Non-innervated Merkel cells and Merkel-neurite complexes in human oral mucosa revealed using antiserum to protein gene product 9.5

Somatosensory innervation of healthy human oral tissues

The oral somatosensory system relays essential information about mechanical stimuli to enable oral functions such as feeding and speech. The neurochemical and anatomical diversity of sensory neurons across oral cavity sites have not been systematically compared. To address this gap, we analyzed healthy human tongue and hard-palate innervation. Biopsies were collected from 12 volunteers and underwent fluorescent immunohistochemistry (≥2 specimens per marker/structure). Afferents were analyzed for markers of neurons (βIII tubulin), myelinated afferents (neurofilament heavy, NFH), and Merkel cells and taste cells (keratin 20, K20). Hard-palate innervation included Meissner corpuscles, glomerular endings, Merkel cell-neurite complexes, and free nerve endings. The organization of these somatosensory endings is reminiscent of fingertips, suggesting that the hard palate is equipped with a rich repertoire of sensory neurons for pressure sensing and spatial localization of mechanical inputs, which are essential for speech production and feeding. Likewise, the tongue is innervated by afferents that impart it with exquisite acuity and detection of moving stimuli that support flavor construction and speech. Filiform papillae contained end bulbs of Krause, as well as endings that have not been previously reported, including subepithelial neuronal densities, and NFH+ neurons innervating basal epithelia. Fungiform papillae had Meissner corpuscles and densities of NFH+ intraepithelial neurons surrounding taste buds. The differing compositions of sensory endings within filiform and fungiform papillae suggest that these structures have distinct roles in mechanosensation. Collectively, this study has identified previously undescribed neuronal endings in human oral tissues and provides an anatomical framework for understanding oral mechanosensory functions.

Somatosensory innervation of the oral mucosa of adult and aging mice

Oral mechanoreception is implicated in fundamental functions including speech, food intake and swallowing; yet, the neuroanatomical substrates that encode mechanical stimuli are not well understood. Tactile perception is initiated by intricate mechanosensitive machinery involving dedicated cells and neurons. This signal transduction setup is coupled with the topology and mechanical properties of surrounding epithelium, thereby providing a sensitive and accurate system to detect stress fluctuations from the external environment. We mapped the distribution of anatomically distinct neuronal endings in mouse oral cavity using transgenic reporters, molecular markers and quantitative histomorphometry. We found that the tongue is equipped with an array of putative mechanoreceptors that express the principal mechanosensory channel Piezo2, including end bulbs of Krause innervating individual filiform papillae and a novel class of neuronal fibers innervating the epithelium surrounding taste buds. The hard palate and gums are densely populated with three classes of sensory afferents organized in discrete patterns including Merkel cell-neurite complexes, Meissner’s corpuscles and glomerular corpuscles. In aged mice, we find that palatal Merkel cells reduce in number at key time-points that correlate with impaired oral abilities, such as swallowing and mastication. Collectively, this work identifies the mechanosensory architecture of oral tissues involved in feeding.

Somatosensory innervation of the oral mucosa of adult and aging mice

Oral mechanoreception is implicated in fundamental functions including speech, food intake and swallowing; yet, the neuroanatomical substrates that encode mechanical stimuli are not well understood. Tactile perception is initiated by intricate mechanosensitive machinery involving dedicated cells and neurons. This signal transduction setup is coupled with the topology and mechanical properties of surrounding epithelium, thereby providing a sensitive and accurate system to detect stress fluctuations from the external environment. We mapped the distribution of anatomically distinct neuronal endings in mouse oral cavity using transgenic reporters, molecular markers and quantitative histomorphometry. We found that the tongue is equipped with an array of putative mechanoreceptors that express the principal mechanosensory channel Piezo2, including end bulbs of Krause innervating individual filiform papillae and a novel class of neuronal fibers innervating the epithelium surrounding taste buds. The hard palate and gums are densely populated with three classes of sensory afferents organized in discrete patterns including Merkel cell-neurite complexes, Meissner's corpuscles and glomerular corpuscles. In aged mice, we find that palatal Merkel cells reduce in number at key time-points that correlate with impaired oral abilities, such as swallowing and mastication. Collectively, this work identifies the mechanosensory architecture of oral tissues involved in feeding.

Innervation of the Human Incisive Papilla: Comparison with Other Oral Regions

Immunohistochemistry for several neurochemical substances was performed on the human incisive papilla and other oral structures. Sodium channel alpha subunit 7 (SCN7A) protein-immunoreactive (IR) Schwann cells and protein gene product 9.5 (PGP 9.5)-IR nerve fibers made nerve plexuses beneath the epithelium of the palate, including the incisive papilla, tongue, and lip. SCN7A immunoreactivity could also be detected in lamellated and nonlamellated capsules of corpuscle endings. Lamellated SCN7A-IR corpuscle endings were mostly restricted to the mucous and cutaneous lips. These endings had thick and spiral-shaped PGP 9.5-IR axons without ramification. Nonlamellated SCN7A-IR corpuscle endings were most numerous in the incisive papilla among the oral regions. On the basis of axonal morphology, the nonlamellated endings were divided into simple and complex types. PGP 9.5-IR terminal axons in the simple type ran straight or meandered with slight ramification, whereas those in the complex type were densely entangled with abundant ramification. Substance P (SP)-, calcitonin gene-related peptide (CGRP)-, and transient receptor potential cation channel subfamily V member 2 (TRPV2)-IR varicose fibers were rarely seen beneath the epithelium of oral structures. The present study indicates that the human incisive papilla has many low-threshold mechanoreceptors with nonlamellated capsules. SP-, CGRP-, and TRPV2-containing nociceptors may be infrequent in the incisive papilla and other oral regions.

Expression of calcitonin gene-related peptide in rat pulp and periodontal tissues by indirect immunofluorescence method

The aim of this study was to investigate the expression of nerve fibers immunoreactive to calcitonin gene-related peptide (CGRP) in pulp and periodontal tissues of rats. Male Sprague-Dawley rats, aged 6 weeks, were sacrificed, and the jaws were excised, demineralized, and processed for indirect immunofluorescence staining. A considerably higher density of nerve fibers immunoreactive to CGRP was found in the dental pulp and gingiva than in periodontal ligament. The majority of pulpal CGRP immunopositive fibers that were located followed blood vessels parallel to the long axis of the root. A subodontoblastic network of fibers IR to CGRP was found in the coronal pulp in rat molars. In the periodontium, CGRP immunopositive fibers were mainly located in the periapical area and close to the alveolar bone. Gingiva was also well supplied with CGRP-IR nerves.

Sensory innervation of the female human umbilical skin: morphological studies

BACKGROUND

Sensory stimuli are conducted by several cutaneous sensory nerves and tactile corpuscles. The latter are specialized sensory organs that represent the starting point of many afferent sensory pathways. To date, our knowledge about the distribution of the sensory innervation in the umbilical skin of females is incomplete.

AIM OF THE STUDY

To elucidate the morphology of the cutaneous innervation of the normal female umbilical skin.

MATERIALS AND METHODS

Biopsies of normal umbilical skin were obtained from female patients undergoing umbilical hernial repair. The specimens were processed for both immunohistological (antibodies against PGP9.5, pan-neuronal marker, and S-100 protein, marker of Schwann cells) and ultrastructural (transmission electron microscopy) examinations.

RESULTS

The authors found abundant genital end-bulb-like structures, numerous epidermal and dermal Merkel cells, Meissner and Ruffini corpuscles, intraepidermal nerve terminals, and multiple free nerve endings surrounding the ducts and acini of the sweat glands.

CONCLUSIONS

The umbilical skin of females has abundant sensory innervation similar to that of the glans penis.

Evidence of fast serotonin transmission in frog slowly adapting type 1 responses

The Merkel cell-neurite (MCN) complex generates slowly adapting type 1 (SA1) response when mechanically stimulated. Both serotonin (5-HT) and glutamate have been implicated in the generation of normal SA1 responses, but previous studies have been inconclusive as to what their roles are or how synaptic transmission occurs. In this study, excised dorsal skin patches from common water frogs (Rana ridibunda) were stimulated by von Frey hairs during perfusion in a tissue bath, and single-unit spike activity was recorded from SA1 fibres. Serotonin had no significant effect on the SA1 response at low (10 µM) concentration, significantly increased activity in a force-independent manner at 100 µM, but decreased activity with reduced responsiveness to force at 1 mM. Glutamate showed no effect on the responsiveness to force at 100 µM. MDL 72222 (100 µM), an ionotropic 5-HT3 receptor antagonist, completely abolished the responsiveness to force, suggesting that serotonin is released from Merkel cells as a result of mechanical stimulation, and activated 5-HT3 receptors on the neurite. The metabotropic 5-HT2 receptor antagonist, ketanserin, greatly reduced the SA1 fibre's responsiveness to force, as did the non-specific glutamate receptor antagonist, kynurenic acid. This supports a role for serotonin and glutamate as neuromodulators in the MCN complex, possibly by activation and/or inhibition of signalling cascades in the Merkel cell associated with vesicle release. Additionally, it was observed that SA1 responses contained a force-independent component, similar to a dynamic response observed during mechanical vibrations.

Merkel cells and permanent disesthesia in the oral mucosa after soft tissue grafts

Connective tissue grafts are routinely procedures in the treatment of gingival defects. The clinical success of the gingival tissue graft procedures anyway should ensure not only the aesthetic integration between the tissues but also the physiological activity of the graft in terms of sensitivity and immunity because the skin and the mucosae constitute the first natural aspecific borders against pathogens. The aim of this paper was to investigate nervous net recovery after connective graft procedure, in relation with sensorial alteration in the injured area. Results showed that there is a close link among the number of Merkel cells and the alteration of sensations. Merkel cells can be found isolated standing in the basal layer, supposed to have neuroendocrine functions in the epithelia or in larger group not associated with nerves; when found in association with nerves they are named Merkel complexes, acting as slow adapter mechanical receptor. Our data can be explained in two ways: Merkel cells increase as a consequence of tissue injury, a sort of "SOS cells" that secrete neuroendocrine signals to guide tissue healing; as an alternative the presence of the Merkel cells could be read as a derailment of tissue regeneration with the stop of cellular differentiation in the direction of an abnormal proliferation, a sort of mad stem cell.

Neuroendocrine cells and associated malignancies of the oral mucosa: a review

Neuroendocrine cells of the oral mucosa constitute an under-recognized component of the diffuse neuroendocrine system with diverse subpopulations and elusive biologic roles in the oral cavity. Primary malignant oral tumors that show a neuroendocrine phenotype display histomorphologic heterogeneity thereby giving rise to a spectrum of lesions in this rare category of oral malignancy. These lesions can be divided into neuroendocrine carcinomas (NECs) of small cell or non-small cell type. The former is further subdivided into the Merkel cell type or the pulmonary type while the latter includes atypical carcinoid tumor and large cell NEC. All histologic subtypes of oral NEC appear to have a strong predilection for men in their fifth or sixth decade and arise predominantly in the non-keratinized oral mucosa. The biologic behavior of oral Merkel cell carcinomas appears to be more aggressive than those of skin. It remains to be determined whether histologic categorization of the remaining tumor subtypes is predictive of patient survival in oral neuroendocrine tumors.

Current considerations about Merkel cells

Since the discovery of Merkel cells by Friedrich S. Merkel in 1875, knowledge of their structure has increased with the progression of new technologies such as electron and laser microscopy, and immunohistochemical techniques. For most vertebrates, Merkel cells are located in the basal layer of the epidermis and characterized by dense-core granules that contain a variety of neuropeptides, plasma membrane spines and cytoskeletal filaments consisting of cytokeratins and desmosomes. The presence of the two latter structures would suggest that Merkel cells originate from the epidermis rather than from the neural crest, even though such a hypothesis is not unanimously accepted. The function of the Merkel cell is also very controversial. For a long time, it has been accepted that Merkel cells with associated nerve terminals act as mechanoreceptors although the transduction mechanism has not yet been elucidated. Merkel cells that do not make contact with nerve terminals have an endocrine function. The present review aims to shed new and comparative light on this field with an attempt to investigate the stimuli that Merkel cells are able to perceive.

An immunohistochemical analysis of human Merkel cell density in gingival epithelium from dentate and edentulous subjects

OBJECTIVE

To investigate whether there is a difference in the density of Merkel cells between the gingiva of dentate and edentate subjects.

METHODS

One hundred and two blocks of human mandibular (n=55) and maxillary (n=47) gingiva from 69 white skinned individuals (44 males, 25 females, mean age 70.1 years, range 42--92 years) were analysed. Twenty-six individuals had teeth remaining in at least one jaw (mean age of dentate 64.5 years, edentate 72.1 years), of which 15 were dentate in both jaws. Overall, 24/55 mandibles and 17/47 maxillae were dentate. Five micrometer sections were stained with a monoclonal antibody to cytokeratin 20 (CK 20) using standard immunoperoxidase or immunoalkaline phosphatase methods. Positive cells were counted in 20 consecutive high power fields using the x 40 objective in three sections from each biopsy, and the results analysed for differences related to age, sex, site and presence of teeth.

RESULTS

CK 20-positive Merkel cells were present either singly or in clusters in the basal epithelial layers, particularly at the tips of rete ridges. Numbers of gingival Merkel cells were significantly higher (p<0.005, Mann-Whitney) in edentate (cells per field mean, median, standard deviation, respectively: 1.39, 0.64, and 1.85) when compared to dentate (0.67, 0.20, and 1.21) specimens. The differences were not statistically significant for any other variable.

CONCLUSION

The data raise the possibility that reduced perception following loss of teeth may be compensated for by an increase in the local Merkel cell population.

Growth-associated protein-43 immunoreactivity in human oral mucosa in dentate subjects, in edentulous patients and after implant-anchored rehabilitation

OBJECTIVES

This study investigates expression of the neural growth-associated protein 43 (GAP-43) in the oral mucosa of (A) normal dentate subjects, (B) edentulous patients rehabilitated with conventional denture and (C) those rehabilitated with mandibular implant-retained overdentures (MIR-OVD), in the long term. This study evaluates morphological changes in the distribution and representation of sensory terminations and corpuscles in the alveolar mucosa under the action of different masticatory or prosthetic loads, in the three clinical groups.

MATERIAL AND METHODS

GAP-43 immunoreactivity (-ir) was compared with the distribution of nerves fibres in the mucosa, as visualised using anti-protein gene product 9.5 (PGP 9.5), a general marker for peripheral nerves and terminals.

RESULTS

GAP-43-ir was found to be highly expressed in the corium and submucosa in specimens from edentulous subjects wearing conventional denture and presenting a reduced number of PGP 9.5-ir nerves in the mucosa, but not in specimens from control subjects or patients wearing MIR-OVD, which on the contrary show a higher number of PGP 9.5-ir mucosal sensory fibres.

CONCLUSION

As the mucosa under traditional denture has been shown to possess reduced innervation and the histological aspect of chronic overloading, these results may be considered indicative of a tentative induction to nerve re-growth in the under-innervated epithelium, or as a response to chronic inflammation. The detection of GAP-43-ir suggests that human oral mucosa presents signs of potential nerve plasticity also in the elderly, and that the type of rehabilitation and the condition of masticatory load transfer to the mucosa have important effects on the nerves underneath.

The role of NT-3 signaling in Merkel cell development

Merkel cells originate from the neural crest. They are located in hairy and glabrous skin and have neuroendocrine characteristics. Together with A beta afferents, Merkel cells form a slowly adapting mechanoreceptor, the Merkel nerve ending, which transduces steady skin indentation. Neurotphin-3 (NT-3) plays important roles in neural crest cell development. We thus sought to determine whether neurotrophin signaling is essential for Merkel cell development in the whisker pad of the mouse. Our data indicate that at embryonic day 16.5 (E 16.5), NT-3 and its receptors, p75 neurotrophin receptor (p75NTR) and tyrosine kinase receptor, TrkC are not expressed at detectable levels in Merkel cells. After a perinatal switch, however, Merkel cells in whiskers of newborn mice are immunoreactive for p75NTR, TrkC and NT-3. Immunoreactivity of all three markers persists into adulthood. By contrast, innervating fibers are intensely p75NTR-immunoreactive in E16.5 whiskers, but no TrkC immunoreactivity is detected. At birth, and at 6 weeks of age, afferent fibers are intensely immunoreactive for both p75NTR and TrkC. In TrkC null whiskers, numerous Merkel cells are present at E16.5, and they are innervated. We draw three major conclusions from these observations: (i) NT-3 signaling through p75NTR or TrkC is not required for the development and prenatal survival of either a major subset or of all Merkel cells, (ii) the postnatal survival of Merkel cells is supported by autocrine or paracrine NT-3, rather than by neuron-derived NT-3, and (iii) Merkel cell-derived NT-3 is not a chemoattractant for innervating A beta fibers, but is likely to be involved in maintaining Merkel cell innervation postnatally.

Distribution of GAP-43 nerve fibers in the skin of the adult human hand

Skin is an important region of somatic sensory input, and is one of the most innervated areas of the human body. In this study, we investigated in human hand skin the distribution of nervous structures immunoreactive for the growth-associated protein 43 (GAP-43) and the protein gene product 9.5 (PGP 9.5). GAP-43 is a neuronal presynaptic membrane protein that is generally considered to be a marker of neuronal plasticity. PGP 9.5 is a neuron-specific soluble protein that is widely used as general marker for the peripheral nervous system. The entire neural network of the dermis and epidermis was stained with antibody to PGP 9.5. In the dermis, there were fewer GAP-43-immunostained nerve fibers than PGP 9.5-immunostained nerve fibers, whereas in the epidermis the numbers were equal. Only some Merkel cells and Meissner corpuscles were GAP-43-immunoreactive. In conclusion, our results show that GAP-43 protein is expressed in a subset of PGP 9.5-immunoreactive nerve structures.

Friedrich Sigmund Merkel and his "Merkel cell", morphology, development, and physiology: review and new results

Merkel nerve endings are mechanoreceptors in the mammalian skin. They consist of large, pale cells with lobulated nuclei forming synapse-like contacts with enlarged terminal endings of myelinated nerve fibers. They were first described by F.S. Merkel in 1875. They are found in the skin and in those parts of the mucosa derived from the ectoderm. In mammals (apart from man), the largest accumulation of Merkel nerve endings is found in whiskers. In all vertebrates, Merkel nerve endings are located in the basal layer of the epidermis, apart from birds, where they are located in the dermis. Cytoskeletal filaments consisting of cytokeratins and osmiophilic granules containing a variety of neuropeptides are found in Merkel cells. In anseriform birds, groups of cells resembling Merkel cells, with discoid nerve terminals between cells, form Grandry corpuscles. There has been controversy over the origin of Merkel cells. Results from chick/quail chimeras show that, in birds, Merkel cells are a subpopulation of cells derived from the neural crest, which thus excludes their development from the epidermis. Most recently, also in mammals, conclusive evidence for a neural crest origin of Merkel cells has been obtained. Merkel cells and nerve terminals form mechanoreceptors. Calcium ions enter Merkel cells in response to mechanical stimuli, a process which triggers the release of calcium from intracellular stores resulting in exocytosis of neurotransmitter or neuromodulator. Recent results suggest that there may be glutamatergic transmission between Merkel cell and nerve terminal, which appears to be essential for the characteristic slowly adapting response of these receptors during maintained mechanical stimuli. Thus, we are convinced that Merkel cells with associated nerve terminals function as mechanoreceptor cells. Cells in the skin with a similar appearance as Merkel cells, but without contact to nerve terminals, are probably part of a diffuse neuroendocrine system and do not function as mechanoreceptors. Probably these cells, rather than those acting as mechanoreceptors, are the origin of a highly malignant skin cancer called Merkel cell carcinoma.

Expression of neuropeptides and growth-associated protein 43 (GAP-43) in cutaneous and mucosal nerve structures of the adult rat lower lip after mental nerve section

The reinnervation of the adult rat lower lip has been investigated after unilateral section of the mental nerve. Rats were sacrificed at 4, 7, 9, 14, 30, and 90 days after the operation. A further group of animals with section of the mental nerve and block of the alveolar nerve regeneration, was sacrificed at 14 days. Specimens were processed for immunocytochemistry with antibodies against PGP 9.5, GAP-43 or neuropeptides (CGRP, SP and VIP). Four days after nerve section, axonal degeneration seems evident in the mental nerve branches and inside skin and mucosa. GAP-43 immunoreactivity is intense in the mental nerve 7 days after nerve section and it reaches its maximal expression and distribution in peripheral nerve fibres at 14 days. At 30 days, the decline in its expression is associated with the increase of PGP9.5-, SP-, and CGRP immunopositivity. VIP is observed only in perivascular fibres at all times observed. Present results suggest that, after sensory denervation of the rat lip, nerve fibres in skin and mucosa remain at lower density than normal. The different time courses in the expression of neuropeptides and GAP-43 suggest a possible early involvement of GAP-43 in peripheral nerve regeneration.

Merkel-like cells in Malassez epithelium in the periodontal ligament of cats: an immunohistochemical, confocal-laser scanning and immuno electron-microscopic investigation

The cellular heterogeneity of Malassez epithelium (ME) residing in the periodontal ligament has recently been reported, and the presence and coexistence of the neuropeptides calcitonin gene-related peptide (CGRP), substance P (SP) and vasoactive intestinal peptide (VIP) in single cells in ME has been shown (1). However, the identity of these neuroendocrine cells has so far not been verified. This study was undertaken in order to elucidate the identity of the neuroendocrine cells in ME by means of transmission electron microscopy, confocal scanning microscopy and immunohistochemistry using antibodies to protein gene product (PGP) 9.5 and cytokeratin 20 (CK). Gingival tissue was included in the study as a positive control for identification of Merkel-like cells in oral epithelium. CK 20 immunopositive cells were present in both Malassez epithelium and in basal cell layers of gingival epithelium showing a distribution consistent with PGP 9.5 labelled cells in both epithelia. The results from PGP 9.5 immuno electron microscopy clearly evidenced the presence of single, intensely labelled cells and some nerve fibres invested between the Malassez epithelial cells. The conformity of the immunopositive cells in Malassez and gingival epithelium verified by double immunolabelling with PGP 9.5 and CK 20, indicates that the labelled neuroendocrine cells are identical in ME and in gingival epithelium. This demonstrates that Malassez epithelium not only exhibits neuroendocrine cells, but additionally that the neuroendocrine cells represent Merkel-like cells.

Recent progress in studies on Merkel cell biology

Merkel cells ubiquitously distribute in the skin of vertebrates, from cyclostomes to mammals. It is well known that mammalian Merkel cells coupled with axon terminals of type I sensory nerve fibers form slowly adapting mechanoreceptors, Merkel endings, within the epidermis. However, there are still many unresolved problems in the biology of Merkel cells. We reviewed recently acquired knowledge about the histochemical nature of Merkel cell granules, the morphological heterogeneity of Merkel cells and the roles of neurotrophins and their receptors for the development and survival of the cells. We discuss the functional significance of Merkel cell granules and the heterogeneity of Merkel cell populations.

Oral sensory papillae, chemo- and mechano-receptors, in the snake, Elaphe quadrivirgata. A light and electron microscopic study

The oral sensory papillae of the snake (Elaphe quadrivirgata), comprising a compound sensory system located along the tooth rows, were studied by light microscopy, immunohistochemistry for neuron specific enolase and S 100 protein, and scanning and transmission electron microscopy. Each sensory papilla exhibited a single taste bud and free nerve endings in the epithelium, and Meissner-like corpuscles, branched coiled terminals, and lamellated corpuscles in the connective tissue. The taste buds consisted of four types of cells; the type III cells, exclusively synapsing onto intragemmal nerves, were identified as gustatory in function. The gustatory cells included dense-cored and clear vesicles in the cytoplasm. These vesicles were accumulated both in the presynaptic and infranuclear regions, suggesting dual functions: the synaptocrine and paracrine/endocrine release of signal substances. The free nerve endings constantly contained mitochondria and frequent clear vesicles. The Meissner-like corpuscles were located in the uppermost zone of the connective tissue. These corpuscles consisted of nerve fibers and lamellar cells. The nerve fibers, rich in mitochondria, were folded and layered on each other. The branched coiled terminals were localized in the connective tissue along the side wall of the papillae. Nerve fibers, free from a Schwann-cell covering, swelled up to make terminals which accumulated mitochondria and glycogen particles. The lamellated corpuscles were associated with the nerve-fiber bundles in the connective tissue. Consisting of a central nerve axon and lamellar cells encircling it, these corpuscles resembled mammalian Vater-Pacini corpuscles, except that they lacked a capsule. These findings demonstrated that the snake sensory papilla represents one of the most specialized, compound sensory systems among vertebrates, which may play an important role in receiving chemical and mechanical information on prey.

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.

Tactile hyperesthesia, altered epidermal innervation and plantar nerve injury in the hindfeet of rats housed on wire grates

The effects of wire grates on nerve injury and recovery were examined in rats housed in cages with sawdust-covered solid flooring. For the first 3 weeks of the study, 20 rats were housed on sawdust alone and 20 rats were housed in cages with wire grates placed over the sawdust. For the remaining 9 weeks, 10 animals housed on sawdust had wire grates added to their cages, while grates were removed from the cages of 10 animals. The effects of tactile stimulation on hindpaw plantar skin was measured weekly using the Von Frey filament test. Intraepidermal innervation using PGP 9.5 immunostaining and plantar nerve histology were assessed at the end of the 12-week study. After just 1 week on grates, hindpaw withdrawal thresholds were already markedly decreased and remained low until the grates were removed at 3 weeks. Thresholds returned to normal by 4 weeks after removal of the grates. Wire grates also induced increases in PGP 9.5 immunoreactive intraepidermal fine nerve endings that were normalized after grate removal. Demyelination, Wallerian degeneration and Renaut bodies were induced in the medial plantar nerve in rats housed in cages with wire-grate flooring. Nerve injury was largely resolved after 9 weeks on sawdust flooring. These data demonstrate that wire grates rapidly induce hindpaw tactile hyperesthesia and plantar neuropathy in rats and emphasize a risk of using wire-grate cage flooring in studies assessing hindlimb function and structure.

Cellular and subcellular distribution of 7B2 in porcine Merkel cells

BACKGROUND

Merkel cells are neuroendocrine cells located in the skin and oral mucosa of various mammalian species. These cells express multiple peptides as well as serotonin. Although the precise function of Merkel cells is still unknown, different studies support its role as mechano-electric transducer. 7B2 granin (secretogranin V) is a polypeptide isolated from the pituitary gland and present in the dense-cored granules of neuronal and paraneuronal cells.

METHODS

The expression of the 7B2 in Merkel cells of pig snout skin was analysed by immunohistochemical techniques. The streptavidin-biotin peroxidase complex procedure was employed for light microscopy. A postembedding method using immunoglobulin-colloidal gold complexes was employed for the ultrastructural studies.

RESULTS

Immunoreactivity for 7B2 was observed in virtually all Merkel cells, both in epidermis and vibrissae. The immunostaining was shown in the basal side of cytoplasms where neuroendocrine granules were accumulated. Immunoelectron microscopy allowed us to demonstrate that 7B2 labelling was located on the electrondense granules. Nuclei and epidermal nerve terminals associated with merkel cells did not show immunoreactivity.

CONCLUSIONS

The polypeptide 7B2 is present in the dense-cored granules of Merkel cells. This result is consistent with the possible role for 7B2 in secretory granules' processing. To our knowledge this is the first evidence of 7B2 protein in Merkel cells.

Immunohistochemical localization of nerve fibres during development of embryonic rat molar using peripherin and protein gene product 9.5 antibodies

Nerve fibres were localized during the initiation and early morphogenesis of the first molar tooth in rat embryos by immunoperoxidase detection of the intermediate-filament protein peripherin and protein gene product 9.5 (PGP 9.5). Nerve fibres from the trigeminal ganglion were detected in the developing first branchial arch of E12-14 embryos. Nerves were not seen in the vicinity of the developing tooth germ before the buid stage (E15), when they were seen around the condensed dental mesenchyme. During transition from the bud to the cap stage (E15), nerve fibres were detected not only in the area of the future dental follicle but also in the mesenchyme next to dental epithelium on the buccal side of the tooth germ. During later cap and bell stages nerve fibres persisted in the dental follicle, but they were not seen in the epithelial dental organ or dental papilla mesenchyme. Absence of trigeminal nerve fibres from the presumptive tooth-bearing area indicates that they are not involved in the initiation of rat tooth development. In addition, the localization of nerve fibres shows that there are some differences in the innervation of rat teeth compared with human and mouse teeth. These results provide data for further studies on the regulation of embryonic rat tooth innervation.

Sensory innervation of the cat knee articular capsule and cruciate ligament visualised using anterogradely transported wheat germ agglutinin-horseradish peroxidase

Wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) was injected into the dorsal root ganglia (L5-S1) of the cat and used as an anterograde tracer substance for intra-axonal labelling of peripheral nerve endings in joint capsule and cranial (anterior) cruciate ligament (CCL). We believed that the high specificity of WGA-HRP for neural tissue along with the high visibility of its reaction product could help resolve controversies concerning the sensory innervation of the cruciate ligaments. Substantial amounts of WGA-HRP were transported in tibial nerve axons to the level of the knee. However, using standard HRP histochemistry we found that the capsular tissue and ligament synovia disintegrated during the incubation reaction. This problem was avoided by air drying the tissue slices on glass slides prior to reaction. Abundant labelling occurred in the posterior capsule with dense filling of axons and terminal endings. Sensory endings displayed features consistent with Ruffini endings and pacinian corpuscles. Sensory endings were located throughout the CCL in its sagittal plane, in the subsynovial layers and between collagen fascicles. In each CCL we observed 5-17 ovoid and elongated endings with dense terminal arborisations. These endings were between 100 and 150 microns long, were encapsulated, and gave rise to 1 or 2 axons. Large (up to 1.5 mm in maximum extent) elongated regions of dense, inhomogeneous labelling were found in the body of several CCLs. These resembled Golgi tendon-like endings, with the exception of their large size. We conclude that anterograde transport of HRP to the knee is a useful technique for labelling mechanoreceptors and axons in knee tissue. However, recently developed immunohistochemical analysis of peripheral tissue using protein gene product 9.5 appears to be the method of choice and should be employed for further study of human and animal cruciate ligament innervation.

Protein gene product 9.5-like and calbindin-like immunoreactivity in the nasal respiratory mucosa of perinatal humans

BACKGROUND

Protein gene product 9.5 (PGP) and calbindin-D28k (calbindin) are neuroendocrine markers that have been localized to neuroendocrine cells in the developing tracheobronchial epithelium. Neuroendocrine cells may play some role in the development of the tracheobronchial epithelium. Little is known about the development of the nasal respiratory epithelium (RE).

METHODS

Nasal respiratory mucosa from fetal and newborn humans was examined to determine immunoreactivity for PGP and calbindin.

RESULTS

At all stages studied, cells of different morphologies displayed PGP-like immunoreactivity (-LI) and calbindin-LI. Columnar immunoreactive cells for both markers predominated, but labeled cells of different shapes were also observed. Most labeled columnar cells were in the RE at its border with olfactory epithelium (OE); a few similarly labeled columnar cells also appeared in this OE. In the lamina propria, PGP-LI was also seen in numerous thin branching fibers. Some of these branches penetrated into the epithelium, where fiber varicosities appeared to contact cells, some of which also exhibited PGP-LI.

CONCLUSION

This study demonstrates that during development the human nasal RE contains different cell types, as illustrated by the assortment of epithelial cells displaying PGP-LI and calbindin-LI among unlabeled cells. Because PGP and calbindin immunoreactivities were found within neuroendocrine cells in previous studies, the present results indicate that the developing human nasal RE also may support a number of neuroendocrine cells. Furthermore, at least some of these cells may form synaptic contacts with nerve fibers from outside the epithelium.

Changes in the density of protein gene product 9.5-immunoreactive nerve fibres in human oral mucosa under implant-retained overdentures

Reduced oral sensitivity and impaired masticatory cycles have been demonstrated in edentulous humans wearing removable dentures, and there is some evidence that these patients have a decreased innervation of the oral mucosa. Clinical and electrophysiological evidence shows that sensory performance improves after oral rehabilitation with implant-retained overdentures. The aim of this study was to compare the density of mucosal innervation in edentulous patients with that in dentate controls and to evaluate changes in the number or type of sensory receptors following placement of endosseous implants in these edentulous individuals. The mucosal innervation was evaluated by immunohistochemical assays for the neurospecific marker protein gene product 9.5, and the innervation pattern was compared with that of dentate controls. Morphometric analysis of the immunohistochemical material demonstrated a decrease in numbers of sensory receptors in the mucosa of edentulous patients and a significant increase in the number of nerve fibres in the mucosa covering the distal edentulous mandibular ridges supporting the prostheses after implant-retained rehabilitation. In contrast, there were only minor increases in the number of nerves in the peri-implant mucosa. These changes in innervation appear to be related either to the new biomechanical situation created by implant support, which favours more physiological tissue conditions, or to an adaptive mechanism in the peripheral processing of sensory stimuli. These changes may explain, at least partially, the clinically observed differences in sensory skills before and after implant placement.

Protein gene product 9.5-immunoreactive nerves and cells in human oral mucosa

BACKGROUND

Current conflicting information on the innervation of the human oral cavity indicates technical problems such as different detectability of the neural structures according to the various staining methods used and difficulties in reproducibility. The possibility of intraoral regional differences has not been properly considered.

METHODS

Human biopsies of mucosa from different intraoral regions were prepared for immunohistochemistry using protein gene product 9.5 (PGP 9.5; a marker for neuronal structures).

RESULTS

Nerves were found consistently in all the biopsies. The neural pattern showed clear regional differences. Intraepithelial nerve fibers were found in the gingiva, labia, palate, within certain fungiform papillae, and in some salivary excretory ducts. Organized nerve endings were found in varying frequencies in all but one (sublingual) region, appearing as lamellar (Meissner-like), coiled or glomerular neural structures. Merkel cell-neurite complexes were observed in the buccal, gingival, and palatal epithelia. Immunoreactive cells with many similarities to Merkel cells but without a neural connection were also encountered.

CONCLUSIONS

Conflicting results from earlier innervation studies of the oral cavity could be attributed to regional innervation differences. The distribution of the nerves also casts doubt on some of the present theories concerning the function(s) of intraoral nerves, such as the free nerve endings and the Merkel cell-neurite complexes.

Merkel cells do not require trophic maintenance from the nerves in adult human skin

A 34-year-old Japanese man with hereditary sensory neuropathy was examined to evaluate the distribution, density and inter-relationship between Merkel cells and peripheral nerves in the skin. An epidermal sheet of affected plantar skin showed numerous CAM 5.2-reactive Merkel cells, whereas PGP 9.5-reactive peripheral nerves were completely absent in the epidermis and dermis. These findings strongly suggest that Merkel cells do not require trophic maintenance from nerves in adult human skin.

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.

Nerve fibres and cells immunoreactive to neurochemical markers in developing rat molars and supporting tissues

The distribution of nerve fibres immunoreactive to calcitonin gene-related peptide (CGRP), substance P (SP) and neuropeptide Y (NPY) was compared to the general neurochemical markers for nerves and neuroendocrine cells protein gene product 9.5 (PGP 9.5) and neurone-specific enolase (NSE), by use of the avidin-biotin peroxidase complex method in developing dental structures in rats aged 13 to 27 days. A substantially greater part of the nerve fibres was immunoreactive to CGRP and SP than to NPY. In the bell stage, nerve fibres immunoreactive to PGP 9.5, CGRP and SP were found in the dental follicle but not in the dental papilla and stellate reticulum. In the advanced bell stage, after initiation of dentine and enamel formation, PGP 9.5, CGRP- and SP-immunoreactive fibres were found in the dental papilla, while the first NPY-immunoreactive fibres were observed in the papilla when root formation started. Concomitant with the beginning of root development, a subodontoblastic nerve plexus was gradually formed and PGP 9.5-, CGRP- and SP-immunoreactive fibres were found within the dentinal tubules. From the start of root formation, CGRP-, SP- and NPY-immunoreactive nerves were shown in the developing periodontal ligament, although a mature distribution pattern was not observed until root formation was nearly completed. Ameloblasts, odontoblasts and cell-like structures in the outer enamel epithelium and within the dental lamina were PGP 9.5-immunoreactive at the bell stage. As the tooth matured, the immunolabelling gradually decreased, but was still present in some odontoblasts after tooth eruption. NSE-immunoreactive, cell-like structures were found in the periphery of the dental follicle, and persisted close to alveolar bone in the periodontal ligament when the tooth reached occlusion. Hence, it may be concluded that sensory nerves containing SP and CGRP are present in the pulp in advance of sympathetic nerves immunoreactive to NPY.

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

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