Ultrastructure of Merkel cells in the hard palate of the squirrel monkey (Saimiri sciureus)

Dendritic atoh1a+ cells serve as transient intermediates during zebrafish Merkel cell development and regeneration

Sensory cells often adopt specific morphologies that aid in the detection of external stimuli. Merkel cells encode gentle touch stimuli in vertebrate skin and adopt a reproducible shape characterized by spiky, actin-rich microvilli that emanate from the cell surface. The mechanism by which Merkel cells acquire this stereotyped morphology from basal keratinocyte progenitors is unknown. Here, we establish that dendritic Merkel cells (dMCs) express atonal homolog 1a (atoh1a), extend dynamic filopodial processes, and arise in transient waves during zebrafish skin development and regeneration. We find that dMCs share molecular similarities with both basal keratinocytes and Merkel cells, yet display mesenchymal-like behaviors, including local cell motility and proliferation within the epidermis. Furthermore, dMCs can directly adopt the mature, microvilliated Merkel cell morphology through substantial remodeling of the actin cytoskeleton. Loss of Ectodysplasin A signaling alters the morphology of dMCs and Merkel cells within specific skin regions. Our results show that dMCs represent an intermediate state in the Merkel cell maturation program and identify Ectodysplasin A signaling as a key regulator of Merkel cell morphology.

Merkel-like cell distribution in the epithelium of the human vagina. An immunohistochemical and TEM study

Human Merkel cells (MCs) were first described by Friedrich S. Merkel in 1875 and named "Tastzellen" (touch cells). Merkel cells are primarily localized in the basal layer of the epidermis and concentrated in touch-sensitive areas. In our previous work, we reported on the distribution of MCs in the human esophagus, so therefore we chose other parts of the human body to study them. We selected the human vagina, because it has a similar epithelium as the esophagus and plays very important roles in reproduction and sexual pleasure. Due to the fact that there are very few research studies focusing on the innervation of this region, we decided to investigate the occurrence of MCs in the anterior wall of the vagina. The aim of our research was to identify MCs in the stratified squamous non-keratinized epithelium of the human vagina in 20 patients. For the identification of Merkel cells by light microscopy, we used antibodies against simple-epithelial cytokeratins (especially anti-cytokeratin 20). We also tried to identify them using transmission electron microscopy. Our investigation confirmed that 10 (50 %) of 20 patients had increased number of predominantly intraepithelial CK20 positive "Merkel-like" cells (MLCs) in the human vaginal epithelium. Subepithelial CK20 positive MLCs were observed in only one patient (5%). We tried to identify them also using transmission electron microscopy. Our investigation detected some unique cells that may be MCs. The purpose of vaginal innervation is still unclear. There are no data available concerning the distribution of MCs in the human vagina, so it would be interesting to study the role of MCs in the vaginal epithelium, in the context of innervation and epithelial biology.

Stem cell niche structure as an inherent cause of undulating epithelial morphologies

The spatial organization of stem cells into a niche is a key factor for growth and continual tissue renewal during development, sustenance, and regeneration. Stratified epithelia serve as a great context to study the spatial aspects of the stem cell niche and cell lineages by organizing into layers of different cell types. Several types of stratified epithelia develop morphologies with advantageous, protruding structures where stem cells reside, such as rete pegs and palisades of Vogt. Here, multistage, spatial cell lineage models for epithelial stratification are used to study how the stem cell niche influences epithelial morphologies. When the stem cell niche forms along a rigid basal lamina, relatively regular morphologies are maintained. In contrast, stem cell niche formation along a free-moving basal lamina may prompt distorted epithelial morphologies with stem cells accumulating at the tips of fingerlike structures that form. The correspondence between our simulated morphologies and developmental stages of the human epidermis is also explored. Overall, our work provides an understanding of how stratified epithelia may attain distorted morphologies and sheds light on the importance of the spatial aspects of the stem cell niche.

Response of Merkel cells in the palatal rugae to the continuous mechanical stimulation by palatal plate

The aim of the present study was to investigate the responses of Merkel cells that are numerous in the palatine rugae, due to the continuous mechanical stimulation exerted by the palatal plate. Forty golden hamsters were used in this experiment. The palatal plate was made of adhesive resin and it was set on the palate of the animal. To exert a continuous pressure, a 0.8 mm elevation on the internal surface of the palatal plate was created at the middle portion of the fourth palatine ruga. Thereafter, the number of Merkel cells in the mucosa was calculated by immunohistochemical observation. Morphological changes of Merkel cells were examined by electron microscopy. There was significant difference among the control and any of the treated groups on the number of CK20 positive Merkel cells (p < 0.05) and that numbers were decreased at the sites where continuous mechanical stimulation was exerted. Degeneration of the cytoplasm mitochondria and nerve endings, and a decrease in both the number of neurosecretory granules and cytoplasmic processes were observed. Furthermore, the presence of nuclear chromatin aggregation and fragmentation was recognized. The continuous mechanical stimulation by the palatal plate affected the responses of Merkel cells and nerve endings, thus inducing a decrease in the number of Merkel cells. A portion of these changes was also associated with the expression of apoptosis.

Ultrastructures of mechanoreceptors in the oral mucosa

The present review describes the fine structures of lamellated mechanoreceptive corpuscles, Merkel cell-neurite complexes and free nerve endings in the oral mucosae of mammals, with special attention to axon terminals and lamellar cells. The mechanoreceptive nerve endings of the oral mucosa were studied using histochemistry, immunohistochemistry and transmission electron microscopy techniques. The organized mechanoreceptive corpuscles are present in the mucosae of gingiva, cheek, tongue and soft and hard palate. They are elongated or globular in shape, being located in the connective tissue papillae. The capsule is composed of several layers of cytoplasmic extensions of perineural cells. Numerous bundles of collagen fibers are noted at the periphery of the corpuscle. The lamellated corpuscles are surrounded by several layers of superimposed flattened capsular cell processes. The interlamellar spaces are 0.2-0.4 micron in width and filled with thin fibrillar collagen fibers embedded in the amorphous substance. The lamellar cells contain rich microtubules and are characterized by the presence of caveolae on the surface plasma membrane. The terminal axon contains an abundance of mitochondria and small clear vesicles (20-50 nm in diameter). There are neurofilaments in the center of the axon terminal. Intermediate-type junctions are seen between the adjacent lamellar cells and between the axon and adjacent lamellae. The free nerve endings are found in the subepithelial regions, very close to the basal laminae of mucosal epithelium. They are surrounded by a thin cytoplasm of Schwann cells. Sometimes Schwann cell basal larinae become multilayered. Merkel cells are present within the basal layer of mucosal epithelium and contain characteristic electron-dense granules that are located almost exclusively at the side of cytoplasm in contact with axon terminals. Intermediate-type junctions are noted between axon terminals and Merkel cells.

Developmental dependency of Merkel endings on trks in the palate

Immunohistochemistry for protein gene product 9.5 was performed on Merkel cells in the palate of wildtype and knockout mice for trkA, trkB or trkC. In wildtype mice, numerous Merkel cells were observed at the top of anterior four rugae. In the posterior four rugae, Merkel cells were fewer and mostly located at the base of rugae. In knockout mice for trkA, trkB and trkC, Merkel cells at the top of rugae mostly disappeared although those at the base of rugae remained unchanged. Therefore, the number of Merkel cells in anterior four rugae decreased. In posterior four rugae, however, the number of Merkel cells in the mutant mice was similar to that for wildtype mice. Immunohistochemistry for S100 also demonstrated that the loss of genes for trkA, trkB and trkC caused the absence of the immunoreactive innervation of Merkel cells. The normal development of Merkel endings at the top of palatal rugae is probably dependent on trkA, trkB and trkC.

Identification of Merkel cells by an antibody to villin

Merkel cells represent a population of epithelial cells in the skin and oral mucosa. Although Merkel cells are reliably distinguishable from other epithelial cells at the ultrastructural level, these cells are usually not discernible by standard light microscopy and need special techniques for their identification. Villin is an actin-crosslinking protein that is associated with the actin filament cores of brush border microvilli. In this study we show that an antibody against villin is an excellent marker of Merkel cells and their microvilli even at the light microscopic level. The surrounding keratinocytes and subepithelial connective tissue cells do not show any significant affinity for the antibody against villin. Confocal laser micrographs reconstructed from serial images 0.5 microm thick of Merkel cells that were immunostained with villin clearly reveal the three-dimensional morphology of Merkel cells and their microvilli. The presence of villin in Merkel cell microvilli lends support to the idea that these cells might have a mechanoreceptor function.

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.

The Merkel cell as a possible mechanoreceptor cell

Merkel cells (MCs) are abundant at the basal layer of various skin in vertebrates, and make synaptic contacts with nerve endings to form the Merkel cell-neurite complex (MCN-complex). It has been established that the MCN-complex is involved in slowly adapting mechanoreception, cutaneous afferents of which are called SAI units in mammals or Ft-I units in frogs. However, the MC function has been the focus of attention, and some hypotheses propose that the site of mechanoreception is at the nerve terminals but not at the MC. In the present review, the possibility that MCs are the mechanoreceptors was focused on recent findings. Irradiation of quinacrine-loaded MCs in the rat hairy skin using excitation light degenerates the MCs selectively with the nerve terminals left intact. Correspondingly, SAI units decrease tonic discharges rapidly, but phasic responses remain intact. Blocking synaptic transmission in the MCN-complexes by divalent or alkyl Ca antagonists in mammals or frogs heavily decreases the tonic mechanical responses of the afferent units, but the phasic responses are rather resistant. Application of anodal current on the Ft-I receptive spots produces tonic discharges as in hair cells or taste cells, while the threshold to elicit the first spike is lower with cathodal than anodal stimulation, in contrast with other secondary sensory cells. These findings indicate that MCs are mechanoreceptors to yield tonic responses, while the nerve terminals may transduce the transient phase. Further studies, particularly on mechanically-gated ionic channels in the MC membrane and on transmitters between the MCs and nerve terminal, are necessary to establish the MC as mechanoreceptors.

Innervation of the hard palate in the rat studied by anterograde transport of horseradish peroxidase conjugates

The innervation of the rat hard palate and the bordering part of the soft palate was studied after anterograde transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) and to choleragenoid (B-HRP) in separate experiments. WGA-HRP labeling showed leakage from several types of nerve endings, whereas B-HRP did not. Both conjugates gave rise to heavy labeling of a variety of nerve endings. Intragemmal and, especially, perigemmal fibers were labeled in chemosensory corpuscles, which were most common in the medial wall of the incisive canal and in the most anterior part of the soft palate. Ruffini endings of different sizes were labeled in the incisive papilla. Other subepithelial endings forming elongated expanded profiles with medium- to large-caliber source fibers were most common in protruding parts of the palate. Labeled intraepithelial endings included Merkel endings, which were most frequent in the incisive papilla and the rugae. Other labeled profiles were medium-caliber afferents giving rise to irregular, beaded, and sometimes branched endings often located far superficially in the epithelium. Such endings were present both within and between protruding parts of the palate. Fine-caliber intraepithelial endings were labeled almost exclusively in WGA-HRP experiments.

A combined occurrence of neuroendocrine carcinoma of the skin and a benign appendageal neoplasm

A 76-year-old Japanese male presented with a subcutaneous nodule in the dorsal cubital region of the right arm. The center of the nodule showed typical features of neuroendocrine carcinoma of the skin. The peripheral portion showed epithelial islands of squamoid cells with horn cysts suggestive of benign appendageal tumor with pilar differentiation. The coexistence of two skin neoplasms in the same lesion would suggest that both tumors developed from common pluripotential stem cell.

The origin and fate of Merkel cell granules--an ultrastructural study

In order to elucidate the origin and fate of Merkel cell granules (MCGs), electron microscopic studies were carried out in fetal rat skin at day 20 of gestation. In addition to the ordinary processing of the tissue, we incubated a part of the tissue with a solution containing ionophore A23187 in order to capture the rarely observable exocytotic event. Based on our findings, a hypothetical model for the life cycle of Merkel cell granules is proposed as follows: I) Granule morphogenesis takes place in the rough endoplasmic reticulum and the GERL, from where immature granules are budding off, thereby exhibiting a bristle-like coat. II) Maturation and storage of MCGs takes place in the cytoplasm. III) Following stimulation, MCGs are released. IV) After the exocytotic granule release, MCG membranes are retrieved in the form of coated pits.

Extensive changes in cytokeratin expression patterns in pathologically affected human gingiva

The stratified squamous epithelium of the oral gingiva and the hard palate is characterized by a tissue architecture and a cytoskeletal composition similar to, although not identical with, that of the epidermis and fundamentally different from that of the adjacent non-masticatory oral mucosa. Using immunocytochemistry with antibodies specific for individual cytokeratins, in situ hybridization and Northern blots of RNA with riboprobes specific for individual cytokeratin mRNAs, and gel electrophoresis of cytoskeletal proteins of microdissected biopsy tissue samples, we show changes in the pattern of expression of cytokeratins and their corresponding mRNAs in pathologically altered oral gingiva. Besides a frequently, although not consistently, observed increase in the number of cells producing cytokeratins 4 and 13 (which are normally found as abundant components in the sulcular epithelium and the alveolar mucosa but not in the oral gingiva) and a reduction in the number of cells producing cytokeratins 1, 10 and 11, the most extensive change was noted for cytokeratin 19, a frequent cytokeratin in diverse one-layered and complex epithelia. While in normal oral gingiva cytokeratin 19 is restricted to certain, sparsely scattered cells of --or near--the basal cell layer, probably neuroendocrine (Merkel) cells, in altered tissue of inflamed samples it can appear in larger regions of the basal cell layer(s) and, in apparently more advanced stages, also in a variable number of suprabasal cells. Specifically, our in situ hybridization experiments show that this altered suprabasal cytokeratin 19 expression is more extended at the mRNA than at the protein level, indicating that cytokeratin 19 mRNA synthesis may be a relatively early event during the alteration. These changes in cytokeratin expression under an external pathological influence are discussed in relation to other factors known to contribute to the expression of certain cytokeratins and with respect to changes occurring during dysplasia and malignant transformation of oral epithelia.

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.

The development of the Merkel cells in the tentacles of Xenopus laevis larvae

The Merkel cells in the larval tentacles of Xenopus laevis were examined by TEM. Different forms of Merkel cells were found, depending on the age of the larvae or the location in the tentacles. These forms have the appearance of intermediate states between Merkel cells and superficial epidermal cells; thus an epidermal origin for the Merkel cells seems more likely than an immigration from the neural crest. The forms differ in (1) their location in the epidermis, (2) their shape, (3) the number and extension of their desmosomes, (4) the content and distribution of dense-core granules, and (5) the outgrowth of their finger-like processes. Also the relation to a nerve ending is different. By marking Merkel cells with quinacrine, fluorescence spots were observed between the superficial and basal epidermal cells or, in the very tip, within the superficial epidermal cells. These latter spots represent immature Merkel cells, as confirmed by TEM. This indicates a development of Merkel cells from superficial epidermal cells and migration towards the basal layer. Dermal Merkel cells were never observed.

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.

Formation of epidermal and dermal Merkel cells during human fetal skin development

The origin of Merkel cells is still a matter of debate, specifically the question of whether they are derived from epithelial cells of the epidermis or from immigrated neural crest cells. As an argument for the latter hypothesis the occurrence of dermal, nerve-associated Merkel cells in human fetal skin has often been mentioned. Therefore, we analyzed the distribution of Merkel cells in epidermis and dermis of plantar skin of human embryos and fetuses, ranging in gestational age between 7 and 17 weeks. Merkel cells were identified by immunocytochemistry on frozen sections using antibodies against simple epithelium-type cytokeratins and by electron microscopy. In the 17-week-old fetus, 17% of the total cutaneous (epidermal and dermal) Merkel cells were located in the upper dermal compartment, whereas in the 14-week-old fetus only 3.9% of the Merkel cells were dermal, including some cells that seemed to be in the process of traversing the dermal-epidermal junction. Thirteen-week-old fetuses showed even fewer dermal Merkel cells. Twelve-week-old fetuses exhibited 660 epidermal Merkel cells per 100 mm total section length, but none in the upper or deep dermis. In 7- to 9-week embryos, no Merkel cells were recognized. However, at this stage, but not in later stages, the basal cells of the plantar epidermis expressed certain simple epithelium-type cytokeratin polypeptides. These results speak against an invasion of Merkel cells or putative neural crest-derived precursor cells into the epidermis via a dermal passage. They suggest that in plantar skin Merkel cells arise, between weeks 8-12, from precursor stages of epithelial cells of the early fetal epidermis which still express simple epithelium-type cytokeratins. The results further suggest that in subsequent stages of skin development some epidermal Merkel cells detach from the epithelium and migrate into the upper dermis where some of them may associate with small nerves.

A neuroendocrine (Merkel) cell carcinoma with coexisting intraepidermal squamous cell carcinoma of the skin. Its growth accelerated by an extrinsic factor

A case of neuroendocrine (Merkel) cell carcinoma with coexisting intraepidermal squamous cell carcinoma of the skin was studied histologically, immunohistochemically and ultrastructurally as well as with tissue-culture and transplantation into nude mice. The primary tumor found in the lower leg of a 68-year-old Japanese man had remained thumb-sized for five years and, after contusion, had begun to enlarge rapidly up to 5 cm in size during one month. The patient died of metastatic neuroendocrine cell carcinoma nine months after excision of the primary tumor. Histologically the primary tumor was composed of neuroendocrine cell carcinoma extending down to subcutaneous adipose tissue and a small amount of intraepidermal squamous cell carcinoma, not associated with a wide range of necrosis, hemorrhage, granulation tissue or fibrosis. The tumor cells of the former were diffusely positive for neuron-specific enolase. They contained a few secretory granules, 100 nm in diameter. The tumor cells both cultured in media and transplanted into nude mice died two months later. The present case is the first report of Merkel cell carcinoma in which the growth accelerated by an extrinsic factor was proved. Histogenesis of neuroendocrine cell carcinoma with coexisting squamous cell carcinoma is also discussed.

A study of intermediate filaments (cytokeratin, vimentin, neurofilament) in two cases of Merkel cell tumor

In two cases of Merkel cell tumor, the study of intermediate filaments, using monoclonal antibodies (vimentin, cytokeratin, neurofilaments), confirmed the double differentiation (neuroendocrine and epithelial) of this tumor as previously observed in histological, electron microscopical and histochemical analyses. Labelling of the tumor cells was positive with monoclonal antibodies against neurofilament proteins and cytokeratin.

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

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

Recent advances in oral mucosal research

Major progress in investigation of the normal structure and function of the oral mucosa has been made within the last ten years and has come principally from the application of various techniques developed in basic science disciplines to specific mucosal problems. However, it is apparent that many gaps still exist in our knowledge of the oral mucosa and, although it is to be expected that different workers will have distinct views on which of these are the most significant, some basic areas for further investigation can clearly be identified. For example, little is known about epithelial control systems and their disturbance by epithelial disease processes, about the nature of the interactions occurring during development and maintenance of the oral mucosal epithelia, or about the epithelial cell surface and its role in normal function. The specific properties and behavior of the cell populations of the subepithelial connective tissues appears to be poorly understood and the existence and significance of functional changes in mucosa with age and malnutrition are uncertain. It is increasingly apparent that successful progress in such investigations involves approaches using diverse methodologies. For example, epithelial-mesenchymal interactions are likely to involve multiple mesenchymal factors acting in concert to establish and maintain epithelial form and, because of this complexity, the nature of the inductive influences is not likely to be elucidated in model systems unless individual variables can be rigidly controlled. Defining the cellular and acellular elements in mesenchyme and reconstructing a functional mesenchyme from purified components may not be a simple task, but with current methods for cultivating mucosal keratinocytes and fibroblasts, as well as for purifying various components of the ground substance, it should be possible to initiate such a program of study. Some of the most dramatic advances made over the past 5-6 years in epidermal research have come about through the utilization of newly developed biochemical investigative techniques, examples of which include the use of gene cloning to study the organization of the keratin gene family, and the use of immuno-fluorescence with monoclonal antibodies to discern when various keratin proteins appear during differentiation.(ABSTRACT TRUNCATED AT 400 WORDS)

Merkel cells and Merkel cell tumors. Ultrastructure, immunocytochemistry and review of the literature

Certain monomorphic cellular tumors that occur in the dermis have been called trabecular carcinomas or Merkel cell tumors. Forty-six cases have been reported to date and the literature on these is reviewed here, with six additional cases reported. Cytologic features include sparse cytoplasm, dispersed chromatin with inconspicuous nucleoli in round nuclei and many mitoses. Trabeculae and pseudorosettes may be identified. Electron microscopy is required for definitive diagnosis. Like normal Merkel cells, tumor cells contain electron-dense granules (80-200 nm), 10 mm filaments and desmosomes. Filament-rich cytoplasmic spikes were found in four tumors. These resemble corresponding protrusions of normal Merkel cells and have not been described in other APUDomas.

Merkel cell carcinoma of the skin: the structure and origin of normal Merkel cells

A series of 15 Merkel cell tumours of skin is reported. They occur dominantly on the head and neck and on the extremities of elderly women, frequently presenting as a reddish nodule. Three cases were associated with squamous carcinoma at the same site, an association deserving further study. There are two main patterns: the commoner one takes the form of a trabecular carcinoma in the dermis mimicking metastatic carcinoma, including oat-cell carcinoma and neuroblastoma: a dissociated-cell form mimicks malignant lymphoma. The triad of vesicular nuclei with very small nucleoli, abundant mitotic activity and apoptosis is so characteristic as to be virtually pathognomonic in conjunction with structural features. Argyrophilia is common, but Bouin fixation is necessary to demonstrate it regularly. Small round secretory granules (89 +/- 18 nm) with narrow haloes, and an abundance of intermediate size filaments are among the ultrastructural hallmarks. There is a close similarity between better differentiated tumour cells and normal Merkel cells. The neural crest origin of MC is in doubt both on the basis of studies of the development and regeneration of MC and from the study of Merkel cell tumours.

The morphology and distribution of Merkel cells in primate gingival mucosa

The morphology and distribution of Merkel cells in primate gingival mucosa have been studied by correlated light and electron microscopic techniques. The gingival mucosa is composed of a stratified squamous epithelium with a dense underlying connective tissue stroma. The epithelium inter-digitates with the underlying connective tissue forming long interconnected rete ridges. Merkel cells and their associated axons are abundant in gingival mucosa where they are located, either individually or in clusters, at the base of epithelial rete ridges. These cells have an identical morphology to Merkel cells described by others in the hard palate, hairy skin, glabrous skin and eyelid. While individual Merkel cells are found throughout the gingival mucosa, Merkel cell clusters are most numerous in the mandibular lingual gingival mucosa. When correlating this finding with data from other investigators, it appears that Merkel cell clusters are located preferentially in the masticatory mucosa in intimate contact with the tongue and thus may function as an important source of somatosensory feedback providing valuable information regarding the position of the tongue in the oral cavity.

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

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

Merkel cell receptors: structure and transducer function

An electron microscopic and electrophysiological investigation was made of Merkel cell-neurite complexes in the sinus hair follicles of the cat. These mechanoreceptors respond with very precise phase locking to heavy-frequency vibratory stimuli as well as to static hair displacements. The mechanoelectric transduction process is faster than that known for any other somatic mechanoreceptor. These data show that the nerve endings themselves and not the Merkel cells are the mechanoelectric transducer elements in these receptors.