eight of the 15 fine branching afferent fibres were tested with non-electrical stimuli of both periodontal ligament and dental pulp by application of heat, cold and potassium chloride. Three of the 4 C fibres could be activated with at least one of these stimuli applied to both tissues. In one case receptive fields were located in both periodontal ligament and dental pulp. The remaining 5 slowly conducting fibres were activated only from one type of tissue. The results suggest that a small percentage (6%) of the slowly conducting nerve fibres in the inferior alveolar nerve innervate both the periodontal ligament and the dental pulp. According to their response behaviour they might be involved in nociception.

Immunocytochemical study of nerve fibers containing substance P in the junctional epithelium of rats

Nerve fibers with substance P-like immunoreactivity (SP-IR) in the junctional epithelium (JE) of 32-42-d-old rats were examined by both light and electron microscopy using the avidin-biotin-peroxidase complex method. The density of nerve fibers with SP-IR was highest in the middle portion of the JE; however, a few fibers were localized in the coronal portion of the JE and close to the enamel surface. Also, rich innervation was found especially in the basal cell layer of the JE. Unmyelinated axons with SP-IR in the connective tissue underlying the JE were enveloped by Schwann cells but lost their Schwann cell sheath almost completely in the JE. The axons often formed varicosities with SP-IR as terminals in various areas of the JE. The terminals contained numerous large granular vesicles, small clear vesicles and a few mitochondria, and were surrounded by the cytoplasmic processes of the junctional epithelial cells. These terminals were sometimes located close to neutrophils in the JE; the minimum gap distance between the terminals and the processes of junctional epithelial cells or neutrophils was about 20 nm. A few terminals with SP-IR came close to the enamel surface, and the minimal distance between the terminals and the enamel surface was about 5 microns. SP-IR in the nerve terminals in the JE fixed with 0.1% or 0.25% glutaraldehyde was distributed diffusely in the axoplasm or was confined to the granular vesicles. These findings show that substance P is contained in the large granular vesicles in the nerve terminals, and suggest that these terminals may function as modulators of junctional epithelial cells and neutrophils.

An immunohistochemical and monastral blue-vascular labelling study on the involvement of capsaicin-sensitive sensory innervation of the junctional epithelium in neurogenic plasma extravasation in the rat gingiva

Nerve fibres immunoreactive for substance P (SP) and calcitonin gene-related peptide (CGRP) were located preferentially at the base of the junctional epithelium. Occasional fibres were observed in close proximity to the subepithelial, small blood vessels. The vascular connective tissue papillae projecting into the epithelium were more densely surrounded by SP- and CGRP-immunoreactive fibres in the interdental col than in other regions of the gingiva. In some cases, hyperplasia of the junctional epithelium was noted in the interdental col where the connective tissue papillae were invaded by widened vessels, indicating severe irritation. SP- and CGRP-immunoreactive fibres around these papillae showed increases in their immunoreactivity and thickness, with some fibres terminating as large expansions. Double immunohistochemical staining revealed the co-existence of SP and CGRP in all nerve fibres within and under the junctional epithelium. Capsaicin pretreatment eliminated most of the immunoreactivity for both peptides. Intravenous infusion of capsaicin or SP caused increased permeability in vessels underlying the junctional epithelium, as indicated by Monastral blue labelling. Labelled vessels were arranged not only in a network extending under the epithelium but also in loops protruding into the connective tissue papillae. These labelled vessels were most abundant in the interdental col, where vascular loops with more complex configurations exhibited strong staining in their walls. In the case of hyperplasia of the junctional epithelium in the interdental col, widened vessels showing extensive labelling in their walls were observed. In capsaicin-pretreated animals, capsaicin-induced extravasation was abolished, while the effect of SP was still observed. These findings provide evidence that capsaicin-sensitive sensory nerves supplying the junctional epithelium are involved in neurogenic plasma extravasation in the rat gingiva. The enhancement of neurogenic plasma extravasation in the col may be vascular response associated with a higher susceptibility of this region to gingival inflammation.

Axonal transport of fluorescent carbocyanine dyes allows mapping of peripheral nerve territories in gingiva

Sensory innervation of gingival tissue can cause neurogenic inflammation that depends on the extent of the branching area of the peripheral nerve fibers. We designed the present study to determine whether single trigeminal axons branch to both the buccal and palatal gingiva of maxillary molars of adult rats. Accumulation via retrograde transport of DiI (red) or DiA (green) fluorescent carbocyanine dyes in neurons of trigeminal ganglia was evaluated 7 days after applying one dye to the buccal sulcus and the other to the palatal sulcus. Both dyes were absorbed through the junctional epithelium, and the two sites each labeled similar numbers and sizes of neurons in the lateral zone of the maxillary division (44% from buccal and 46% from palatal gingiva). Double-labeled neurons had the same size (32.5 +/- 6.70 microns, mean circumference +/- S.D.) and location as single-labeled neurons, and they were 9% of the total. This study shows that exogenous dyes can diffuse into mucosa and thereby allow in vivo mapping of sensory nerve branching patterns to several intact tissues per animal. We found that 9% of the labeled cells extended to both the buccal and palatal gingiva. Thus, inflammation that spreads from one gingival region to the other could have a neurogenic mechanism involving trigeminal sensory neurons that extend their peripheral branches to innervate both buccal and palatal gingiva of adult rat molars.

Immunocytochemical localization of tyrosine hydroxylase and gamma-aminobutyric acid in the mesencephalic trigeminal nucleus of the cat: a light and electron microscopic study

BACKGROUND

Recent studies conducted on the rat have demonstrated that the mesencephalic trigeminal nucleus (MTN) neurons, involved in the proprioceptive transmission, contain some neuroactive substances, including classical and amino acid neurotransmitters. In addition, there is evidence that serotonin could not act as a neurotransmitter at the first synaptic relay in the cat MTN. In the present study, we aimed to examine two other possible neurotransmitter systems, i.e., catecholamines and gamma-aminobutyric acid (GABA), and the relationships between GABA-immunoreactive (IR) neurons and tyrosine hydroxylase (TH)-IR axonal varicosities in the MTN of the cat.

METHODS

To ensure the localization of immunoreactive structures, the experiments were carried out at the light and electron microscopic level using single immunostaining for TH and GABA alone. The correlation between GABA-IR cell bodies and TH-IR fibers was investigated by means of double-labeling immunogold and peroxidase technique for GABA and TH.

RESULTS

Light microscopically, a few GABA-IR neurons were observed in the cat MTN. These small-size, labeled cells, most likely interneurons, were apposed to unstained large mesencephalic trigeminal cells. Most of the large nonreactive MTN neurons were closely surrounded by fine TH-IR varicose or nonvaricose fibers and dot-like structures, presumably nerve terminals. Under the electron microscope, TH-IR fibers were not seen in synaptic contact and only rarely appeared to be in close proximity to neuronal profiles of small GABAergic cells, which contained gold particles.

CONCLUSIONS

Taken together with earlier studies from other laboratories, the present findings suggest that GABAergic system might play an indirect role in the proprioceptive information processing in the cat MTN by interactions of GABA-immunoreactive neurons with the systems that control the transmission of selected sensory information. In contrast, the presence of TH-IR fibers in direct apposition to the majority of MTN neurons provide further evidence that presumed GABAergic interneurons have extensive interactions with catecholamine varicosities and raise the possibility that catecholamines could modify the transmission on these neurons.

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)

Evidence for the existence of intraepithelial nerve endings in the junctional epithelium of rat molars: an immunohistochemical study using protein gene product 9.5 (PGP 9.5) antibody

Innervation of the junctional epithelium was investigated in rat molars by means of immunohistochemistry for protein gene product 9.5 (PGP 9.5) at light and electron microscopic levels. In comparison with our previous study on same tissues using neurofilament protein (NFP)-antibody, the PGP 9.5-immunostaining further disclosed numerous nerve fibers in the gingiva of rat molars and revealed the existence of a well-developed plexus of PGP 9.5-positive nerve fibers. The interproximal portion also contained numerous nerve fibers. Observation of horizontal sections revealed a denser innervation toward the inner junctional gingival epithelium than the outer marginal epithelium. The nerve fibers, beaded in appearance and extending from the nerve bundles in the lamina propria, penetrated into the junctional epithelial layer and were distributed throughout the junctional epithelium, with some nerves being located near the epithelial surface. Non-neuronal cells showing PGP 9.5-immunoreactivity were absent in the junctional epithelium. In immunoelectron microscopy, the axoplasm of nerves in the gingiva was filled with electron-dense reaction products of PGP 9.5, except for the cell organellae. The nerve fibers were devoid of Schwann cell investment and terminated among the epithelial cells in the junctional epithelium, frequently beneath the epithelial surface. The intraepithelial nerve endings contained various kinds of vesicles including large-cored ones, supporting the presence of peptidergic innervation shown by previous studies. These findings confirmed the usefulness of PGP 9.5-immunohistochemistry for the identification of delicated nerve fibers in dental tissue, and suggested the dense network of nerve fibers that may serve as sensory receptor and other functions in the junctional epithelium.

Regeneration of junctional epithelium and its innervation in adult rats: a study using immunocytochemistry for p75 nerve growth factor receptor and calcitonin gene-related peptide

Junctional epithelium (JE) is a rapidly proliferating tissue that connects the gum to the tooth, that provides a free surface for bidirectional movement of substances between the body and the oral cavity, and that participates in defense against bacterial infection. It is innervated by numerous sensory nerve fibers that are immunoreactive (IR) for neuropeptides such as calcitonin gene-related peptide (CGRP), and for low affinity nerve growth factor receptor (p75-NGFR). Basal epithelial cells of the JE and of adjacent sulcular epithelium also have intense p75-NGFR-IR. In the present study we removed a wedge of the free gingiva and JE from the anterior side of the maxillary first molar of adult rats, and then studied the return of nerve fibers during tissue regeneration from 1-63 days after gingivectomy. The nerve fibers entered the adjacent healing sulcular epithelium before innervating the new JE, in both cases prior to return of epithelial cell p75-NGFR-IR. The regenerating nerve fibers completely bypassed the zone of epithelial down-growth (long junctional epithelium, LJE) that was briefly present along the tooth from 1-3 weeks after injury. The LJE did not have p75-NGFR-IR and was gradually replaced by a modified thicker regenerated junctional epithelium (RJE). The RJE was attached along the injured root surface, had numerous nerves in basal layers, and it had begun to regain p75-NGFR-IR staining of basal epithelial cells by 22 d. Regenerating nerve fibers at 6-10 d had unusually weak CGRP-IR and greatly increased p75-NGFR-IR. Both nerve stains had returned to normal by 3-6 weeks. The intense p75-NGFR-IR of regenerating nerves was found on both axonal and Schwann cell membranes using electron microscopic immunocytochemistry. In both the normal and regenerating JE, nerve fibers were rare in the attachment layers next to the anterior side of the maxillary first molar, compared to well-innervated basal layers. The complete avoidance of LJE by regenerating nerve fibers and its lack of p75-NGFR-IR suggest that its functions do not require innervation and that it does not make neurotrophic growth factors.

Central connections of trigeminal primary afferent neurons: topographical and functional considerations

This article reviews literature relating to the central projection of primary afferent neurons of the trigeminal nerve. After a brief description of the major nuclei associated with the trigeminal nerve, the presentation reviews several early issues related to theories of trigeminal organization including modality and somatotopic representation. Recent studies directed toward further definition of central projection patterns of single nerve branches or nerves supplying specific oral and facial tissues are considered together with data from intraaxonal and intracellular studies that define the projection patterns of single fibers. A presentation of recent immunocytochemical data related to primary afferent fibers is described. Finally, several insights that recent studies shed on early theories of trigeminal input are assessed.

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.

Immunohistochemical study of nerve fibres with substance P- or calcitonin gene-related peptide-like immunoreactivity in the junctional epithelium of developing rats

The beginning of innervation in the junctional epithelium of maxillary first molars was examined in gingival tissues from 19 to 32-day-old rats. Substance P- or calcitonin gene-related peptide (CGRP)-like immunoreactivity was demonstrated by the avidin-biotin peroxidase complex method. In 19-day-old rats, nerve fibres with substance P- or CGRP-like immunoreactivity were seen in the connective tissue and oral epithelium, but not in the reduced enamel epithelium, which would be transformed into the junctional epithelium. In 21-day-old rats, the fibres with substance P- or CGRP-like immunoreactivity formed a plexus in the oral sulcular epithelium and thin varicose fibres were seen for the first time entering the adjacent reduced enamel epithelium. These fibres also penetrated the middle portion of the reduced enamel epithelium, but did not reach the cuboidal reduced ameloblasts. More nerve fibres had CGRP-like immunoreactivity than substance P-like immunoreactivity. In 23-day-old rats, many fibres with both immunoreactivities were seen in the basal layers of the junctional epithelium, but only a few were seen in its superficial layers. In 28-32-day-old rats, numerous fibres with both immunoreactivities were distributed in the whole junctional epithelium and showed a similar pattern of innervation. For all immunoreactive fibres, the density in the middle portion in the junctional epithelium was the highest. The nerve plexus was formed in the basal layers and some fibres with a varicose appearance were found in the superficial layers.

A light and electron microscopic anterograde WGA-HRP tracing study on the sensory innervation of junctional and sulcular epithelium in the rat molar

The sensory innervation of junctional and oral sulcular epithelium was investigated by use of anterograde transport of wheat-germ agglutinin-horseradish peroxidase from the trigeminal ganglion. By light microscopy, labeled intra-epithelial nerve fibers were observed forming a dense plexus in the apical two-thirds of the junctional epithelium, with some fibers located near the enamel space. Occasional fibers extended coronally to the sulcus bottom. By electron microscopy, labeled intra-epithelial axon terminals or varicosities were demonstrated to be in close contact with both junctional epithelial cells and neutrophils. These varicosities, which were occasionally surrounded by the cytoplasmic processes of epithelial cells or neutrophils, frequently contained large granular and small clear vesicles. In contrast to the junctional epithelium, the oral sulcular epithelium was sparsely innervated, except for the transition region between the oral sulcular epithelium and the junctional epithelium, where a dense innervation by labeled intraepithelial fibers was found. These fibers extended as far as the stratum spinosum. Electron microscopy revealed mitochondria-filled profiles of varicosities between epithelial cells. This study shows differences in the distribution and ultrastructure of sensory nerves between the junctional and oral sulcular epithelia, and further provides morphological evidence that sensory nerves in the junctional epithelium come into contact not only with epithelial cells but also with neutrophils.

The presence of nerve fibres in the coronal odontoblast layer of teeth at various stages of root development

Many studies have investigated the presence of nerve fibres in the coronal odontoblast layer of mature teeth. However, little information is available on the presence of these structures in immature human teeth. The pulps of 18 human maxillary and mandibular third molars at various stages of development were collected, sectioned, and stained by the immunoperoxidase technique. The results showed the presence of numerous myelinated nerve fibres in the coronal odontoblast layer of teeth with fully formed roots. Similar observations were made in teeth with half formed roots, and in those without roots. On the basis of these results, it appears that a lack of response to vitality tests in teeth with incompletely formed roots is not wholly due to the absence of nerve fibres in the coronal odontoblast layer.

Distribution and morphological characteristics of axons in the periodontal ligament of cat canine teeth and the changes observed after reinnervation

The distribution and morphological characteristics of myelinated and non-myelinated axons innervating the lower canine periodontal ligament (PDL) in adult cats have been analysed. After perfusion fixation and decalcification, the teeth were slit transversely, divided into segments, and embedded in plastic. Ultrathin sections of each segment were examined in the electron microscope and used to reconstruct the whole PDL at 1, 4, 7, and 9 mm from the tooth apex. One millimeter from the tooth apex there were a mean of 920 myelinated axons and 1,415 non-myelinated axons. The numbers of axons declined toward the tooth crown. Bundles of myelinated and small non-myelinated axons lay adjacent to the blood vessels midway between the bone and cementum. Isolated myelinated axons appeared to have split away from these main nerve bundles and entered the avascular zone of the ligament, where they lost their myelin sheaths to become large non-myelinated axons rich in mitochondria. These non-myelinated axons sometimes appeared to be linked to collagen fibres and were thought to be the mechanoreceptor terminals. Twelve weeks after sectioning and inferior alveolar nerve, the total number of axons innervating the periodontal ligament was 50% of that found in the contralateral controls. The large non-myelinated axons had smaller mean diameters and contained fewer mitochondria, a change which may be consistent with a reduction in mechanoreceptor excitability.

The innervation of human teeth and gingival epithelium as revealed by means of an antiserum for protein gene product 9.5 (PGP 9.5)

The innervation of human teeth and oral mucosa has been studied in the past by different methods, none of which offered a clear description of the precise morphology of nerve fibers and terminals and of nerve organization as a whole. Recently, interesting findings have been obtained by means of immunohistochemical investigations for neurofilaments and S-100 proteins. A new brain-specific molecule, protein gene product 9.5 (PGP 9.5), has been used for the first time in the present research to investigate the distribution of nerves in human oral mucosa and decalcified teeth, about which there is a paucity of information. The data provided in this study, confirming previous work in other species, may be of value for understanding the anatomy of human oral innervation. In the oral mucosa, the antiserum labels nerve fibers, corpuscles, and neuroendocrine (Merkel) cells. In sections of decalcified teeth, numerous PGP 9.5 positive fibers are demonstrated in the pulp and in the inner 100 microns of dentin. The novel nerve tissue protein used, PGP 9.5, thus appears to be a reliable marker for studies of nerve fibers in human tissues and not to be affected by decalcification procedures. It could then be used for investigations on the innervation of normal and pathological calcified human tissues.

The distribution of vasoactive intestinal polypeptides and calcitonin gene-related peptide in the periodontal ligament of mouse molar teeth

The distribution of VIP- and CGRP-containing nerve fibres was examined by indirect immunofluorescence. There were many such fibres in the lower third of the ligament, some around the blood vessel close to the socket wall. In the middle third of the ligament, some CGRP-containing fibers entered from the lateral wall of the socket; this type of fibre was more numerous in the lower third than in middle third. There were some VIP-containing fibres but no CGRP-containing fibres in the ligament surrounding the furcation of the molar roots.

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.

Immunohistochemical study of neurochemical markers in gingiva obtained from periodontitis-affected sites

Immunohistochemical methods have been used to study the occurrence of neuronal markers in human gingiva from periodontitis-affected sites. In periodontitis-affected buccal gingiva densely distributed neurofilament (NF)-immunoreactive (IR) fiber bundles were observed in the deeper parts of the propria, while NF-IR single fibers occurred in the superficial propria and occasionally in the buccal epithelium. Periodontitis-affected gingiva obtained from interproximal sites showed only sparsely distributed NF-IR fibers. Single nerve fibers immuno-reactive to the peptides substance P and calcitonin gene-related peptide occurred close to or within the epithelium in both buccal and interproximal gingiva. Around blood vessels neuropeptide Y-, peptide histidine-isoleucine amide- and vasoactive intestinal polypeptide-IR fibers were occasionally observed, while clusters of gamma-melanocyte-stimulating hormone-IR cells were found in the propria, in addition to gamma-melanocyte-stimulating hormone IR nerve fibers. Somatostatin-IR dendritic cells were seen in epithelium and propria of buccal and interproximal gingiva, although a high variability in the number of SOM-IR cells was observed. All neuronal markers studied showed a similar distribution in material obtained from young patients with clinically healthy gingivae, although the number of NF-IR fibers in the propria in these subjects was lower. The results demonstrate that in gingiva obtained from periodontitis-affected sites several different biologically active peptides occur in both nerve fibers and cells. At least some of these substances could possible play a role in the inflammatory process. However, since clinically normal gingiva was shown to contain nerve fibers and cells expressing immunoreactivity to the substances studied, no unique periodontitis-induced expression of the neuronal markers studied was found. Thus, any alteration of these substances during the periodontitis process remains to be elucidated.

Comparison of trigeminal receptor location and structure in the periodontal ligament of different types of teeth from the rat, cat, and monkey

The periodontal ligament is richly innervated by mechanoreceptors whose cell bodies are located either in the trigeminal ganglion (TG) or the mesencephalic (MS) trigeminal nucleus. Both are sensitive to stretch of the ligament induced by tooth movement, but their thresholds, central connections, and functional significance differ. This study compared the location of TG and MS receptors in the periodontal ligament of cat teeth after labeling each by anterograde axonal transport. We also compared the location and ultrastructure of the feline TG receptors with labeled TG receptors in the periodontal ligament of monkey teeth and rat incisors in order to determine their location and ultrastructural properties. We found that the MS and TG receptors had a different distribution in the periodontal ligament of cat teeth; the MS terminals were concentrated below and next to the base of the roots, whereas the TG receptors were most numerous around the middle of the roots. The TG receptors of monkey teeth had a similar location to the feline TG receptors, but those of rat incisors were very different. Rat incisors are curved, continuously erupting teeth, and their TG receptors were located primarily on the lingual side in the alveolar (nonerupting) portion of the ligament. Ultrastructural comparisons found that most mechanoreceptors in the periodontal ligament of all the teeth had an unencapsulated branched Ruffini-like structure. The TG receptors in the rat incisor ligament were the largest; those of monkey had the most varied form. Some coiled or encapsulated receptors were found in the monkey and cat ligament, but not in the rat incisor ligament. The TG receptors appear to be located at sites that would be most easily stretched during tooth contact. The different sites and intensity of the stretch forces occurring during the use of different types of teeth may determine the variations in the size and location of the TG mechanoreceptors and of their associated support cells. The different distribution of MS receptors may contribute to their response thresholds and static properties, which differ from those of TG receptors.

Innervation of periodontal ligament and dental pulp in the rat incisor: an immunohistochemical investigation of neurofilament protein and glia-specific S-100 protein

Nervous elements in the periodontal ligament and dental pulp of rat incisors were investigated by means of immunohistochemistry for neurofilament protein (NFP) and glia-specific S-100 protein. The periodontal ligament in the incisors was densely innervated by NFP-immunoreactive nerve fibers; the distribution of the nerve fibers and their terminations differed markedly from those in molars. NFP-positive, thick nerve bundles entered the lingual periodontal ligament through slits located in the mid-region of the alveolar socket, and immediately formed numerous Ruffini-like corpuscles. In the labial periodontal ligament, all of the NFP-immunoreactive nerve fibers terminated in free endings. The restricted location of the stretch receptor, Ruffini-like corpuscle, in the lingual periodontal ligament appears to be an essential element, because this region is regularly extended during mastication. The nervous elements were restricted to the alveolar half of the periodontal ligament in every region; they avoided the dental half of the periodontal ligament, which presumably moves continuously with the tooth. Pulpal nerve fibers in incisors also showed a characteristic distribution different from those in molars; individual nerve fibers with beaded structures ran in the center of the pulp toward the incisal edge, and did not form the subodontoblastic nerve plexus of Raschkow. Immunostaining for S-100 protein revealed a distribution pattern of nervous elements similar to that for NFP, suggesting that the nerves supplying the periodontal ligament and dental pulp were mostly covered by a Schwann sheath.

Numerous nerves with calcitonin gene-related peptide-like immunoreactivity innervate junctional epithelium of rats

We found an unusually dense concentration of nerves with calcitonin gene-related peptide-like (CGRP-L) immunoreactivity in junctional epithelium of rat gingiva. Adjacent oral epithelium was only sparsely innervated. The CGRP-L innervation may affect the local inflammatory reactions, leukocyte transmigration, and serum outflow that occur in junctional epithelium, as well as having somatosensory functions.

Distribution of nerve fibers immunoreactive to neurofilament protein in rat molars and periodontium

The distribution of nerve fibers in molars, periodontal ligament and gingiva of the rat shows a complex pattern. Decalcified material including the alveolar bone was sectioned in three different planes and stained by means of immunohistochemistry for detection of the neurofilament protein (NFP); the immunoreactive neural elements were clearly visualized in three-dimensional analyses. NFP-positive nerve fibers formed a subodontoblastic plexus in the roof area of the dental pulp; some of them entered the predentin and dentin directly through the dentinal tubules. This penetration was found mainly in the pulp horn, and was limited to a distance of about 100 micrometers from the pulpo-dentinal junction. In the periodontal ligament, NFP-positive nerve fibers were found densely distributed in the lower half of the alveolar socket. Two types of nerve terminals were recognized in the periodontal ligament: free nerve endings with tree-like ramifications, and expanded nerve terminals showing button- or glove-like shapes. The former tapered among the periodontal fibers, some even reaching the cementoblastic layer. The latter were located, frequently in groups, within the ligament restricted to the lower third of the alveolar socket. A well-developed plexus of NFP-positive nerves was revealed in the lamina propria of the free gingiva, the innervation being denser toward the epithelium of the gingival crevice. The characteristic distribution of NFP-immunoreactive nerve fibers revealed in this study is discussed in relation to region-specific sensations in the teeth and surrounding tissues.

An interpretation of dental innervation based upon the pattern of calcitonin gene-related peptide (CGRP)-immunoreactive thin sensory axons

Calcitonin gene-related peptide (CGRP) is a recently characterized neuroactive substance that is expressed in a large proportion of small- to medium-diameter sensory ganglion neurons whose central terminals lie in the superficial spinal and medullary dorsal horn. This restricted distribution within the peripheral nervous system suggests a prominent role for the peptide in nociceptive processing. The mammalian tooth pulp, which receives a relatively homogeneous afferent input from thin (putative nociceptive) fibers originating from this subpopulation of trigeminal ganglion cells, thus affords an ideal target zone in which to examine peripheral nociceptive mechanisms. The large percentage of these neurons displaying CGRP-like immunoreactivity (CGRP-LI) furthermore provides a valuable tool to study its thin-fiber afferent innervation. CGRP-LI has been localized within intact, decalcified specimens of rat, cat, monkey, and human teeth and associated dental structures. A remarkably robust CGRP-LI innervation of molar pulp and dentin was revealed in all species, with fibers coursing both in fascicles and individually, in variable relation to blood vessels and pulpal stroma. Our methods enabled tracing of a large number of axons through Raschkow's plexus and odontoblast layer into dentinal tubules. Paralleling anterograde axonal transport studies, a greater share of fibers was found in coronal vis-à-vis radicular dentin. In the rat, this fiber pattern stood in contrast both to incisor dentin, which appeared devoid of CGRP-LI, and to the abundant labeled axons in gingiva and periodontal tissues. Surgical deafferentation of rat mandible resulted in widespread depletion of CGRP-LI, while superior cervical ganglionectomy was without effect, confirming the sensory nature of the CGRP-LI fibers. Neonatal capsaicin treatment greatly attenuated the immunostaining, providing evidence for CGRP-LI localization in chemosensitive unmyelinated afferents. The great density of CGRP-LI axons demonstrated is considered in contrast to the restricted range and extent of sensory stimuli to which teeth are presumably subjected, and in relation to the diverse ongoing trophic, regulatory, and reparative processes in tooth structures. It is therefore suggested that these fibers may be subserving prominent efferent roles in dental pulp not directly related to nociception.

Sensory nerve endings of the incisive papilla of rat hard palate studied by peroxidase cytochemical methods

This study utilized peroxidase cytochemical methods to survey and analyze the distribution, morphology, and functional relationship of the various components of sensory nerve endings in a focal region of the anterior hard palate (incisive papilla) of the rat. The studies included an anatomical survey of this relatively unknown oral structure, confirmation of the trigeminal origin (labeled by retrograde axonal transport of WGA-HRP injected into the incisive papilla) of the sensory nerve endings, and a complete analysis of the distribution of these nerve endings (labeled by anterograde axonal transport of HRP or WGA-HRP injected into the trigeminal ganglion). Three methods of fixation and two methods of cytochemical reaction were used for selection of an optimal technique for these studies. The results showed distinct patterns of peroxidase-labeled sensory nerve endings at three regions of the incisive papilla: a) dome region (ventral), where labeled nerve endings formed three parallel channels in association with three surface ridges; b) chemosensory corpuscle enriched region (medial to incisive canal), where 82% of the 30-40 chemosensory corpuscles were labeled; and c) lateral labium (lateral to incisive canal), where labeled nerve endings formed a circumscribed network guarding the orifice of incisive canal. The discrete organization of multiple sensory nerve endings in the incisive papilla of the rat may provide an easily accessible model system for various studies in sensory physiology.

Sensory innervation of periodontal ligament of rat molars consists of unencapsulated Ruffini-like mechanoreceptors and free nerve endings

The trigeminal ganglion (TG) of adult rats was injected with 3H-amino acids to label periodontal receptors by axonal transport; 20-24 hours after injection, samples of molar ligament were prepared for autoradiography and electron microscopy. Four types of neurites labeled from TG were found in the avascular ligament fiber regions: large, complex, Ruffini-like endings, lacking a capsule, but with finger extensions touching ligament collagen; smaller Ruffini-like endings, lacking a capsule and neural fingers; free bundles of unmyelinated axons; and free, small, myelinated axons. The vascular channels plus associated loose connective tissue that perforate the ligament contained labeled preterminal ensheathed axons, small Ruffini endings, and free unmyelinated or small myelinated axons. The incidence of labeled endings was about 5 X greater next to the lower third of the root than in the upper two-thirds or beneath the root. The TG myelinated axons (diameter range 2-15 microns) entered the ligament in sheathed nerve bundles; these branched to form numerous small preterminal axons that were surrounded by a periaxonal fluid space and a perineurial sheath. Terminal axons branched from nodes of Ranvier, left the preterminal chamber, and followed an extended branching course through the collagen fibers. Large, complex Ruffini-like endings had numerous mitochondria and were partially covered by special lamellar Schwann cells and complex basal lamina; vesicles and multivesicular bodies were found near exposed regions of the receptor. Smaller Ruffini-like endings lacked neural fingers and had a simpler structure and less elaborate Schwann cells. The structure of Ruffini-like endings was highly varied; thus a structural continuum may exist from the largest, most complex to the smallest, simplest Ruffini-like receptor. The TG unmyelinated axons entered the ligament in ensheathed bundles; they then branched into free bundles that were found in the avascular ligament or near blood vessels. No encapsulated receptors were found.

The distribution and origin of substance P-like immunoreactivity in the rat molar pulp and periodontal tissues

Rat mandibles were fixed in Zamboni fixative and demineralized in a mixture of EDTA and fixative. Substance P-like immunoreactivity was demonstrated by indirect immunofluorescence in molar pulp, periodontal ligament and gingiva. Substance P (SP) containing nerve fibres with varicosities were observed in the pulp horn and root pulp in general located around blood vessels. Some SP-containing fibres penetrated into the predentine and dentine. In the periodontal ligament, SP fibres were localized along the blood vessels in the middle and apical regions. Many SP-containing fibres were associated with the blood vessels in the lamina propria of gingiva. After inferior alveolar nerve section, SP-positive nerve fibres in the pulp and periodontal ligament disappeared completely. In gingiva the number of SP fibres decreased but not all fibres disappeared. Removal of the superior cervical ganglion did not affect the distribution of SP-containing nerve fibres.

Relationship between occlusal contacts and jaw-closing muscle activity during tooth clenching: Part I

Electromyographic recordings from the anterior temporal muscle fibers bilaterally, the posterior temporal muscle fibers bilaterally, the superficial masseter muscle bilaterally, and the left medial pterygoid muscle were used to study the effects of changing the location, size, and direction of effort on specific contact points during maximal clenching tasks in human subjects. Vertical clenching efforts in the natural or simulated intercuspal position generally showed the highest muscle activities for all the muscles recorded. When the contact point moved posteriorly along the arch from incisors to molars, the activity in the ipsilateral temporal muscles was seen to increase, while the activity in the ipsilateral medial pterygoid and the masseter muscles bilaterally was seen to decrease during vertical clenching tasks. Eccentric efforts on specific contact points generally resulted in lower activity than the corresponding vertical effort. This was usually seen in all muscles, but not all values were significant. The ipsilateral temporal and contralateral pterygoid muscles showed the most activity during maximal clenches in lateral direction with little contribution from the other muscles. The temporal muscles showed the most activity in retrusive clenching, with activity in the other muscles nearly nonexistent. The medial pterygoid and masseter muscles were found to be the most active muscles during protrusive and incisal clenching, while the temporal muscle activity was low. When the size and number of contacts were increased anteriorly, a generalized increase in muscle activity was seen. The same trend occurred posteriorly but was not as consistent or significant. Cross-arch contacts were associated with a slight but significant bilateral increase in masseter muscle activity and an increase in temporal muscle activity ipsilateral to the cross-arch contact when maximum vertical clenches were performed. However, no significant increases were observed when the effort was directed laterally. The findings of this electromyographic study on change of the contact point, size of contact point, and the direction of effort applied on a contact point confirm their specific associations with the activity of muscle groups. Significant data have also been made available for a biomechanic approach of the investigation of degenerative joint changes.

Fine structure of subepithelial "free" and corpuscular trigeminal nerve endings in anterior hard palate of the rat

Axonally transported protein labeled many trigeminal nerve endings in subepithelial regions of the anterior hard palate of the rat. Sensory endings were most numerous in the lamina propria near the tips of the palatal rugae where large connective tissue and epithelial papillae interdigitated. Two kinds of sensory ending were found there: "free" endings, and a variety of corpuscular endings. The "free" sensory endings consisted of bundles of unmyelinated axons separated from the connective tissue by relatively unspecialized Schwann cells covering part or all of their surface and a completely continuous basal lamina; they were commonly found running parallel to the epithelium or near corpuscular endings. The corpuscular sensory endings all had a specialized nerve form, specialized Schwann cells, and axonal fingers projecting into the corpuscular basal lamina or connective tissue. There were at least four distinct types of corpuscular ending: Ruffini-like endings were found among dense collagen bundles, and they had a flattened nerve ending with a flattened Schwann lamella on either side. Meissner endings had an ordered stack of flattened nerve terminals with flattened Schwann cells and much basal lamina within and around the corpuscle. Simple corpuscles were single nerve endings surrounded by several layers of concentric lamellar Schwann processes. Glomerular endings were found in lamina propria papillae or encircling epithelial papillae; they were a tangle of varied neural forms each of which had apposed flattened Schwann cells, and a layer of basal lamina of varied thickness. Fibroblasts often formed incomplete partitions around Meissner and simple corpuscles. The axoplasm of all kinds of subepithelial sensory endings contained numerous mitochondria and vesicles, as well as occasional multivesicular bodies and lysosomes; the axoplasm of all endings was pale with few microtubules and neurofilaments. The specialized lamellar Schwann cells had much pinocytotic activity. Four kinds of junctions were found between the corpuscular sensory endings and the lamellar Schwann cells: (1) symmetric densities that resemble desmosomes; (2) asymmetric densities with either the neuronal or glial membrane more dense; (3) neural membrane densities adjacent to Schwann parallel inner and outer membrane densities; and (4) sites of apparent Schwann endocytosis associated with neural blebs. The "free" sensory endings only made occasional desmosome-like junctions with their Schwann cells.(ABSTRACT TRUNCATED AT 400 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)

Dental nerve regeneration in rats. II. Autoradiographic studies of nerve regeneration to molar pulp and dentin

The autoradiographic technique was used to analyze the degeneration and regeneration of sensory nerves to rat molars and gingiva following cut or crush injury to the right inferior alveolar nerve. At 2 days after nerve injury there was almost complete denervation of the first molar, partial denervation of the second molar, and minimal effect on the innervation to the third molar and gingiva. The degree of sensory deficit and recovery for these same rats had been previously determined. Reinnervation of the first molar was analyzed in terms of axon number and location, intensity of axon labeling, and type of nerve injury. At 6 days, neither the cut injury nor crush injury rats had any reinnervation of their first molars. By 7 days, 3 of 4 rats had axons reinnervating first molars; in those teeth there was approximately one-fourth of the normal number of axons in the pulp, and very few axons in the dentin. These rats still had as large a molar sensory deficit as the 7 day rat and 6 day rats that had no reinnervation. By 3 weeks there were one-half to three-fourths of the normal axon numbers in the pulp, one-fourth to one-half of the normal axon numbers in dentin; and sensitivity was at least half-recovered. By 6 weeks, numbers of axons in the pulp and dentin were either normal or slightly less than normal; axons had grown back into dentin to the same depth as in normal teeth; and complete recovery of sensitivity had occurred. The regenerating axons had greater than normal labeling intensity at 1 week and 3 weeks in all rats. Those with the crush nerve injury had somewhat greater numbers of reinnervating axons at 1 week and 3 weeks than the cut injury rats. A structure-function comparison for the molars showed that return of sensitivity correlated with reinnervation of both pulp and dentin.

Fine structure and axonal transport labeling of intraepithelial sensory nerve endings in anterior hard palate of the rat

The fine structure of intraepithelial free nerve endings, Merkel receptors, and special (possibly chemosensory) corpuscular receptors has been examined in anterior hard palate of the rat. Samples were taken from the two anterior large rugae as well as from the more rostral incisive papilla. Most nerve endings were concentrated in regions in which the epithelium and connective tissue interdigitated extensively. The nerve endings contained relatively pale cytoplasm, a variety of vesicles, and numerous mitochondria. Distinctions could be made among different endings depending on their intraepithelial location, or on the degree of specialization of the neighboring epithelial cells. Simple (free) nerve endings entered the epithelium between epithelial papillae; upon entering, their basal lamina became associated with the epithelial basal lamina in various ways, and their Schwann cell sheath also entered the epithelium for a short length. Free nerve endings were found at various sites among the regular epithelial cells in the basal layers, the stratum granulosum, and stratum spinosum, and next to the stratum corneum border; they did not form specialized junctions with the epithelial cells. Intraepithelial nerve endings at all levels could be labeled by axonal transport from the trigeminal ganglion. Some nerve endings in anterior hard palate were associated with specialized epithelial cells, either in Merkel complexes or in apparent chemosensory corpuscles; both were labeled by axonal transport from the trigeminal ganglion. The specialized epithelial cells in the corpuscular receptors formed possible efferent and afferent synapses with the nerve endings. The nerve endings in the Merkel complexes formed junctions with the specialized Merkel epithelial cells. Both the Merkel complexes and corpuscular receptors were found in their respective characteristic epithelial papillae, which were surrounded by a dense plexus of subepithelial terminal axons and nerve endings.

A fine-structural investigation on the extent of perineurial investment of the nerve supply to the pulp in rat molar teeth

The fine structure of the tissues immediately adjacent to nerve fibres supplying the rat molar pulp has been examined and compared with that of perineurium of other peripheral nerves. In the periapical region, almost all nerve fibres were invested with typical perineurium; only a few Schwann cell units (unmyelinated axons) lacked perineurium. In the pulp at the apex of the distal root, most fibres were invested by less typical perineurium but some were not invested at all. In the pulp at the coronal end of the root, nerve fibres were completely invested or incompletely invested with attenuated perineurium or lacked investment altogether. In the coronal pulp, there was virtually no perineurium at all. Thus rat molar pulp nerve fibres lack perineurium for distances of about 0.5-3.0 mm, distances greater than in situations such as skin or muscle. In normal peripheral nerves, the perineurium behaves as a barrier to the passage of substances from the immediate environment of the nerve fibres (endoneurium) to the surrounding environment (epineurium) and vice versa. Its absence around pulp nerve fibres raises questions about the nature of the pulp environment and may help to explain the sensitivity of pulp nerves to chemical, osmotic and other stimuli.

Autoradiographic demonstration of ipsilateral and contralateral sensory nerve endings in cat dentin, pulp, and periodontium

In order to determine the location of sensory nerve ending in cat teeth, 3H-proline and 3H-leucine were injected into the left trigeminal ganglion of eight cats aged 6.5-10 months; 24 hours was allowed for axonal transport of radioactive protein to dental nerve endings, and the endings were then detected by autoradiography. The pulps of most ipsilateral (left) teeth contained some labeled axons. These axons ended in the odontoblastic layer and predentin of roots and crown; at the tip of the pulp horn of each cusp, nerve endings also extended as far as 150 micrometer into dentinal tubules. Labeled nerve endings were extremely rare in contralateral (right) teeth; only one tooth of 83 studied (eight cats) contained heavily labeled axons, and one other had faintly labeled axons. Both labeled contralateral teeth were central maxillary incisors. Their labeled axons were unbranched in the root and arborized in the crown to end among odontoblasts and many adjacent dentinal tubules. Labeled periodontal nerve endings were most numerous in the apical one-third of the ligament, with some endings extending as far as the gingiva. The nerve endings in the periodontal ligament were often clustered and appeared to end freely between the collagen bundles; their radioactivity varied in the same way as that of pulp nerves in the adjacent root.

Neuro-histological reactions following tooth extractions

The neuro-histological reactions after tooth extraction were investigated in the extraction alveolus, the mandibular nerve and the trigeminal ganglion. In the ganglion, nerve cell bodies showing signs of retrograde reaction (chromatolysis and nuclear displacement) were seen 12 h after extraction. Maximal number of reacting cells were registered in the first postoperative week. Three weeks after extraction the number of reacting cells were at a normal low level. In the mandibular nerve no signs of axon degeneration could be demonstrated. In the alveolus, initial traumatic axon degeneration was followed by regeneration 2 days after extraction. Within the first postoperative week the alveolus was filled with connective tissue, in which many long thin axons were seen. Cancellous bone then filled the alveolus; the axons were thereby gathered - concentrated - into fascicles in the central part, with a direction towards the limbus. However, this was only passed by very few axons. Four months after extraction signs of axon degeneration were seen, and 2 months later the myelin sheaths also displayed degenerative signs. Ten months after extraction a minor area of connective tissue with a content of few axons and vessels was found at the bottom of the former alveolus. The histological appearance was of a small traumatic neuroma.

Catecholamine-containing and acetylcholinesterase-positive nerve fibres in the rabbit conjunctiva

The innervation of the rabbit conjunctiva was investigated histochemically. Nerves containing acetylcholinesterase (AChE) were demonstrated using a copper thiocholine technique. Sympathetic nerve fibres were demonstrated using either the formaldehyde-induced fluorescence (FIF) or glyoxylic acid induced fluorescence (GIF) method. In addition, GIF and AChE-reactions were performed consecutively for the demonstration of fluorescent and AChE-positive nerves in the same specimen. The number of AChE-positive nerve fibres exceeded that of fluorescent fibres. Both fibre types were more numerous in the stroma of the limbal and tarsal conjunctiva than in the fornix. In the epithelium, only a few AChE-positive nerves were observed. These are assumed to be sensory. Most nerves seemed to contain both catecholamine-containing and AChE-positive fibres. However, nerves fibres containing only AChE-activity were also found. The blood vessels of the conjunctiva were innervated by both fluorescent and AChE-positive nerves, the distribution and number of which differed slightly.