The Merkel cell as a possible mechanoreceptor cell

Morphological evaluation of Merkel cells and small lamellated sensory receptors in the equine foot

OBJECTIVE To examine the equine foot for the presence of sensory receptors including Merkel cells and small lamellated Pacinian-like corpuscles (SLPCs). SAMPLE Forefeet obtained from 7 horses following euthanasia for reasons other than foot disease. PROCEDURES Disarticulated feet were cut into either sagittal sections or cross sections and immersed in neutral-buffered 4% formalin. Following fixation, samples were obtained from the midline of the dorsal aspect of the hoof wall and from the frog (cuneus ungulae) between the apex and central sulcus. The formalin-fixed, paraffin-embedded hoof wall and frog sections were routinely processed for peroxidase immunohistochemistry and stained with H&E, Alcian blue, and Masson trichrome stains for histologic evaluation. RESULTS Sensory myelinated nerves and specific receptors were identified within the epidermal and dermal tissues of the equine foot including the hoof wall laminae, coronet, and frog. Merkel cells were identified with specific antisera to villin, cytokeratin 20, and protein gene product 9.5 in coronet epidermis and hoof wall. These cells were interspersed among basilar keratinocytes within the frog, coronary epidermis, and secondary epidermal laminae. The SLPCs were present within the superficial dermis associated with the central ridge of the frog (ie, frog stay). Numerous S100 protein and protein gene product 9.5 immunoreactive sensory nerves in close proximity to these receptors were present throughout the dermal tissues within both the frog and hoof wall. CONCLUSIONS AND CLINICAL RELEVANCE The presence of Merkel cells and SLPCs that are known to detect tactile and vibrational stimuli, respectively, further defined the diverse range of neural elements within the equine foot.

Prognostic factors in Merkel cell carcinoma patients undergoing sentinel node biopsy

INTRODUCTION

Debate remains about prognostic factors in primary Merkel cell carcinoma (MCC). We investigated clinicopathological factors as determinants of survival in patients with MCC submitted to sentinel node biopsy.

METHODS

Sixty-four consecutive patients treated for a primary MCC were identified from a prospectively maintained database at Fondazione IRCCS Istituto Nazionale dei Tumori, Milan. Time to events outcome were described by product limit estimators and proportional hazards model was used to investigate the association between outcome and potential predictors.

RESULTS

The most common site of primary tumor was lower limbs (56.3%). The size of primary lesion was ≤2 cm in 67.2% of cases. Presence of residual disease after the diagnostic surgical excision was observed in 28% of cases. All patients received sentinel node biopsy (SNB) and a SN positivity was detected in 26.6%. The median follow up was 78 months. Disease recurrence occurred in 17 patients (26.6%). In the SN negative group 10 recurrences occurred (21.3%), whereas 7 (41.2%) were found in SN positive one. Nine patients SN negative (19.1%) died of disease and 3 (17.6%) among SN positive. SN status was not associated with survival (p = 0.78). Neither age, gender, size and site of primary tumor resulted predictors of patients' outcome. The presence of residual tumor in the specimen of the wide local excision, after the diagnostic surgical excision, was the only variable associated with survival (p = 0.03).

CONCLUSIONS

Presence of residual tumor in the specimen of the wide local excision is the main prognostic factor in MCC patients.

Diversification and specialization of touch receptors in skin

Our skin is the furthest outpost of the nervous system and a primary sensor for harmful and innocuous external stimuli. As a multifunctional sensory organ, the skin manifests a diverse and highly specialized array of mechanosensitive neurons with complex terminals, or end organs, which are able to discriminate different sensory stimuli and encode this information for appropriate central processing. Historically, the basis for this diversity of sensory specializations has been poorly understood. In addition, the relationship between cutaneous mechanosensory afferents and resident skin cells, including keratinocytes, Merkel cells, and Schwann cells, during the development and function of tactile receptors has been poorly defined. In this article, we will discuss conserved tactile end organs in the epidermis and hair follicles, with a focus on recent advances in our understanding that have emerged from studies of mouse hairy skin.

The anatomy, function, and development of mammalian Aβ low-threshold mechanoreceptors

Touch sensation is critical for our social and environmental interactions. In mammals, most discriminative light touch sensation is mediated by the Aβ low-threshold mechanoreceptors. Cell bodies of Aβ low-threshold mechanoreceptors are located in the dorsal root ganglia and trigeminal ganglia, which extend a central projection innervating the spinal cord and brain stem and a peripheral projection innervating the specialized mechanosensory end organs. These specialized mechanosensory end organs include Meissner's corpuscles, Pacinian corpuscles, lanceolate endings, Merkel cells, and Ruffini corpuscles. The morphologies and physiological properties of these mechanosensory end organs and their innervating neurons have been investigated for over a century. In addition, recent advances in mouse genetics have enabled the identification of molecular mechanisms underlying the development of Aβ low-threshold mechanoreceptors, which highlight the crucial roles of neurotrophic factor signaling and transcription factor activity in this process. Here, we will review the anatomy, physiological properties, and development of mammalian low-threshold Aβ mechanoreceptors.

Neurotransmitters and synaptic components in the Merkel cell-neurite complex, a gentle-touch receptor

Merkel cells are an enigmatic group of rare cells found in the skin of vertebrates. Most make contacts with somatosensory afferents to form Merkel cell-neurite complexes, which are gentle-touch receptors that initiate slowly adapting type I responses. The function of Merkel cells within the complex remains debated despite decades of research. Numerous anatomical studies demonstrate that Merkel cells form synaptic-like contacts with sensory afferent terminals. Moreover, recent molecular analysis reveals that Merkel cells express dozens of presynaptic molecules that are essential for synaptic vesicle release in neurons. Merkel cells also produce a host of neuroactive substances that can act as fast excitatory neurotransmitters or neuromodulators. Here, we review the major neurotransmitters found in Merkel cells and discuss these findings in relation to the potential function of Merkel cells in touch reception.

Diminutive digits discern delicate details: fingertip size and the sex difference in tactile spatial acuity

We have observed that passive tactile spatial acuity, the ability to resolve the spatial structure of surfaces pressed upon the skin, differs subtly but consistently between the sexes, with women able to perceive finer surface detail than men. Eschewing complex central explanations, we hypothesized that this sex difference in somatosensory perception might result from simple physical differences between the fingers of women and men. To investigate, we tested 50 women and 50 men on a tactile grating orientation task and measured the surface area of the participants' index fingertips. In subsets of participants, we additionally measured finger skin compliance and optically imaged the fingerprint microstructure to count sweat pores. We show here that tactile perception improves with decreasing finger size, and that this correlation fully explains the better perception of women, who on average have smaller fingers than men. Indeed, when sex and finger size are both considered in statistical analyses, only finger size predicts tactile acuity. Thus, a man and a woman with fingers of equal size will, on average, enjoy equal tactile acuity. We further show that sweat pores, and presumably the Merkel receptors beneath them, are packed more densely in smaller fingers.

Distribution and ultrastructure of Merkel cell of the fishing bat (Myotis ricketti)

The distribution and ultrastructure of Merkel cells were described in detail in piscivorous bats through immunohistochemistry and transmission electron microscopy techniques. The findings indicated that Merkel cells are commonly found in raised-domes, hair follicles and in the basal epidermis of the skin from their back, abdomen, intercrural membranes, wing membranes and footpads. However, the density of Merkel cells is significantly higher in the footpad than in other places. These results suggested that there may be a link between Merkel cells and tactile sense, and also might imply that raised-domes with air-flow sensitive hairs played an important role in adjusting flying gestures by monitoring the air flow around the body. The ultrastructure of Merkel cells is similar to other vertebrates except having more intermediate filaments and larger granules.

Merkel cells as putative regulatory cells in skin disorders: an in vitro study

Merkel cells (MCs) are involved in mechanoreception, but several lines of evidence suggest that they may also participate in skin disorders through the release of neuropeptides and hormones. In addition, MC hyperplasias have been reported in inflammatory skin diseases. However, neither proliferation nor reactions to the epidermal environment have been demonstrated. We established a culture model enriched in swine MCs to analyze their proliferative capability and to discover MC survival factors and modulators of MC neuroendocrine properties. In culture, MCs reacted to bFGF by extending outgrowths. Conversely, neurotrophins failed to induce cell spreading, suggesting that they do not act as a growth factor for MCs. For the first time, we provide evidence of proliferation in culture through Ki-67 immunoreactivity. We also found that MCs reacted to histamine or activation of the proton gated/osmoreceptor TRPV4 by releasing vasoactive intestinal peptide (VIP). Since VIP is involved in many pathophysiological processes, its release suggests a putative regulatory role for MCs in skin disorders. Moreover, in contrast to mechanotransduction, neuropeptide exocytosis was Ca²⁺-independent, as inhibition of Ca²⁺ channels or culture in the absence of Ca²⁺ failed to decrease the amount of VIP released. We conclude that neuropeptide release and neurotransmitter exocytosis may be two distinct pathways that are differentially regulated.

Fingerprint lines may not directly affect SA-I mechanoreceptor response

Understanding how skin microstructure affects slowly adapting type I (SA-I) mechanoreceptors in encoding edge discontinuities is fundamental to understanding our sense of touch. Skin microstructure, in particular papillary ridges, has been thought to contribute to edge and gap sensation. Cauna's 1954 model of touch sensibility describes a functional relationship between papillary ridges and edge sensation. His lever arm model proposes that the papillary ridge (exterior fingerprint line) and underlying intermediate ridge operate as a single unit, with the intermediate ridge acting as a lever which magnifies indentation imposed at the papillary ridge. This paper contests the validity of the lever arm model. While correctly representing the anatomy, this mechanism inaccurately characterizes the function of the papillary ridges. Finite element analysis and assessment of the critical anatomy indicate that papillary ridges have little direct effect on how SA-I receptors respond to the indentation of static edges. Our analysis supports a revised (stiff shell-elastic bending support) interpretation where the epidermis is split into two major layers with a stiff, deformable shell over an elastic bending support. Recent physiological, electrophysiological, and psychophysical findings support our conclusion that the function of the intermediate ridge is distinct from the function of the papillary ridge.

Swelling-activated Ca2+ channels trigger Ca2+ signals in Merkel cells

Merkel cell-neurite complexes are highly sensitive touch receptors comprising epidermal Merkel cells and sensory afferents. Based on morphological and molecular studies, Merkel cells are proposed to be mechanosensory cells that signal afferents via neurotransmission; however, functional studies testing this hypothesis in intact skin have produced conflicting results. To test this model in a simplified system, we asked whether purified Merkel cells are directly activated by mechanical stimulation. Cell shape was manipulated with anisotonic solution changes and responses were monitored by Ca²⁺ imaging with fura-2. We found that hypotonic-induced cell swelling, but not hypertonic solutions, triggered cytoplasmic Ca²⁺ transients. Several lines of evidence indicate that these signals arise from swelling-activated Ca²⁺-permeable ion channels. First, transients were reversibly abolished by chelating extracellular Ca²⁺, demonstrating a requirement for Ca²⁺ influx across the plasma membrane. Second, Ca²⁺ transients were initially observed near the plasma membrane in cytoplasmic processes. Third, voltage-activated Ca²⁺ channel (VACC) antagonists reduced transients by half, suggesting that swelling-activated channels depolarize plasma membranes to activate VACCs. Finally, emptying internal Ca²⁺ stores attenuated transients by 80%, suggesting Ca²⁺ release from stores augments swelling-activated Ca²⁺ signals. To identify candidate mechanotransduction channels, we used RT-PCR to amplify ion-channel transcripts whose pharmacological profiles matched those of hypotonic-evoked Ca²⁺ signals in Merkel cells. We found 11 amplicons, including PKD1, PKD2, and TRPC1, channels previously implicated in mechanotransduction in other cells. Collectively, these results directly demonstrate that Merkel cells are activated by hypotonic-evoked swelling, identify cellular signaling mechanisms that mediate these responses, and support the hypothesis that Merkel cells contribute to touch reception in the Merkel cell-neurite complex.

Current considerations about Merkel cells

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

Touch

Light touch, a sense of muscle position, and the responses to tissue-damaging levels of pressure all involve mechanosensitive sensory neurons that originate in the dorsal root or trigeminal ganglia. A variety of mechanisms of mechanotransduction are proposed. These ranges from direct activation of mechanically activated channels at the tips of sensory neurons to indirect effects of intracellular mediators, or chemical signals released from distended tissues, or specialized mechanosensory end organs. This chapter describes the properties of mechanosensitive channels present in sensory neurons and the potential molecular candidates that may underlie. Mechanically regulated electrical activity by touch and tissue damaging levels of pressure in sensory neurons seems to involve a variety of direct and indirect mechanisms and ion channels, and the involvement of specialized end organs in mechanotransduction complicates matters even more. Imaging studies are providing useful information about the events in the central nervous system associated with touch pain and allodynia (a pathological state where touch becomes painful this type of activity).

Metabotropic glutamate receptor antagonists selectively enhance responses of slowly adapting type I mechanoreceptors

There is evidence that glutamate may participate as a transmitter at the junction between Merkel cells and the nerve terminals of slowly adapting type I (St I) units. We recorded extracellularly from the deep vibrissal nerve of an isolated rat vibrissa preparation in vitro. Five second trapezoid stimulus ramp deflections of the hair shaft were used to evoke responses. We bath-applied two compounds, which we planned would interfere with glutamatergic transmission. (2S)-2-Amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495) was used at concentrations up to 100 microM to block all known metabotropic glutamate (mGlu) receptors. The racemic mixture (RS)-4-carboxy-3-hydroxyphenylglycine ((RS)-4C3HPG) was used up to 100 microM to block ionotropic and Group I metabotropic glutamate receptors, and as an agonist at Group II mGlu receptors. Unexpectedly, both compounds had rapid onset excitatory effects on mechanically-evoked responses. (RS)-4C3HPG increased responses, with a mean 146% of control (P < 0.05) in a concentration-dependent manner. LY341495 increased responses, with a mean 128% of control (P < 0.05). With (RS)-4C3HPG in particular, it was noted that the static component (the firing during the last 1 s plateau) was preferentially enhanced relative to the dynamic component (firing during the first 0.5 s). Rapid recovery was seen after wash. Slowly adapting type II units, which have no junctional transmission, were completely unaffected by these compounds up to 200 microM. These results suggest that mGlu receptors play a role in Merkel cell-neurite complex mechanotransduction, although other explanations are considered.

A continuum mechanical model of mechanoreceptive afferent responses to indented spatial patterns

Information about the spatial structure of tactile stimuli is conveyed by slowly adapting type 1 (SA1) and rapidly adapting (RA) afferents innervating the skin. Here, we investigate how the spatial properties of the stimulus shape the afferent response. To that end, we present an analytical framework to characterize SA1 and RA responses to a wide variety of spatial patterns indented into the skin. This framework comprises a model of the tissue deformation produced by any three-dimensional indented spatial pattern, along with an expression that converts the deformation at the receptor site into a neural response. We evaluated 15 candidate variables for the relevant receptor deformation and found that physical quantities closely related to local membrane stretch were most predictive of the observed afferent responses. The main outcome of this study is an accurate working model of SA1 and RA afferent responses to indented spatial patterns.

Merkel cell carcinoma

Background

Merkel cell carcinoma (MCC) is an unusual primary neuroendocrine carcinoma of the skin. MCC is a fatal disease, and patients have a poor chance of survival. Moreover, MCC lacks distinguishing clinical features, and thus by the time the diagnosis is made, the tumour usually have metastasized. MCC mainly affects sun-exposed areas of elderly persons. Half of the tumours are located in the head and neck region.

Methods

MCC was first described in 1972. Since then, most of the cases reported, have been in small series of patients. Most of the reports concern single cases or epidemiological studies. The present study reviews the world literature on MCC. The purpose of this article is to shed light on this unknown neuroendocrine carcinoma and provide the latest information on prognostic markers and treatment options.

Results

The epidemiological studies have revealed that large tumour size, male sex, truncal site, nodal/distant disease at presentation, and duration of disease before presentation, are poor prognostic factors. The recommended initial treatment is extensive local excision. Adjuvant radiation therapy has recently been shown to improve survival. Thus far, no chemotherapy protocol have achieved the same objective.

Conclusion

Although rare, the fatality of this malignancy makes is important to understand the etiology and pathophysiology. During the last few years, the research on MCC has produced prognostic markers, which can be translated into clinical patient care.

Receptors and transporter for serotonin in Merkel cell-nerve endings in the rat sinus hair follicle. An immunohistochemical study

Serotonin (5-HT) has been a candidate for neurotransmitters in cutaneous type I mechanoreceptors (i.e., Merkel cell-nerve endings). Although recent electrophysiological studies have suggested the presence of the 5-HT2 and 3 receptors in the Merkel cell-nerve endings, the histological localization of these receptors are obscure. We thus immunohistochemically examined the presence of 5-HT1, 2, 3 receptors in Merkel cell-nerve endings in sinus hair follicles of the rat whisker pad. We also studied the immunohistochemical localization of the 5-HT transporter to confirm the site of 5-HT secretion. For this purpose, we used antibodies for the 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C and 5-HT3 receptors, and for the 5-HT transporter, as well as antibodies for cytokeratin 20 (as a marker of Merkel cells) and neurofilament H (a marker of type I sensory nerve terminals). The immuno-stained sections were analyzed under a laser-scanning microscope. It was found that the sensory nerve terminals in the Merkel cell-nerve endings showed strong positive immunoreactions of 5-HT1A and 1B receptors but not 5-HT2A, 2C, and 3 receptors. Furthermore, both the Merkel cells and related axon terminals showed strong immunoreactions of the 5-HT transporter. These findings support the idea that 5-HT molecules are released from the Merkel cells during mechanical reception and indirectly regulate neural actions of sensory neurons via 5-HT1 receptors. The localization of the 5-HT transporter found in this study also suggests a possibility that axon terminals in the Merkel cell-nerve endings also release 5-HT.

Immunohistochemical reactions of receptors to met-enkephalin, VIP, substance P, and CGRP located on Merkel cells in the rat sinus hair follicle

The role of Merkel cells in type I cutaneous mechanoreceptors remains enigmatic though mechanical transduction or neuromodulation function has been proposed. It has been shown that mammalian Merkel cells express immunohistochemical reactions of met-enkephalin, VIP, substance P, and CGRP, though the reactivity differs between species. If any one of these peptides acts as a transmitter or modulator for Merkel nerve terminals, these structures must have a specific receptor for the substance. We therefore studied the immunohistochemical localization of the above-mentioned neuropeptides and their receptors in Merkel cell-nerve endings in rat whisker pads. Specimens were doubly stained with polyclonal antibodies to neuropeptides and their receptors combined with a monoclonal antibody to cytokeratin 20, which was used for the labeling of Merkel cells. Merkel cells in the rat sinus hair follicles showed positive immunoreactions for all peptides studied, whereas the immunoreactions of receptors to these peptides were localized on Merkel cell membranes but not on the axon terminals. These results suggest that neuropeptides released from Merkel cells act on Merkel cells themselves by an autocrine mechanism.

NT3 expressed in skin causes enhancement of SA1 sensory neurons that leads to postnatal enhancement of Merkel cells

To determine the role of NT3 in the postnatal maturation of Merkel cell (MC) sensory neurite complexes (touch domes), we examined the development of their neural and end-organ components in wild-type and transgenic mice that overexpress NT3 (NT3-OE). Touch domes are sensory complexes of the skin that contain specialized MCs innervated by slowly adapting type 1 (SA1) neurons. Touch domes are dependent on NT3 and, though formed in newborn mice that lack NT3, are severely depleted during postnatal maturation. Mice that overexpress NT3 in the skin have larger touch domes characterized by enhanced neural innervation and MC number. In this study, we asked how this NT3-mediated enhancement occurs, whether through stimulatory effects of NT3 on the SA1 neuron, or the MC, or both. The innervation density and number of MCs associated with each touch dome were measured in wild-type and transgenic animals at postnatal times. In newborn NT3-OE mice, touch dome innervation was enhanced. Surprisingly, however, the number of MCs was lower in newborn NT3-OE animals than in wild-type littermates, and equivalent numbers were not reached until postnatal day 8 (PN8). Not until the PN12 and PN16 time points did MCs increase in NT3-OE mice. To examine the neural dependence of MCs in NT3-OE mice, touch domes were chronically denervated by resecting dorsal cutaneous nerves. Both wild-type and NT3-OE animals showed similar depletion in the number of MCs associated with touch domes. These data indicate that NT3 is not a survival factor for MCs and that the NT3-mediated enhancement of MC number is indirect and neurally dependent.

The role of NT-3 signaling in Merkel cell development

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

Presence of β-arrestin-1 immunoreactivity in the cutaneous nerve fibers of rat glabrous skin

beta-Arrestin-1 (betaArr1) plays a major role in the desensitization and internalization of G protein-coupled receptors. We previously localized betaArr1 in the sensory neurons of rat lumbar 4 and 5 dorsal root ganglia (DRG) and reported the predominant presence of betaArr1 in the small-diameter DRG neurons that are often implicated with nociception. Because of betaArr1's crucial role in regulating the initiation of cellular signaling, in the current study we evaluated the distribution of betaArr1 in the peripheral sensory terminals where various receptors are present. Western blotting confirmed the presence of betaArr1 immunoreactivity in the rat skin. Sciatic nerve ligation demonstrated that betaArr1 is transported peripherally from the DRG, and immunohistochemistry showed betaArr1 immunoreactivity in the glabrous skin of the rat hindpaw. In the glabrous skin, strong betaArr1 immunoreactivity was detected in nerve fibers in the dermal nerve plexus and dermal papillae. Fine varicose immunoreactive fibers were found in the epidermis. In addition, betaArr1 was observed in specialized sensory receptors such as Meissner corpuscles. Our observations thus indicate that betaArr1 may be involved in modulation of specific tactile stimulation from the skin in addition to nociception.

Immunohistochemical expressions of mGluR5, P2Y2 receptor, PLC-beta1, and IP3R-I and -II in Merkel cells in rat sinus hair follicles

We previously found that Merkel cells (MCs) of the rat and monkey show a strong immunoreaction of the alpha-subunit of Gq protein. The Galphaq-subunit isoform activates isozymes of phospholipase C-beta (PLC-beta), which produces inositol-(1,4,5)-triphosphate (IP3) which mobilizes intracellular Ca(++) from calcium stores via IP3 receptors. Glutamate and adenosine triphosphate (ATP), which are candidates for neurotransmitters in Merkel endings, are known to couple to Galphaq. Although MCs showed positive immunoreactions of metabotropic glutamate receptor 5 (mGluR5) in our preliminary study, these cells were not reactive to all antibodies to PLC-beta isozymes. We, therefore, reinvestigated immunohistochemical affinities to MCs of antibodies to PLC-beta isozymes and mGluRs using frozen sections of rat sinus hair follicles that were briefly postfixed in formaldehyde. We also studied the immunohistochemical expressions of P2Y receptors for ATP and IP3 receptor subtypes using similar sections. Merkel cells showed positive immunoreactions of PLC-beta1 and mGluR5. It was also found that MCs show positive immunoreactions of P2Y2, IP3R-I, and IP3R-II receptors. These results suggest that the Galphaq isoform in MCs couples to both the P2Y2 receptor and mGluR5 and regulates the intracellular Ca(++) concentration via the PLC-beta-IP3 cascade.

5-HT2 and 3 receptor antagonists suppress the response of rat type I slowly adapting mechanoreceptor: an in vitro study

Previous experiments have shown an increase in rat type I mechanoreceptor responsiveness during arterial serotonin (5-hydroxytryptamine) infusion and the presence of serotonin immunostaining in Merkel cells. The current findings demonstrate that the 5-HT(2) antagonists ritanserin and ketanserin, as well as the 5-HT(3) antagonist MDL 72222, reduce type I response to a standardized mechanical stimulus in an in vitro skin preparation. In addition, ritanserin blocked the enhancement of type I response produced by 5-HT. These experiments suggest that serotonin is released during mechanical distortion of the Merkel cell membrane and alters action potential generation by the type I ending. In addition, it is possible that serotonin, released from outside the type I complex, influences mechanoreceptor responsiveness. For example, serotonin generated during inflammatory events could enhance type I response to mechanical stimulation and thereby increase symptoms of mechanical allodynia.

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.

Evidence for glutamate receptor mediated transmission at mechanoreceptors in the skin

The functional role of Merkel cells in the mechanosensitivity of the slowly adapting type I responses has been a controversial issue for many years. Here we show, for the first time, that glutamate receptor-mediated transmission is largely responsible for the static component of the slowly adapting type I response. An isolated sinus hair preparation was used to study the two types (I and II) of slowly adapting units. A broad spectrum ionotropic glutamate receptor antagonist kynurenate (1-10 mM) caused reliable and dose-dependent reductions in the static component of type I unit responses to mechanical stimulation. In addition, an amino acid transmitter candidate aspartate applied to the preparation selectively increased responses in type I units but not responses in type II units. This evidence establishes that the Merkel cell is a mechano-electric transducer, and challenges prevailing views that the Merkel cell acts merely as a support or target cell in the epidermis.

Merkel cells in the human oesophagus

Merkel cells (MCs) are well recognized in the basal layers of the skin and oral mucosa, but this paper describes for the first time the presence of MCs in the human oesophagus. These cells are not identified in neonatal oesophagus, but are seen singly and in clusters in adult specimens. Application of stereological techniques shows that MCs are more numerous in the mid-oesophageal region. Cells expressing established markers of MCs have also been demonstrated in two out of six primary small cell carcinomas of the oesophagus. Further investigation of the role of MCs in oesophageal innervation and epithelial biology will be of interest.

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.