The morphology and distribution of Merkel cells in primate gingival mucosa

Merkel cells of human oral mucosa express the pluripotent stem cell transcription factor Sox2

Merkel cells are neuroendocrine cells associated to a neural sensitive ending and localized primarily in the epidermis, although they are also found in oral mucosa. Sox2 or SRY-box2 is a key transcription factor important in the maintenance of embryonic neural crest stem cell pluripotency. Sox2 has been described in Merkel cells of skin and in Merkel cell carcinomas, but not specifically in oral Merkel cells. The aims of the present study were to analyze the density of Merkel cells in human oral mucosa and to study the expression of Sox2 in these cells. For these purposes, immunohistochemical analyses for Sox2 and CK20 (the best marker for Merkel cells) were automatically performed on sections of normal human oral mucosa. Double immunofluorescence for Sox2 and CK20 was also performed. To analyze the density of Merkel cells, CK20 positive cells were counted in each sample and the length of the epithelial apical edge was measured (cells/mm). Merkel cells, demonstrated by CK20 immunoreactivity, were found in 95% of oral mucosa specimens studied (n=21). Mean density of Merkel cells in oral mucosa was 1.71±2.34 cells/mm. Sox2 immunoreactivity was found in the nuclei of scattered cells located at the basal layer. Serial sections immunostained for Sox2 and CK20 showed that Sox2-positive cells of oral mucosa coexpressed CK20, confirming that they were Merkel cells. Immunofluorescence for Sox2 and CK20 showed colocalization of both markers, demonstrating that virtually all oral Merkel cells expressed Sox2. This transcription factor could play a role in Merkel cell maturation and maintenance.

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.

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.

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.

Characterization of the peri-implant epithelium in hamster palatine mucosa: Behavior of Merkel cells and nerve endings

The purpose of this study was to investigate the relationship between Merkel cells and nerve elements during tissue regeneration after receiving dental implants. Golden hamsters were divided into 3 groups and titanium alloy implants were fixed in their left-side maxilla through the third palatine rug. Animals were sacrificed at 1, 2, 3, 4, 5, 6, and 7 days after the implantation and tissues were characterized at the immunohistochemical and morphological levels. CK 20 and PGP 9.5 antibodies which react with Merkel cells and nerve fibers were used. Immunohistochemically, no CK 20-positive Merkel cells were seen in the peri-implant epithelium throughout the 7 days. However, starting at day 4, PGP 9.5-positive nerve fibers appeared in the connective tissue, and by day 7, nerve fibers had invaded the more superficial layer of the peri-implant epithelium compared to the mucosa removal control group. At the electron microscopic level, the intercellular spaces of the regenerating epithelium in the mucosa removal control group were small. In contrast, intercellular spaces of the peri-implant epithelium tended to be wide and regenerating nerve fibers invaded those intercellular spaces. In both the mucosa removal control group and the implantation group, the basal lamina and connective tissues regenerated completely. However, clear Merkel cells containing neurosecretory granules were not observed. Taken together, our results indicate that Merkel cells in the hamster palatine mucosa do not regenerate in the peri-implant epithelium. However, regenerative nerve fibers seem to play essential roles as part of the defense and sensory systems around the peri-implant epithelium to compensate for the weakened defense mechanism.

An immunohistological study of cytokeratin 20 in human and mammalian oral epithelium

Cytokeratin (CK) 20 is a low molecular-weight intermediate filament reportedly expressed only by benign and malignant gastrointestinal epithelium, urothelium and Merkel cells. The main aims here were to map its expression in normal oral mucosa of humans and other mammals, and to determine whether it was expressed by abnormal human oral epithelium. Salivary and odontogenic epithelium were also analysed. An immunoperoxidase method was used on wax-embedded and cryostat sections. In addition, double-labelling experiments were undertaken to determine the association between CK 20 expression and that of CK 8/18 or S100 protein. Normal human oral mucosa from four sites, together with abdominal skin, was studied in autopsy samples from 32 individuals. CK 20-positive, basally situated, round or angular cells, consistent with Merkel cells, were recorded in 24/32 (75.0%) samples of mandibular gingiva, 25/32 (78.1%) samples of hard palate, 7/32 (21.9%) samples of buccal mucosa, 0/32 samples of lateral border of tongue, and 2/32 (6.3%) samples of abdominal skin. Double-labelling showed that all CK 20-positive Merkel cells also expressed CK 8/18 and S100. The only other cells to express CK 20 were human taste buds. There was no expression by dysplastic or invasive oral epithelium from biopsy samples. Colonic mucosa showed luminal-cell positivity in man, marmoset, ferret, rabbit and guinea-pig, but oral mucosa was universally negative in non-human species. It is concluded that in oral mucosa CK 20 is a specific marker of Merkel cells and taste buds, that Merkel cells are more frequently present in keratinized than non-keratinized oral mucosa, that CK 20-positive Merkel cells are also S100-positive, that there may be interspecies variations in CK 20 polypeptide composition and that, by contrast to urothelium, CK 20 has no value in the diagnosis of oral epithelial dysplasia.

Neuroendocrine cells in Malassez epithelium and gingiva of the cat

Malassez epithelium has been designated as epithelial cell rests, the biological significance of which is still under debate. This study was designed to analyze Malassez epithelium for the presence of neuroendocrine cells. Gingival tissue was included as a positive control. Using immunohistochemistry, confocal and light microscopy, Malassez epithelium and gingival epithelium from mature cats (n = 5) were examined for cells containing the neuropeptides calcitonin gene-related peptide (CGRP), substance P (SP), and vasoactive intestinal peptide (VIP). Both Malassez epithelium and the basal epithelial cell layers in gingival rete pegs regularly displayed cells immunoreactive to CGRP, SP, and VIP. The immunopositive cells were most frequently present in the epithelial cell clusters and strands of Malassez located in the cervical half of thc periodontal ligament. Double immunolabeling revealed cellular co-expression of CGRP or SP with VIP, and the neuropeptides were co-localized in the cellular compartments. Labeled cells in both epithelia were occasionally supported by immunoreactive nerve fibers. This study shows that cells immunoreactive to CGRP, SP, and VIP arc located within the cat Malassez epithelium. The localization of neuroendocrine cells verifies the diversity of this epithelium and confirms that Malassez epithelium is composed of different cell types, in common with epithelia from other locations. The presence of neuroendocrine cells in Malassez epithelium strongly suggests biological functions of this tissue, and the neuropeptide content may thus indicate endocrine functions of the cells.

Merkel cell distribution in human hair follicles of the fetal and adult scalp

The distribution of Merkel cells in fetal and adult terminal hair follicles of human scalp was studied immunohistochemically using cytokeratin (CK) 20 as a specific Merkel cell marker. In hair follicles of adult scalp, abundant Merkel cells were found enriched in two belt-like clusters, one in the deep infundibulum and one in the isthmus region. No Merkel cells were found in the deep follicular portions including the bulb, or in the dermis. In early fetal hair follicles (bulbous peg stage), Merkel cells were only detected in the basal layer of the developing infundibulum but not in deeper follicular areas. In later stages, Merkel cells were also present in the isthmus and bulge. No Merkel cells were seen in the dermis around developing hair follicles. Nerve growth factor receptor was not only present in nerves but was found to be widely distributed within fetal skin. In adult skin, this receptor was localized to the basal cell layers of the outer root sheath of the bulb and the suprabulbar area, but was not detectable in the areas containing Merkel cells. The present study localizing Merkel cells within the permanent hair follicle structures close to their possible stem cells suggests that they have paracrine functions.

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

Merkel cells are non-keratinized cells present in many different epithelia, but whose origin and functional role are still controversial. They were here investigated by means of antisera to the neural and neuroendocrine markers protein gene product 9.5 (PGP 9.5), and neurone-specific enolase. The expression of both markers in Merkel cells of human gingival and palatal mucosa was confirmed. Merkel cell-neurite complexes and isolated non-innervated Merkel cells had a similar morphology when stained by either antiserum. Merkel-neurite complexes were clustered in relatively large numbers in the lingual gingiva, thus constituting structures closely similar to the 'touch domes' in the skin. Clusters of non-innervated cells showing the same immunohistochemical features as Merkel cells were also demonstrated. In other areas of the oral mucosa, the innervated and non-innervated elements were only occasionally seen but there were many encapsulated Meissner-like receptors. When comparing the two different antisera, anti-PGP 9.5 appeared to provide a more consistent labelling of small fibres inside the epithelium and of bulb-like terminals on Merkel cells.

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 neural dependency of Merkel cell development in the rat: the touch domes and foot pads contrasted

We have used the quinacrine labeling technique and electron microscopy to study the development of the Merkel cell population in the skin of the rat and how this is affected by denervation produced at birth and at various times thereafter. An unexpected difference was found between the Merkel cells of glabrous and hairy skin. In the paw pads of rats aged 1 day or older the Merkel cells differentiated normally and survived quantitatively in the absence of their nerves. In the touch domes however, denervation at 1-4 days prevented the differentiation of the normal Merkel cell population and led to the disappearance of all or most of the Merkel cells that were already present. The Merkel cells in touch domes of the lower leg were affected by denervation like those of the back skin, differing strikingly from the Merkel cells of the footpads, even though the hairy skin of the leg and the glabrous skin of the foot are innervated by the same anatomical nerve. In adult rats, axons regenerating to denervated paws reinnervated epidermal Merkel cells of the pads and restored essentially normal mechanosensitivity to them; thus the Merkel cells of mammalian glabrous skin, like their counterparts in the wholly glabrous skin of lower vertebrates (S. A. Scott, E. Cooper, and J. Diamond, 1981, Proc. R. Soc. London B211, 455-470; K. M. Mearow and J. Diamond, 1988, Neuroscience 26, 695-708), can act as targets for ingrowing nerves. However, even though the differentiation of Merkel cells in hairy skin is nerve dependent, they probably have in common with the Merkel cells of glabrous skin the role of acting as final targets for nerves during development and regeneration.

Organization of subcortical pathways for sensory projections to the limbic cortex. II. Afferent projections to the thalamic lateral dorsal nucleus in the rat

Afferent projections to the thalamic lateral dorsal nucleus were examined in the rat by the use of retrograde axonal transport techniques. Small iontophoretic injections of horseradish peroxidase were placed at various locations within the lateral dorsal nucleus, and the location and morphology of cells of origin of afferent projections were identified by retrograde labeling. For all cases examined, subcortical retrogradely labeled neurons were most prominent in the pretectal complex, the intermediate layers of the superior colliculus, and the ventral lateral geniculate nucleus. Labeled cells were also seen in the thalamic reticular nucleus and the zona incerta. Within the cerebral cortex, labeled cells were prominent in the retrosplenial areas (areas 29b, 29c, and 29d) and the presubiculum. Labeled cells were also seen in areas 17 and 18 of occipital cortex. Peroxidase injections in the dorsal lateral part of the lateral dorsal nucleus result in labeled neurons in all of the ipsilateral pretectal nuclei, but especially those that receive direct retinal afferents. Labeled cells were also seen in the ventral lateral geniculate nucleus and the rostral tip of laminae IV-VI of the superior colliculus. In contrast, peroxidase injections in ventral medial portions of the lateral dorsal nucleus result in fewer labeled pretectal cells, and these labeled cells are found exclusively in the pretectal nuclei that do not receive retinal afferents. Other labeled cells following injections in the rostral and medial portions of the lateral dorsal nucleus are seen contralaterally in the medial pretectal region and nucleus of the posterior commissure, and bilaterally in the rostral tips of laminae IV and V of the superior colliculus. Camera lucida drawings of HRP labeled cells reveal that projecting cells in each pretectal nucleus have a characteristic soma size and dendritic branching pattern. These results are discussed with regard to the type of sensory information that may reach the lateral dorsal nucleus and then be relayed on to the medial limbic cortex.

Merkel-neurite complexes in the fungiform papillae of two species of monkeys

Merkel-neurite complexes in tongues of Japanese and cynomolgus monkeys were examined by means of light and electron microscopy. Merkel-neurite complexes were found preferentially in the epithelium of fungiform papillae located at the tip of the tongue. It appears that the anterior fungiform papillae of the monkey are highly adapted for both taste and mechanical sensation.

Identification of Merkel cells in oral epithelium using antikeratin and antineuroendocrine monoclonal antibodies

Merkel cells are a rare cell type located at or near the basal lamina of oral epithelia and epidermis. They are associated with nerves, and may function as touch receptors. Merkel cells are difficult to identify by routine light microscopy, but have been identified by electron microscopy and specific antibodies. We demonstrate here that Merkel cells can be identified by immunohistochemistry, using a monoclonal antibody (LK2H11, Lloyd and Wilson, 1983) to neuroendocrine granules, and a monoclonal antibody (35 beta H11, Gown and Vogel, 1982) to a 54-kD keratin generally located in simple epithelia but not in stratified epithelia. Human oral tissue was fixed in Carnoy's fixative, and adjacent serial sections were stained with the two antibodies. Individual positively-stained cells were generally located at the bases of rete ridges. Cells in the same position in adjacent sections stained with both antibodies. Essentially all cells (94%) staining for neuroendocrine granules also gave a positive reaction for the 54-kD keratin, evidence that a single Merkel cell contains neuroendocrine granules and the 54-kD keratin. Staining of epithelial sheets of human oral tissues revealed a linear distribution of Merkel cells along the bases of rete ridges. We concluded that antibodies to simple epithelial keratins and neuroendocrine granules may be excellent immunohistochemical markers for Merkel cells, thus facilitating future studies to examine changes in Merkel cells and their distribution in pathologic conditions. The observation that Merkel cells can be stained with an antikeratin antibody strongly supports the epithelial origin of these cells.

Bidirectional differentiation in a large cell pleomorphic primary endocrine carcinoma of the skin (a variant of malignant Merkel cell tumour)

A primary endocrine carcinoma of the skin is described in which the tumour cells showed the unusual features of large size and marked nuclear pleomorphism. Ultrastructural examination revealed combined features of squamous and endocrine differentiation in the same cells, a phenotype which has been previously anticipated but only recently described in endocrine carcinomas of the skin. In addition, immunocytochemical staining showed the co-expression of cytokeratin and neurofilament. The globular masses of cytokeratin corresponded to paranuclear whorls of intermediate filaments and there was also strong and diffuse staining for neuron specific enolase. Many tumour cells stained for calcitonin, VIP, ACTH, and S100 protein. Large tumour cells with markedly pleomorphic nuclei and co-expression of cytokeratin and neurofilament add to the expanding morphological spectrum of primary endocrine carcinomas of the skin.

Touch domes and Merkel cells in hamster cheek pouch epithelium

Minute dome-shaped elevations on the epithelial surface of hamster buccal pouch were counted in full-thickness whole mounts of detached pouch epithelium. Domes were examined by light and electron microscopy. Each dome consisted of thickened epithelium containing numerous Merkel cells with associated unmyelinated terminal axons. The pouch domes have structural characteristics in common with Haarscheiben, or touch domes, described in hair-bearing skin in man and other mammals and must not be confused with focal hyperplastic or precancerous lesions.

Neuroendocrine carcinoma of skin with simultaneous cytokeratin expression

An unusual tumor of the skin was removed from the thigh of a 52-year-old white male. By light microscopy, the tumor was composed of intermediate and small cells in sheets and clusters. Ultrastructural study of the tumor cells showed numerous dense core granules and dendritic cell processes as well as intermediate filaments and cell junctions frequently within the same cells. Most of the tumor cells were stained intensely by antibodies to neurone-specific enolase (NSE), a marker of cells of the central and peripheral nervous system. The neuropeptides met-enkephalin and vasoactive intestinal peptide (VIP) were also found in tumor cells. Immunohistochemistry furthermore demonstrated cytokeratin. Both the ultrastructural appearance and keratin content of this tumor set it apart from conventional Merkel cell (or trabecular) carcinoma of the skin in a manner analogous to bipartite (i.e., epidermoid and small cell) carcinoma of lung. The production of neuropeptides simultaneously with the production of keratin establishes this as a bipartite skin tumor (i.e., ectodermal and neuroectodermal phenotype). We suggest that at least some primary neuroendocrine tumors of the skin arise from multipotential ectodermal cells not of neural crest origin, as has been proposed for small cell carcinoma of lung.