Merkel cells as human cutaneous neuroreceptor cells

Effects of potassium channel modulators on the responses of mammalian slowly adapting mechanoreceptors

Introduction

slowly adapting mechanoreceptors in the skin provide vital tactile information to animals. The ionic channels that underlie their functioning is the subject of intense research. Previous work suggests that potassium channels may play particular roles in the activation and firing of these mechanoreceptors.

Objective

We used a range of potassium channel blockers and openers to observe their effects on different phases of mechanoreceptor responses.

Methods

Extracellular recording of neural activity of slowly adapting mechanoreceptors was carried out in an in vitro preparation of the sinus hair follicles taken from rat whisker pads. A range of potassium (K⁺) channel modulators were tested on these mechanoreceptor responses. The channel blockers tested were: tetraethylammonium (TEA), barium chloride (BaCl₂), dequalinium, 4-aminopyridine (4-AP), paxilline, XE 991, apamin, and charybdotoxin.

Results

Except for charybdotoxin and apamin, these drugs increased the activity of both types of slowly adapting units, St I and St II. Generally, both spontaneous and evoked (dynamic and static) activities increased. The channel opener NS1619 was also tested. NS1619 clearly decreased evoked activity (both dynamic and static) while leaving spontaneous activity relatively unaffected, with no clear discrimination of effects on the two types of St receptor

Conclusion

These findings are consistent with the targets of the drugs suggesting that K⁺ channels play an important role in the maintenance of spontaneous firing and in the production of and persistence of mechanoreceptor activity.

Dermal and Intraepidermal Merkel Cell Carcinoma With Squamous Cell Carcinoma: A Report of a Rare Case With Special Reference to the Touch Dome

ABSTRACT

In skin containing hair follicles, specialized epithelial structures known as "touch domes (TDs)" are located where the Merkel cells are clustered. We explored the histogenetic relationship between intraepidermal and dermal Merkel cell carcinomas (MCCs) and investigated which transformed progenitor cells can develop into intraepidermal MCC. We encountered an association between an extremely rare case of dermal and intraepidermal MCC with squamous cell carcinoma, which was examined using standard immunohistochemical methods with various epithelial, neuroendocrine, and TD markers including several immunohistochemical markers. Differential expression levels of CK20 and CD56 were found between intraepidermal and dermal MCCs, indicating molecularly distinct MCC populations. CK15 and CK17, expressed in TDs, were partially expressed in the intraepidermal neuroendocrine component at the tumor periphery in intraepidermal MCC with squamous cell carcinoma. These differences may suggest that the origin of dermal and intraepidermal MCCs is different under pathological conditions. We hypothesize that intraepidermal MCC is derived from tissue-specific stem cells localized within TDs.

The importance of the neuro-immuno-cutaneous system on human skin equivalent design

The skin is a highly complex organ, responsible for sensation, protection against the environment (pollutants, foreign proteins, infection) and thereby linked to the immune and sensory systems in the neuro-immuno-cutaneous (NIC) system. Cutaneous innervation is a key part of the peripheral nervous system; therefore, the skin should be considered a sensory organ and an important part of the central nervous system, an 'active interface' and the first connection of the body to the outside world. Peripheral nerves are a complex class of neurons within these systems, subsets of functions are conducted, including mechanoreception, nociception and thermoception. Epidermal and dermal cells produce signalling factors (such as cytokines or growth factors), neurites influence skin cells (such as via neuropeptides), and peripheral nerves have a role in both early and late stages of the inflammatory response. One way this is achieved, specifically in the cutaneous system, is through neuropeptide release and signalling, especially via substance P (SP), neuropeptide Y (NPY) and nerve growth factor (NGF). Cutaneous, neuronal and immune cells play a central role in many conditions, including psoriasis, atopic dermatitis, vitiligo, UV-induced immunosuppression, herpes and lymphomas. Therefore, it is critical to understand the connections and interplay between the peripheral nervous system and the skin and immune systems, the NIC system. Relevant in vitro tissue models based on human skin equivalents can be used to gain insight and to address impact across research and clinical needs.

Merkel cell carcinoma: recent progress and current priorities on etiology, pathogenesis, and clinical management

PURPOSE

To expedite improved understanding, diagnosis, treatment, and prevention of Merkel cell carcinoma (MCC), a rare malignancy of cutaneous neuroendocrine cells that has a 28% 2-year mortality rate.

METHODS

This article summarizes a workshop that discussed the state-of-the-art research and priorities for research on MCC and on a new human polyomavirus (ie, MCPyV) recently discovered in 80% of MCC tumors.

RESULTS

Normal Merkel cells are widely distributed in the epidermis near the end of nerve axons and may function as mechanoreceptors or chemoreceptors. Malignant MCC cells typically stain for cytokeratin 20 as well as for other epithelial and neuroendocrine markers. MCC subtypes, which are based on histology, on cell line growth properties, and on gene expression profiles, have been reported but have not been linked to prognosis. Clinical management has been empiric. MCPyV is clonally integrated at various sites in the human genome of MCC tumors, with truncating mutations in the viral, large T antigen gene that interrupt viral replication. MCPyV seroprevalence may be high, as with previously known human polyomaviruses. MCC risk is increased 11-fold with AIDS and with other cell-mediated immune deficiencies, B-cell neoplasms, and ultraviolet radiation exposure.

CONCLUSION

Development and validation of a range quantitative polymerase chain reaction and serologic assays for detection of MCPyV, as well as an infectious clone of the virus, would clarify the fundamental biology, natural history, and epidemiology of the virus, of MCC, and of other diseases. Contingent on standardized histologic diagnosis and staging of MCC, consortia are needed to clarify the risks and benefits of sentinel lymph node biopsy, adjuvant radiation therapy, and salvage therapies; consortia are needed also for epidemiologic studies of MCC etiology.

Neuroendocrine (Merkel Cell) Carcinomas of the Skin: An Ultrastructural Study of Nine Cases

Nine neuroendocrine tumors of the skin were examined by light and electron microscopy. The patients (7 males and 2 females) had an average age of 57 years. Seven tumors were located in the head and neck region. Light microscopic examination showed sheets of cells. In 4 cases, a tendency to form small groups of cells was observed. Contact with the epidermis was seen in only 2 cases. The dominant ultrastructural feature was the presence of cytoplasmic processes that contained membrane-bound granules 100-200 nm. Three tumors recurred locally, and in 7 patients, regional nodal metastases occurred. Three patients died of disseminated disease.

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.

Histochemistry of the human hair follicle

Immunohistochemistry of the human hair follicle is of increasing interest in hair research. The data on antigen distribution in the different epithelial and mesenchymal structures of this unique skin appendage are superfluous now. In the present chapter, I will concentrate on selected aspects related to hair follicle differentiation, epithelial-mesenchymal interactions, proliferation and metabolic activity. Hair diseases are common. Not unusually, hair growth and structure reflect systemic disturbances. Basic knowledge of hair anatomy and histochemistry is required for their rational evaluation. Immunohistochemistry is a valuable tool for microanatomy of the hair apparatus. It offers a link between the biochemical data and structural components of hair follicles, which may help to better understand physiology of hair growth and hair diseases.

Merkel cells of the terminal hair follicle of the adult human scalp

Human scalp skins were treated with 20 mM ethylenediaminetetraacetic acid and terminal hair follicles were extracted with the epidermis. Some terminal hair follicles were morphologically preserved well and provided opportunity to examine three-dimensional distribution of CAM5.2 (K8, 52.5 kD) reactive Merkel cells. In anagen terminal hair of the scalp numerous immunoreactive Merkel cells were distributed in the presumptive bulge area. Distinct swelling as in the bulge of human vellus hair was usually absent; however, in rare instances anagen terminal hair demonstrated unilateral prominent swelling with dense aggregation of Merkel cells. In telogen hair the bulge becomes indistinguishable from the regressed end of the club hair follicle but Merkel cells continued to be abundant. We found morphologic variation of the bulge such as formation of knoblike swellings and villous projections. Interestingly, Merkel cells were also located in these structures. Palisading stockade-like nerve endings were observed surrounding the follicular epithelium at the sebaceous gland level. Merkel cells were sparse in this follicular segment. Variable number of Merkel cells were also scattered in the infundibulum of terminal hair in no association with peripheral nerves.

Sensory innervation of the hairy skin (light- and electronmicroscopic study

The sense of touch develops early in phylogeny and is one of the most important senses for the survival of the animal. Touch organs of hairy skin in mammals include the so-called "Haarscheiben" (also Pinkus corpuscles) and all types of hair follicles with their nerve endings. The touch organs of the skin consist of a mechanical transducing component and the sensory component. The epithelium and its derivatives like hair follicles and sebaceous glands are the mechanical transducing component transmitting the mechanical forces like pressure or touch to the second component--the sensory nerve endings. In mammalian hairy skin all sinus and guard hairs and many vellus hairs are touch organs. The sinus hair is a typical example of a touch organ. All mammals except humans are equipped with these highly differentiated touch organs. The hair follicle is almost completely embedded in a blood sinus and equipped with more than 2,000 sensory nerve endings. All sinus and guard hairs are equipped with free nerve endings (nociceptors), Merkel nerve endings (slowly adapting [SA I] mechanoreceptor units-pressure detectors), palisades of lanceolate nerve endings (velocity detectors), and pilo-Ruffini corpuscles (tension receptors). In most of the sinus hairs lamellated corpuscles of Pacini type could be found (rapidly adapting receptors-acceleration detectors). Most vellus hairs are equipped with free and lanceolate nerve endings. Some of the vellus hairs of the upper portion of the body (head, upper extremity) are innervated by Merkel nerve endings. The presence of pilo-Ruffini nerve endings in vellus hairs is very unusual.

A high concentration of Merkel cells in the bulge prior to the attachment of the arrector pili muscle and the formation of the perifollicular nerve plexus in human fetal skin

The distribution of Merkel cells in human fetal hair follicles was studied using whole mounts of separated epidermis with attached hair follicles. The technique had the advantage of enabling the elucidation of the spatial relationships of Merkel cells with other cells in the skin. In a 16-week-old fetus the hair anlagen had formed one or two epithelial swellings of variable size. In a 17-week-old fetus sebaceous glands and the bulge of the hair follicle were recognizable and immunoreactive. Merkel cells were present in the bulge and surrounding the acrotrichium (intraepidermal follicular canal). In a 20-week-old fetus the sebaceous gland and bulge were well formed and immunoreactive Merkel cells were concentrated in the bulge and infundibulum. In vertical sections of a 20-week-old fetus immunoreactive Merkel cells were also situated in the vicinity of the bulge. Arrector pili muscles were first observable in a 24-week-old fetus being weakly stained with anti-desmin antibody. In a 24-week-old fetus, nerves were also stained within the arrector pili muscles with S-100 protein antibody. In the presumptive arrector pili muscle immunoreactivity for S-100 protein developed before or at the same time as immunoreactivity for desmin. Merkel cells or their products in the bulge may serve as attractants for the growing arrector pili muscle which contain peripheral nerves. Following our report that dermal Merkel cells influence the formation of the dermal nerve plexus, perifollicular Merkel cells near the bulge may also play an inductive and growth-stimulative role for the perifollicular nerve plexus.

Histochemistry of the human hair follicle with consideration of anagen phases I to VI

The hair follicle is composed of different epithelial layers under participation of mesenchymal and nerval factors. The present study is an attempt to localize differentiation and functional markers in the human hair follicle during anagen phases I to VI. Monoclonal antibody K 8.12 against keratins 13 and 16 showed an increasing reactivity with certain types of the follicle epithelia during anagen I to VI. Ki67 was expressed within the innermost layer of the outer hair root sheath. Scattered Ki67-positive matrix cells could be additionally identified during anagen V and VI but were absent in anagen I to IV. During anagen Merkel cells became more abundant in the bulbar area. Neuropeptide-like immunoreactivity expressed by bulbar (especially matrix) cells were evenly seen during the early anagen I and II. The findings are in favour of a neurohumoral modulatory role during anagen phases accompanied by an increase of expression of certain proliferation-associated antigens like keratin 16 and Ki67 among the complex epithelia of human hair follicles.

The Merkel cell and associated neoplasms in the eyelids and periocular region

Merkel cells are clear oval cells in the epidermis and outer root sheaths of hair follicles, which are probably of epithelial origin, share ultrastructural features with neuroendocrine cells, and are found in association with touch receptors. In the eyelid, they occur singly in the epidermis and external root sheaths of hairs and eyelashes, and in specialized touch spots alternating with eyelashes. Their typical electron microscopical and antigenic features include dense-core granules, intranuclear rodlets, spinous processes, and a positive reaction for specific cytokeratins, epithelial membrane antigen, neuron-specific enolase, chromogranin and synaptophysin. Merkel cell carcinoma probably develops from precursor cells which give rise to keratinocytes and Merkel cells, and nearly one out of ten Merkel cell carcinomas occur in the eyelid and periocular region. They tend to be bulging lesions near the lid margin of elderly patients, reddish in color, and erythematous with telangiectatic vessels. The diagnosis is based on the frequent presence of neurofilaments and paranuclear aggregates of intermediate filaments in addition to features typical of normal Merkel cells. The tumor often mimics lymphoma or undifferentiated carcinoma and frequently invades lymphatic vessels. One third of Merkel cell carcinomas recur, almost two thirds give rise to regional node metastases, and up to one half metastasize widely and result in death. Initial treatment should be prompt and aggressive, with wide resection and routine postoperative irradiation. Although metastatic lesions often respond to radiation therapy and cytostatic drugs, these treatments are mainly of palliative value.

[Clinical aspects and therapy of Merkel cell tumor--report of 4 personal cases and review of the literature]

Four cases of Merkel cell tumor located in the face or on the trunk are presented. In a literature review 339 cases were found and analysed. The Merkel cell tumor is a neuroendocrine tumor of the skin derived from the Merkel cells. It is located most often in the face and neck area. Typically, it is an exophytic node, has a red-blue color and measures 25 mm in diameter on the average. Since it is metastasizing it is a malignant tumor. Regional node metastases were found in 27%, local recurrences occur in 51%, and distant metastases were seen in 32% of the cases. The best therapy is excision with wide margins as is the rule for a malignant skin tumor. While the tumor reacts to radiotherapy, chemotherapy seems to be of no help.

Variability of expression and arrangement of cytokeratin and neurofilaments in cutaneous neuroendocrine carcinomas (Merkel cell tumors): immunocytochemical and biochemical analysis of twelve cases

Twelve specimens of cutaneous neuroendocrine carcinomas (Merkel cell tumors) available as fresh tissue were analyzed for intermediate filament (IF) expression by immunocytochemical and biochemical methods. In immunofluorescence microscopy, most cases were positive for both simple-epithelium-type cytokeratins and the neurofilament L- and M-polypeptides. Several different IF staining patterns ranging from presence of plaque-like structures (fibrous bodies) only to nearly exclusive expression of delicate cytokeratin fibrils could be distinguished. In immunoelectron microscopy the labeling for both cytokeratin and neurofilament polypeptides seemed evenly distributed among the IFs of the fibrous bodies. In primary culture, tumor cells maintained the coexpression of both IF types. Desmoplakin-positive true desmosomes were found in 5 specimens. Biochemically, cytokeratins nos. 8, 18 and, variably, 19, as well as IT protein and, in many specimens, the neurofilament L-protein and a putative neurofilament M-protein were detected. Only traces of the neurofilament H-polypeptide were found. Our results show that a coexpression of cytokeratin IFs and neurofilaments in variable patterns is a characteristic feature of cutaneous neoendocrine carcinomas; occasionally, however, neurofilaments may be very scarce. The biological, histogenetic and diagnostic implications are discussed.

Flow cytometry sorting of unlabelled epidermal Langerhans cells using forward and orthogonal light scatter properties

A variety of techniques based on the presence of specific markers has been proposed to enrich Langerhans cells from epidermal cell suspensions. Computer analysis of multiparameter flow cytometry records involving forward angle and orthogonal scattered light and immunofluorescence of epidermal cells allowed us to determine the scattering properties of Langerhans cells. Unlabelled cells sorted according to these properties were shown to be Langerhans cells by electron microscopy and/or subsequent labelling by anti-HLA-DR monoclonal antibody. The relevance of this method is discussed to sorting viable Langerhans cells which may be used in functional studies and for establishing long-term culture.

Identification of Merkel cells in human skin by specific cytokeratin antibodies: changes of cell density and distribution in fetal and adult plantar epidermis

Merkel cells are special neurosecretory cells which, in adult human skin, are usually very scarce. By immunofluorescence microscopy using antibodies to human cytokeratin polypeptide no. 18, we localized distinct non-keratinocyte cells in the glandular ridges of human fetal and adult plantar epidermis. Using electron and immunofluorescence microscopy, these cells were identified as Merkel cells containing typical neurosecretory granules as well as bundles of intermediate-sized filaments and desmosomes. Two-dimensional gel electrophoresis of the cytoskeletal fractions of microdissected epidermal preparations highly enriched in Merkel cells indicated the presence of cytokeratin polypeptides nos. 8, 18 and 19 which are typical of diverse simple epithelia of the human body. Double immunofluorescence microscopy showed that these human Merkel cells contain neither neurofilaments nor vimentin filaments. In human fetuses of 18-24 weeks of age, conspicuously high concentrations of Merkel cells, reaching a density of approximately 1,700 Merkel cells/mm2 skin, were found in the glandular ridges of plantar skin. The concentration decreased considerably at newborn and adult stages. Thin cell processes (up to 20 microns long) were observed in many fetal epidermal Merkel cells. In addition, we detected isolated Merkel cells deeper in the dermis (i.e. at distances of, at most, 100 microns from the epidermis) in fetal and newborn plantar skin. Our results show that Merkel cells are true epithelial cells which, however, differ profoundly from epidermal keratinocytes in their cytokeratin expression. The findings are discussed in relation to the much disputed question of the origin of Merkel cells. The present data speak against the immigration of Merkel cells from the neural crest, but rather suggest that they originate from epithelial cells of the skin, although most probably not from differentiated keratinocytes.

Dermal Merkel cells in the nevus of Ota and leopard syndrome

We observed dermal Merkel cells around vellus hair follicles in one patient with nevus of Ota and one with leopard syndrome. These Merkel cells were in contact with Schwann cells and nerve endings in the dermis. The question of whether or not Merkel cells exist normally in adult dermis remains unanswered. However, their presence in these abnormal conditions suggests that normally they do occur in the dermis but go undetected because their numbers are so few or they are in a form not readily identifiable by currently available methods.

Use of the fluorescence-activated cell sorter to quantitate and enrich for subpopulations of human skin cells

A variety of immunologic staining techniques were compared in a quantitative study of antigen expression by human epidermal cells. Virtually all nucleated epidermal cells express beta 2-microglobulin, which is associated with HLA-A, -B, and -C antigens, whereas only about 4% expressed T6, an antigen expressed by Langerhans cells but not other cells in the skin. With the fluorescence-activated cell sorter (FACS), epidermal cell suspensions were selectively enriched 10- to 15-fold for T6-positive Langerhans cells. An average of 6.5% of cells were specifically stained by anti-HLA-DR antibody. When dispersed cells stained with anti-DR plus peroxidase were examined with the technique of immunoelectron microscopy, only mononuclear leukocytes (probably Langerhans cells) were stained. After separating HLA-DR positive skin cells with the FACS, the DR-positive population but not the DR-negative population stimulated proliferation of allogeneic responder lymphocytes, indicating that sorted cells are metabolically active. We conclude that HLA-DR antigen is not expressed by keratinocytes in normal human skin cell suspensions and that the FACS can be used to selectively enrich or deplete skin cell suspensions of antigenically distinct subpopulations such as Langerhans cells.

Separation of human skin cells by velocity sedimentation into functionally distinct fractions

The epidermis consists of a heterogeneous population of cells including Langerhans cells, Merkel cells, melanocytes, and keratinocytes in various stages of differentiation. The current study was undertaken to determine if skin cell suspensions can be separated into morphologically and/or functionally distinct fractions. Skin cells were suspended by trypsinization and separated into multiple fractions by velocity sedimentation. Certain fractions reproducibly stimulated proliferation of allogeneic lymphocytes in the skin cell lymphocyte reaction, whereas other fractions, containing larger cells, supported growth of keratinocyte colonies in cell cultures. These results indicate that stimulation in the skin cell lymphocyte reaction and growth of keratinocyte colonies are mediated by distinct cells, separable by velocity sedimentation.

Ultrastructure of Merkel corpuscles and so-called "transitional" cells in the white Leghorn chicken

In the chicken Merkel corpuscles are located in the dermis and consist of specialized Merkel cells, discoid nerve endings and lamellar cells. Merkel cells contain characteristic membrane-bound dense-core granules and bundles of microfilaments. Asymmetric junctions, synapse like, with thickened membranes and clusters of dense-core vesicles were observed between the Merkel cells and the nerve endings. The nerve ending is derived from myelinated nerves and sometimes contains clusters of clear vesicles. A laminar system formed by lamellar cells of the Schwann cell type encloses the Merkel cells and the nerve endings. So called "transitional" cells, showing some of the morphological features of both keratinocytes and Merkel cells, were observed in the basal layer of the epidermis. One was located partly in the epidermis and partly in the dermis. The structure of Merkel corpuscles is compared with that of Merkel cells in other tetrapods. The developmental significance of "transitional" cells and the origin of Merkel cells are discussed.

The ultrastructure of cutaneous type I mechanoreceptors (Haarscheiben) in cats following denervation

Denervation of specialized cutaneous mechanoreceptors (Haarscheiben or domes) in cats was followed after 20 and 25 days by the following alterations in receptor structure: (1) reduced numbers of Merkel cells, (2) Merkel cells degenerating in situ, (3) fewer dense-core granules in the cytoplasm of Merkel cells, (4) an increased number of agranular dendritic cells and Langerhans cells in the dome, (5) the apparent phagocytosis of Merkel cells by Schwann and Langerhans cells, (6) fewer epithelial cell layers over the dome, and (7) a decrease in the number of transitional cells. Skin excised between the domes in the denervated nerve field appeared normal when compared to innervated skin, and it was considered unlikely that the alterations in dome structure were due to generalized nutritional changes in the skin caused by transection of sympathetic axons or to some other side effect of denervation. Since domes are formed in new locations on the skin after nerves have regenerated (Burgess et al., '74), changes in dome structure following nerve transection are probably due to loss of the "trophic" influence of the nerves supplying the dome.