Innervation of taste buds in the canine larynx as revealed by immunohistochemistry for the various neurochemical markers

Laryngeal Chemoreflex in Health and Disease: A Review

The larynx plays a key role in airway protection via the laryngeal chemoreflex (LCR). This involuntary reflex can be evoked when hazardous substances activate mucosal receptors, which send signals to be processed within the brainstem. Although the LCR is meant to be protective, the reflex can become hyperstimulated, even to benign stimuli, which can result in pathological disorders, such as chronic cough and inducible laryngeal obstruction. In this review, we will outline the mechanism of the LCR and its associated pathological disorders.

Optimisation and validation of immunohistochemical axonal markers for morphological and functional characterisation of equine peripheral nerves

BACKGROUND

Horses are affected by various peripheral nerve disorders but defining their aetiology and pathophysiology is hampered by limited understanding of associated morphological and pathological changes and involvement of specific axonal types.

OBJECTIVES

To investigate the hypothesis that selected antibody markers, used in conjunction with various tissue processing methods, would enable identification of axons with different functional modalities within a range of equine peripheral nerves.

STUDY DESIGN

Optimisation and validation study.

METHODS

A range of antibodies were evaluated immunohistochemically via fluorescence confocal microscopy in cadaver equine nerve samples of primary motor, mixed or primary sensory functions (recurrent laryngeal, phrenic and plantar digital) within formalin-fixed paraffin-embedded (FFPE) and formalin-fixed frozen (FFF) tissues subjected to different antigen retrieval protocols.

RESULTS

Immunohistochemistry of FFPE-derived nerve samples with selected antibodies and specific antigen retrieval methods enabled identification of myelinated and unmyelinated axons, cholinergic, sympathetic and peptidergic axons. The recurrent laryngeal and phrenic nerves are composed of myelinated cholinergic (motor), myelinated sensory fibres, unmyelinated adrenergic (sympathetic) axons and unmyelinated peptidergic (sensory) axons. In contrast, as expected, the plantar digital nerve had no myelinated motor fibres being mainly composed of myelinated sensory fibres, unmyelinated sympathetic and unmyelinated peptidergic sensory axons.

MAIN LIMITATION

Attempts specifically to label parasympathetic fibres were unsuccessful in any nerve examined in both FFPE and FFF tissues.

CONCLUSIONS

A panel of antibody markers can be used to reveal morphological and functional properties of equine nerves. Future work should enable better characterisation of morphological changes in equine neuropathies at various stages of disease development.

[Changes in the innervation of the taste buds in diabetic rats]

INTRODUCTION

Abnormal sensations such as pain and impairment of taste are symptoms of approximately 10% of patients having diabetes mellitus.

AIM

The aim of the study was to investigate and quantify the different neuropeptide containing nerve fibres in the vallate papilla of the diabetic rat.

METHODS

Immunohistochemical methods were used to study the changes of the number of different neuropeptide containing nerve terminals located in the vallate papillae in diabetic rats. Diabetes was induced in the rats with streptozotocin.

RESULTS

Two weeks after streptozotocin treatment the number of the substance P, galanin, vasoactive intestinal polypeptide and neuropeptide Y immunoreactive nerve terminals was significantly increased (p<0.05) in the tunica mucosa of the tongue. The number of the lymphocytes and mast cells was also increased significantly. Some of the immunoreactive nerve terminals were located in the lingual epithelium both intragemmally and extragemmally and were seen to comprise dense bundles in the lamina propria just beneath the epithelium. No taste cells were immunoreactive for any of the investigated peptides. Vasoactive intestinal polypeptide and neuropeptide Y immunoreactive nerve fibres were not detected in the taste buds. For weeks after streptozotocin administration the number of the substance P, calcitonin gene related peptide and galanin immunoreactive nerve terminals was decreased both intragemmally and intergemmally. In case of immediate insulin treatment, the number of the immunoreactive nerve terminals was similar to that of the controls, however, insulin treatment given 1 week later to diabetic rats produced a decreased number of nerve fibers. Morphometry revealed no significant difference in papilla size between the control and diabetic groups, but there were fewer taste buds (per papilla).

CONCLUSIONS

Increased number of immunoreactive nerve terminals and mast cells 2 weeks after the development of diabetes was the consequence of neurogenic inflammation which might cause vasoconstriction and lesions of the oral mucosa. Taste impairment, which developed 4 weeks after streptozotocin treatment could be caused by neuropathic defects and degeneration or morphological changes in the taste buds and nerve fibres.

Otolaryngological aspects of sudden infant death syndrome

INTRODUCTION

Sudden infant death syndrome (SIDS) is characterized by the sudden death of an apparently otherwise healthy infant, typically during sleep, and with no obvious case after a thorough post-mortem and scene death examination.

OBJECTIVE

To address the problem from the otolaryngologist's perspective, describe relevant pathologies, discuss controversies and suggest preventive measures in high-risk populations.

METHODOLOGY

A MEDLINE search and hand search were conducted to identify reports published between 1969 and 2011 in the English language on the pathophysiology of SIDS related to the head and neck organs. Search terms included SIDS (MeSH term), SIDS and pathophysiology (text words), and SIDS and autopsy (text words).

DISCUSSION

A growing number of reports suggested head and neck organs involvement in SIDS autopsies. Laryngeal, oropharyngeal, maxillofacial, otologic, cervical vascular abnormalities and infectious etiologies, were recognized and discussed.

CONCLUSIONS

Otolaryngologists should be aware of relevant pathologies, as some are treatable, if identified early enough in infancy. A proactive risk-management approach is warranted in infants presenting with certain abnormalities reviewed here.

Morphological development and expression of neurotrophin receptors in the laryngeal sensory corpuscles

Morphological development of sensory structures in the laryngeal mucosa of postnatal rats was observed by use of immunohistochemistry for protein gene-product 9.5 (PGP9.5). Moreover, expression changes of high affinity neurotrophin receptors, TrkA, TrkB and TrkC, and low affinity neurotrophin receptor p75(NTR) were examined to elucidate the relationship to morphogenesis. Intraepithelial nerve endings and parent axons of the laminar endings with immunoreactivity for PGP9.5 have already appeared in the rat on embryonic day 18 (E18) as well as solitary chemoreceptor cells in the glottic cleft. According to neurotrophin receptors, TrkA immunoreactivity were observed on and after postnatal week 3 (3W) in the nervous sensory structures, that is, free nerve endings, laminar endings and sub- and intragemmal plexuses of the taste buds. In the laminar endings, TrkC immunoreactivity was also observed on and after 3W. According to the laryngeal sensory cells, the solitary chemoreceptor cells were immunoreactive to TrkA, TrkB, and TrkC on and after postnatal day 3 (P3). In the taste buds in arytenoid region, taste cells were immunoreactive for TrkA, TrkB, and TrkC on and after 3W, P14, and 3W, respectively. Immunoreactivity for p75(NTR) was observed on the surface of taste cells on and after P9. The results of the present study suggest that sensory structures in the laryngeal mucosa were developed on perinatal days to involve respiratory reflex, and that neurotrophin receptors may take part in the regulation and maintenance of sensory structures.

Upper airway reflexes in response to gastric reflux

Gastric reflux, and especially laryngopharyngeal reflux, occur several times a day in every infant. Most often, this does not pose any problem. However, in certain conditions, the contact between the refluxate and the upper airway mucosa can trigger several reflexes leading to cardiorespiratory inhibition. This is especially true for the laryngeal chemoreflexes, which are triggered by laryngeal penetration of gastric refluxate. The laryngeal chemoreflexes are held responsible for a subset of apnoeas of prematurity, many apparent life-threatening events, and probably some cases of sudden infant death syndrome. Although a number of experiments in newborn animals, as well as a few clinical studies in human infants, have been performed in the last 40 years to evaluate laryngeal chemoreflexes, their true role in neonatal cardiorespiratory events is still highly debated. In addition, many uncertainties persist with regard to treatment and prevention of their potentially dramatic consequences.

Neurochemical characterization of sea lamprey taste buds and afferent gustatory fibers: presence of serotonin, calretinin, and CGRP immunoreactivity in taste bud bi-ciliated cells of the earliest vertebrates

Neuroactive substances such as serotonin and other monoamines have been suggested to be involved in the transmission of gustatory signals from taste bud cells to afferent fibers. Lampreys are the earliest vertebrates that possess taste buds, although these differ in structure from taste buds in jawed vertebrates, and their neurochemistry remains unknown. We used immunofluorescence methods with antibodies raised against serotonin, tyrosine hydroxylase (TH), gamma-aminobutyric acid (GABA), glutamate, calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), calretinin, and acetylated alpha-tubulin to characterize the neurochemistry and innervation of taste buds in the sea lamprey, Petromyzon marinus L. For localization of proliferative cells in taste buds we used bromodeoxyuridine labeling and proliferating cell nuclear antigen immunohistochemistry. Results with both markers indicate that proliferating cells are restricted to a few basal cells and that almost all cells in taste buds are nonproliferating. A large number of serotonin-, calretinin-, and CGRP-immunoreactive bi-ciliated cells were revealed in lamprey taste buds. This suggests that serotonin participates in the transmission of gustatory signals and indicates that this substance appeared early on in vertebrate evolution. The basal surface of the bi-ciliated taste bud cells was contacted by tubulin-immunoreactive fibers. Some of the fibers surrounding the taste bud were calretinin immunoreactive. Lamprey taste bud cells or afferent fibers did not exhibit TH, GABA, glutamate, or NPY immunoreactivity, which suggests that expression of these substances evolved in taste buds of some gnathostomes lines after the separation of gnathostomes and lampreys.

Expression of ENaC subunits in sensory nerve endings in the rat larynx

We investigated the expression of three subunits of epithelial sodium channel (ENaC), alphaENaC, betaENaC and gammaENaC, in the nodose ganglion and laryngeal mucosa of rat by RT-PCR analysis and immunohistochemistry. PCR products of predicted size for alphaENaC, betaENaC and gammaENaC subunits were amplified from extract of nodose ganglion. Immunohistochemically, nodose ganglion neurons of medium to large diameter were immunoreactive for alphaENaC, betaENaC and gammaENaC. In the deep region of laryngeal submucosal layer, thick nerve fibers without varicosities were immunoreactive for alphaENaC, betaENaC and gammaENaC. In the laryngeal mucosa, terminal arborizations of the nerve endings, that immunoreacted for alphaENaC, betaENaC and gammaENaC were scattered in the lamina propria just beneath the epithelia of epiglottis and laryngeal vestibule. Double immunofluorescence with calretinin revealed that they were laminar nerve endings. Some thick nerve fibers near the laryngeal taste buds were also immunoreactive for betaENaC and gammaENaC, but negative for alphaENaC. In the larynx, ENaC channels may play important roles in mechanotransduction in the laminar endings and in the mechano- and chemotransductions in the taste bud-associated nerve fibers.

Immunohistochemical demonstration of lumbar intervertebral disc innervation in the dog

Low back pain is a common ailment in dogs, particularly in specific breeds such as the German shepherd dog. A number of structures such as facet joint capsules, ligaments, dorsal root ganglia, periosteum, vertebral endplates and meninges have been associated with this condition. Yet, in spite of all diagnostic efforts, the origin of pain remains obscure in a substantial proportion of all cases. A further structure often being involved in vertebral column disorders is the intervertebral disc. The presence of nerves, however, is a precondition for pain sensation and, consequently, structures lacking innervation can be left out of consideration as a cause for low back pain. Nerve fibres have been demonstrated at the periphery of the intervertebral disc in man, rabbit and rat. With regard to the dog, however, the extent of intervertebral disc innervation is still being disputed. The goal of the present study, therefore, was to substantiate and expand current knowledge of intervertebral disc innervation. Protein gene product (PGP) 9.5 was used for immunohistochemical examination of serial transversal and sagittal paraffin sections of lumbar discs from adult dogs. This general marker revealed nerve fibres to be confined to the periphery of the intervertebral discs. These results indicate that even limited pathological processes affecting the outer layers of the intervertebral disc are prone to cause low back pain.

Identification and characterization of a specific sensory epithelium in the rat larynx

A specific laryngeal sensory epithelium (SLSE), which includes arrays of solitary chemoreceptor cells, is described in the supraglottic region of the rat. Two plates of SLSE were found, one on each side of the larynx. The first plate was located in the ventrolateral wall of the larynx, and the second was located in the interarytenoidal region. In SLSE, immunoblotting showed the presence of alpha-gustducin and phospholipase C beta2 (PLCbeta2), which are two markers of chemoreceptor cells. At immunocytochemistry, laryngeal immunoreactivity for alpha-gustducin was localized mainly in solitary chemosensory cells. Double-label immunocytochemistry using confocal microscopy demonstrated that alpha-gustducin-expressing cells in large part colocalize type III IP3 receptor (IP3R3), another key molecule in bitter taste perception. However, some IP3R3-expressing cells do not colocalize alpha-gustducin. At ultrastructural immunocytochemistry, these cells showed packed apical microvilli, clear cytoplasmic vesicles, and cytoneural junctions. SLSE was characterized by high permeability to a tracer due to poorly developed junctional contacts between superficial cells. Junctions were short in length and showed little contact with the terminal web. Ultrastructural analysis showed deep pits among the superficial cells. In SLSE, high density of intraepithelial nerve fibers was found. The lamina propria of the SLSE appeared thicker than that in other supraglottic regions. It was characterized by the presence of a well-developed subepithelial nerve plexus. The immunocytochemical and ultrastructural data suggested that SLSE is a chemoreceptor located in an optimal position for detecting substances entering the larynx from the pharynx or the trachea.

Intraepithelial Nerve Fibers Project Into the Lumen of the Larynx

OBJECTIVES/HYPOTHESIS

Studies on the morphology and location of the sensory receptors in the laryngeal mucosa have resulted in insufficient and sometimes conflicting data. In the present study the authors analyzed the distribution and morphology of sensory nerve plexuses and terminal fibers in the laryngeal mucosa of the rat.

STUDY DESIGN

Two groups of Male Wistar rats were used in this laboratory study; the larynx of the first group were used to analyse the sensitive innervation of its epithelium, whereas the larynx of the second group (controls) were tested for the specificity of the antibodies used.

METHODS

The larynges of the animals were entirely removed after perfusion, and coronal or horizontal sections were immunoprocessed for further randomized analysis of the mucosa. Primary afferents were detected by immunoreaction to two widely recognized markers of sensory nerves, calcitonin gene-related peptide and substance P, and visualized using diaminobenzidine as a chromogen.

RESULTS

The nerve plexuses were more densely distributed in the dorsal half of the vocal folds and in the laryngeal aspect of the epiglottis. Dense networks of fine fibers with many varicosities en passant, immunoreactive for both calcitonin gene-related peptide and substance P, occurred in the lamina propria and along the epithelial thickness. Calcitonin gene-related peptide-immunoreactive and substance P-immunoreactive fibers extended across the epithelium and projected to the laryngeal lumen itself, reaching the space between the cilia.

CONCLUSION

The projection of intraepithelial nerve fibers into the lumen of the larynx indicates that in the absence of mucus, nerve endings may be exposed and thus receive direct stimulation from airborne substances. Furthermore, it suggests that the laryngeal mucosa of the rat may constitute an experimental model for studying the direct activation or manipulation of primary afferents at the periphery and neurogenic inflammation.

Contribution of free nerve endings in the laryngeal epithelium to CO2 reception in rats

The superior laryngeal nerve (SLN) contains CO2-sensitive fibers. In the laryngeal epithelium, two candidates for CO2 reception have been identified, namely the intraepithelial free nerve endings and the taste buds. To elucidate the contribution of free nerve endings to CO2 reception, electrophysiological activities were recorded during various stages of regeneration of nerve endings following SLN-crush in rats. The left SLN was crushed surgically and maintained from 4 to 40 days for regeneration of nerve endings. Laryngeal sections were processed for immunohistochemical staining of protein gene product 9.5 to observe regeneration of free nerve endings and taste buds in the epithelium. By day 4 after SLN-crush, both the free nerve endings and taste buds had disappeared. Regeneration of the free nerve endings was recognized from day 8, while that of the taste buds started at day 16. On day 40, the number of taste buds on SLN-crush side was similar to that on the untreated side. Electrophysiological recording of SLN throughout the regeneration period (excluding day 4), showed response to intralaryngeal 9% CO2 (stimulation or inhibition) whether or not taste buds were present. Our results showed intralaryngeal CO2 reception without taste bud involvement, indicating that the free nerve endings in the laryngeal epithelium are receptive to intralaryngeal CO2.

Taste buds and nerve fibers in the rat larynx: an ultrastructural and immunohistochemical study

We investigated the rat laryngeal taste buds and their innervation by electron microscopy and immunohistochemical methods. Taste buds were densely arranged in the surface facing the laryngeal cavity of the epiglottis, the aryepiglottic fold, and the cuneiform process of the arytenoid cartilages. The cells of the buds were classified into types I, II, III, and basal cells, the ultrastucture of which was almost the same as that previously reported in lingual taste buds. The type III cells that had synaptic contacts with nerve fibers were considered to be sensory cells. Immunohistochemical analysis revealed thick calbindin D28k-immunoreactive fibers and thin varicose fibers immunoreactive for calcitonin gene-related peptide or substance P in and around the taste bud. Serotonin-immunoreactive cells were also observed here. The results revealed the innervation pattern of laryngeal taste buds to be the same as that in lingual taste buds. Carbonic anhydrase (CA) is known to catalyze the hydration of CO2 and dehydration of H2CO3, and seems to be essential in CO2 reception. Immunoreactivity for CAI was detected in slender cells and that for CAIII was observed in barrel-like cells in the laryngeal taste buds. The pH-sensitive inward rectifier K+ (Kir) channel in the cell membrane may be involved in CO2 reception as well. CAII-reactive cells were also reactive to Kir4.1, PGP 9.5 and serotonin. Our results indicated that CAII and Kir4.1 are located in type III cells of the laryngeal taste buds, and supported the idea that the buds may be involved in the recognition of CO2.

Immunoelectron microscopic studies on protein gene product 9.5 and calcitonin gene-related peptide in vallate taste cells and related nerves in the guinea pig

On the basis of our previous report that protein gene product 9.5 (PGP 9.5)-immunoreactive nerve fibers and taste cells and calcitonin gene-related peptide (CGRP)-immunoreactive nerve fibers are found in guinea pig vallate papillae [Huang and Lu (1996b) Arch. Histol. Cytol. 59:433-441]. We speculated that PGP 9.5 might be a marker for taste receptor cells and that CGRP might play an important role in taste transmission. We, therefore, performed an immunohistochemical and ultrastructural analysis of taste cells and related nerves in guinea pig vallate papillae. In the connective tissue of the vallate papilla, the ultrastructural data revealed that the PGP 9.5-immunoreactive nerve fibers were both myelinated and unmyelinated. The CGRP-immunoreactive nerve fibers were unmyelinated and surrounded by the cytoplasm of Schwann cells as were the non-immunoreactive fibers. In the vallate taste buds, only type III cells, which make synaptic contacts with intragemmal nerves, were PGP 9.5-immunoreactive, while the nerve terminals making synaptic contact with the underlying type III cells were CGRP-immunoreactive. From these observations, we conclude that: (1) PGP 9.5 might be a useful specific marker for type III cells in guinea pig vallate taste buds; and (2) CGRP-containing nerve fibers might be primarily involved in the neural transmission of taste stimuli.

Airway receptors

There are many types of afferent receptor in the airways; at least five in the larynx: pressure, drive, cold, irritant and C-fibre; and at least four in the trachea and bronchi: slowly and rapidly adapting stretch receptors (SARs and RARs), C-fibre receptors, and those in neuroepithelial bodies (NEBs). Histologically enough sensory structures have been identified to account for the various patterns of afferent activity, but most correlations are poor. For the larynx, four or more sensory structures have not definitively been identified with afferent discharges and reflex responses. For the trachea and bronchi, only SARs have been clearly identified morphologically and physiologically. The reflexes and afferent discharges from RARs and C-fibre receptors are fairly clear, some at least of the sensory terminals lie in the epithelium, but receptor complexes have not been mapped out. Nerves in NEBs have been identified, but not their local and central reflex actions.

Morphology of the nerve endings in laryngeal mucosa of the horse

To discuss the significance of laryngeal sensation on various disorders of the horse, we studied the morphological and topographical characteristics of sensory structures in the laryngeal mucosa using immunohistochemistry and immunoelectron microscopy. Various sensory structures, i.e. glomerular endings, taste buds and intraepithelial free nerve endings, were found in the laryngeal mucosa by immunohistochemistry for protein gene product 9.5 (PGP 9.5) and neurofilament 200kD (NF200). Glomerular nerve endings were distributed mainly in the epiglottic mucosa; some endings were also found in the arytenoid region arising from thick nerve fibres running through the subepithelial connective tissue. Some terminals directly contacted the epithelial cells. Taste buds were distributed in the epithelium of the epiglottis and aryepiglottic fold. In the whole mount preparation, the taste buds were supplied by the terminal branching of the thick nerve fibres. In some cases, the taste buds were arranged around the opening of the duct of the epiglottic glands. The intraepithelial free nerve endings were found to be immunoreactive for substance P (SP) and calcitonin gene-related peptide (CGRP). These nerve endings were surrounded by the polygonal stratified epithelial cells in the supraglottic region, and by the ciliated cells in the subglottic region. The density of the intraepithelial free nerve endings was highest in the corniculate process of the arytenoid region and lowest in the vocal cord mucosa. The densities of CGRP- and SP-immunoreactive nerve endings in the arytenoid region were (mean +/- s.d.) 30.6+/-12.0 and 10.0+/-4.9 per unit epithelial length (1 mm), respectively and in the vocal fold mucosa, 1.1+/-0.9 and 0.8+/-0.7, respectively. Approximately one half of the CGRP immunoreactive nerve endings were immunoreactive for SP, and most SP-immunoreactive nerve endings were also immunoreactive for CGRP. Well-developed subepithelial plexus with numerous intraepithelial fibres were observed in flat or round mucosal projections that existed on the corniculate process of the arytenoid region. In conclusion, the laryngeal mucosa of the horse seems to have morphology- and/or location-dependent sensory mechanisms against various endo-and exogenious stimuli.

Calbindin D28k-immunoreactive afferent nerve endings in the laryngeal mucosa

The distribution of the calbindin D28k in the laryngeal sensory structures was studied by immunohistochemistry, immunoelectronmicroscopy, and double immunofluorescence with calretinin-immunoreactivity. Moreover, origin of the nerve endings were observed using retrograde tracer, fast blue. Immunoreactivity for calbindin D28k was found in the various types of nerve endings in the larynx, namely, laminar nerve endings, nerve endings associated with the taste buds, intraepithelial nerve endings, and endocrine cells. The laminar endings with calbindin D28k-immunoreactivity were observed in the subepithelial connective tissue. In some endings, terminals were expanded. The laminar endings were also observed in the perichondrium of the epiglottic cartilage. In the epiglottic and arytenoid epithelia, thick nerve fibers with calbindin D28k-immunoreactivity ascending to taste buds and intragemmal nerve fibers were also observed. Within the epithelial layer, intraepithelial free nerve endings with calbindin D28k-immunoreactivity were observed. Furthermore, diffuse endocrine cells were observed within the laryngeal epithelium. By immunoelectron microscopy, immunoreaction products in the endings mentioned above were localized in the cytoplasm of the axon terminals and nerve fibers which contained with numerous mitochondria. Out of the 100 laminar endings, 18 endings were immunopositive for both calbindin D28k and calretinin, 33 were positive for calbindin D28k but negative for calretinin, and 49 were positive for only calretinin in the double immunofluorescence microscopy. The nerve fibers associated with the taste buds and the free nerve endings, which immunostained for calbindin D28k, were not stained with antibody against calretinin. After injection of the fast blue in the laryngeal mucosa, fast blue-labeled cells were mainly observed in the nodose ganglia. Of the total number of labeled cell in the nodose and dorsal root ganglia at the level C1 to Th2, 65.1% occurred in nodose ganglia (572/879, n = 6). In the nodose ganglia, 79.7% of labeled cells (456/572) were immunoreacted for calbindin D28k. The distribution of calbindin D28k-immunoreactivity may be differnt from that of calretinin. It is suggested that calbindin D28k have regulatory role on intracellular calcium concentration in the laryngeal sensory corpuscles.

Sensory receptors of the larynx

The larynx is a highly reflexogenic area, and stimulation with mechanical and chemical stimuli results in a number of protective reflexes. Investigators have used anatomical, behavioral, and neurophysiological techniques to examine the receptors responsible for initiating these reflex responses. Histologic examination has revealed the presence of free nerve endings, Merkel cells, Meissner corpuscles, and taste buds. Mechanoreceptors have been classified in several different ways and are located either in the superficial mucosa or in muscles and laryngeal joints. Recordings from afferent fibers innervating laryngeal mechanoreceptors have revealed that some of them are spontaneously active whereas others are silent until stimulated. Laryngeal mechanoreceptors respond to stimulation with either a rapidly adapting or a slowly adapting response pattern. Often the mechanoreceptors respond to respiratory movement of the larynx, giving bursts of action potentials during inspiration. A large number of taste buds that are anatomically similar to lingual taste buds populates the laryngeal surface of the epiglottis. Taste buds of the larynx respond to a number of chemical stimuli and to water. They do not respond to NaCl solutions close to physiological concentrations (0.154 M) but do respond at both a lower and higher concentration. When water is the solvent for the chemical stimuli, most chemicals initiate a response in laryngeal taste buds. However, when 0.154 M saline is used as a solvent, chemicals that taste bitter or sweet when applied to the tongue are ineffective stimuli. Taste buds of the larynx tend to be stimulated by the pH and tonicity of the stimulating solution and not by the gustatory properties. These results reveal a fundamental difference between the chemoreceptors of the oral cavity and larynx and result in the conclusion that chemoreceptors of the larynx do not play a role in gustation but are adapted to detect chemicals that are not saline-like in composition.

The existence of Merkel cells in the lingual connective tissue of the Surinam caiman, Caiman crocodilus crocodilus (order Crocodilia)

The tongue of the Surinam caiman (a reptilian species) was studied by light microscopy including immunohistochemistry for protein gene product 9.5 (PGP 9.5), and transmission electron microscopy. The connective tissue immediately under taste buds housed a cluster of cells immunoreactive for PGP 9.5. These cells synapsed on nerves, and their cytoplasm contained characteristic granules of 90 nm in the mean diameter, glycogen particles, and bundles of intermediate filaments. In light of these ultrastructural features, they were identified as Merkel cells. The Merkel cells were also surrounded by Schwann cells. These findings indicate that the present Merkel cell-neurite-Schwann cell complex is comparable to the avian Merkel corpuscle. On the basis of the granule localization in the cytoplasm, the caiman Merkel cell was presumed to be involved in not only mechanoreception but also endocrine or paracrine functions.

Morphology of intraepithelial corpuscular nerve endings in the nasal respiratory mucosa of the dog

Corpuscular nerve endings in the nasal respiratory mucosa of the dog were investigated by immunohistochemical staining specific for protein gene product 9.5 by light and electron microscopy. In the nasal respiratory mucosa, complex corpuscular endings, which displayed bulbous, laminar and varicose expansions, were distributed on the dorsal elevated part of the nasal septum and on the dorsal nasal concha. The endings were 300-500 microm long and 100-250 microm wide. Some axons gave rise to a single ending while others branched into 2 endings. Cryostat sections revealed that the corpuscular endings were located within the nasal respiratory epithelium. On electron microscopy, immunoreactive nerve terminals that contained organelles, including mitochondria and neurofilaments, were observed within the epithelial layer near the lumen of the nasal cavity. Some terminals contacted the goblet cell. Such terminal regions were covered by the cytoplasmic process of ciliated cells and were never exposed to the lumen of the nasal cavity. These nerve endings are probably activated by pressure changes.

Neurochemical markers in the nervous plexus of the canine glottis

The structure of the nervous network and the distribution of tyrosine hydroxylase (TH)- and various neuropeptide-containing nerves were immunohistochemically studied in the glottis of the dog. The nervous network in the glottis revealed apparent regional differences in morphology. The nervous network in the cartilaginous vocal fold of the posterior glottis consisted of nerve bundles running parallel to the edge of the vocal fold. Only a small number of nerve bundles were observed in the anterior glottis, specifically in membranous vocal fold. In the subepithelial layer of the posterior glottis, a moderate number of galanin (GAL)-immunoreactive nerve fibers were observed, while only a few fibers were present in the anterior glottis. Numerous vasoactive intestinal peptide (VIP)-, GAL-, methionine-enkephalin (ENK)- and TH-immunoreactive nerve fibers were observed within and around the laryngeal submucosal seromucous gland. Many TH- and neuropeptide Y (NPY)-immunoreactive fibers were arranged around the blood vessels. In the epithelia, free nerve endings with immunoreactivity for substance P (SP) and calcitonin gene-related peptide (CGRP) was observed. Furthermore, nerve cell bodies with SP-, VIP-, GAL-, ENK-, and NPY-immunoreactivity were observed in the deep region of the submucosal layer. The results from the present study suggest that there is autonomic regulation of the glottis. Regional structural differences in the nervous network of the glottis may reflect functional differences.

Morphological and quantitative study of the intrinsic nerve plexuses of the canine trachea as revealed by immunohistochemical staining of protein gene product 9.5

BACKGROUND

The ganglionated nervous plexuses in the trachea play an important role in the regulation of respiration. Although the tracheal tissue of the dog has frequently been used in physiological and pharmacological studies, little is known about the morphology of the intrinsic nervous plexuses in the dog trachea and quantitative data about the ganglia and nerve cell bodies are lacking. The structure of the nervous plexuses and detailed morphometric data about the intrinsic neurons of the dog have not previously been reported.

METHODS

The structure of the intrinsic nerves in the dog trachea was examined by a combination method of digestion with KOH and immunohistochemical attaining of protein gene product 9.5. In addition, areas of nerve cell profiles, numbers of nerve cell bodies per ganglion and densities of nerve cell bodies per ganglion were estimated in preparations from five dogs.

RESULTS

In the dog trachea, the peritracheal plexus, outer submucosal plexus, inner submucosal plexus, and mucosal plexus were identified from adventitia to epithelium in that order. The peritracheal plexus, situated in the lamina adventitia of the trachea, consisted of thick nerve bundles that were densely distributed in the membranous wall. The peritracheal plexus contained ganglia with round or oval nerve cell bodies. In the trachealis muscle, the intramuscular plexus, which was a well-developed, three-dimensional nervous network, was observed. It did not include ganglia and consisted of inter- and intrafascicular networks. The outer submucosal plexus was observed as small clusters in the inside of the trachealis muscle. A few round nerve cells were observed in the outer submucosal plexus as small clusters. The ganglionated inner submucosal plexus was identified in the superficial layer of the submucosal layer. A finer meshwork, namely, the mucosal plexus, was observed in the lamina propria. Among the entire length of the trachea, we counted 2,134-2,873 ganglia (average, 2,389) and 13,902-24,232 nerve cell bodies (average,18,461) were counted in the peritracheal plexuses of five dogs. The densities of ganglia and nerve cell bodies were high near the carina and low near the thoracic inlet. The number of nerve cell bodies per ganglion was estimated as 6.5-8.8 (average, 7.7). The average maximum area of nerve cell profiles was 522 +/- 223 microm2.

CONCLUSIONS

The structure of the tracheal plexuses in the dog is more complex than those in other smaller animals. The constitution of the plexuses seems to be that of the intestine. The numerous nerve cells in the peritracheal plexus in the adventitia can be expected to play important roles in airway regulation and in the pathogenesis of tracheal collapse.