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Merkel cell

Merkel cells are rare, postmitotic epidermal cells located at the dermo-epidermal junction in vertebrates, serving primarily as slowly adapting type I mechanoreceptors that mediate fine touch discrimination and sustained pressure sensation.^[1] These cells are distributed throughout the skin, with higher densities in tactile-sensitive regions such as the fingertips, palms, soles, and touch domes of hairy skin, as well as in certain mucosal tissues including the oral mucosa, esophagus, and genital areas.^[1] They typically occur in small clusters, forming Merkel cell-neurite complexes where they establish synaptic-like contacts with slowly adapting sensory afferents from somatosensory neurons.^[2] Structurally, Merkel cells are ovoid in shape, measuring 10–15 μm in diameter, and possess distinctive features including a large lobulated nucleus, numerous electron-dense cytoplasmic granules (80–100 nm in size) suggestive of neuroendocrine function, and apical microvilli-like projections that extend into the epidermis.^[1] They express specific markers such as cytokeratin 20 (CK20) and neuroendocrine proteins like chromogranin A, which aid in their identification and highlight their dual epithelial and sensory characteristics.^[3] In terms of function, Merkel cells play a central role in mechanotransduction by detecting low-threshold mechanical stimuli through Piezo2 ion channels, which generate rapid, calcium-permeable currents that depolarize the cell and trigger the release of protons, activating acid-sensing ion channels (ASICs) on associated sensory neurons to generate action potentials.^[2]^[4] This process contributes to the encoding of spatial details like texture and edges during touch, as evidenced by optogenetic studies showing sustained afferent firing dependent on Merkel cell activation.^[2] Beyond mechanosensation, emerging evidence suggests involvement in neuroendocrine signaling and possibly cutaneous immune modulation, though these roles remain under investigation.^[1] Embryologically, Merkel cells originate from epidermal keratinocyte progenitors rather than neural crest cells, first appearing in humans around the 8th week of gestation and persisting into adulthood through homeostatic renewal without significant proliferation.^[5] Their development is regulated by transcription factors like Atoh1 and signaling pathways such as Wnt, underscoring their integration into the epidermal lineage while acquiring specialized sensory properties.^[5]

Structure

Morphology

Merkel cells were first described in 1875 by German anatomist Friedrich Sigmund Merkel, who observed them under light microscopy in mammalian skin and termed them "Tastzellen" or touch cells due to their proximity to nerve endings.^[6] These cells are oval-shaped with lobulated nuclei and are primarily located in the basal layer of the epidermis, where they comprise approximately 0.2–5% of the epidermal cell population; this proportion is higher in humans at about 1.5% compared to roughly 0.1% in mice.^[7] They measure 10–15 μm in diameter and feature clear cytoplasm containing tonofilaments, with desmosomal connections to adjacent keratinocytes that anchor them within the epidermal architecture.^[8]^[9] In histological identification, Merkel cells are distinguished by immunohistochemical markers such as cytokeratin 20 (CK20), which typically exhibits a characteristic perinuclear ring-like or dot-like staining pattern, while neurofilament proteins highlight associated neurites in Merkel cell-neurite complexes.^[10] These complexes involve Merkel cells in close association with slowly adapting type 1 (SA1) afferent nerve endings.^[11]

Ultrastructure

Merkel cells exhibit distinctive subcellular features observable through electron microscopy, including dense-core neurosecretory granules within the cytoplasm. These granules measure 80–120 nm in diameter and are predominantly concentrated at the side of the cell in contact with sensory nerve terminals. They contain neuropeptides and other signaling molecules, such as serotonin (5-HT) and ATP, which are heterogeneously distributed and visualized via immunolabeling techniques.^[7]^[12]^[13] These granules facilitate neurotransmitter release during sensory signaling.^[7] At the interface with afferent nerve endings, Merkel cells form synapse-like structures characterized by clusters of dense-core vesicles in the cytoplasm and dense projections on the apposed membranes. These contacts express synaptic proteins, including synapsin and vesicle-associated membrane protein 2 (VAMP2), supporting vesicle docking and exocytosis.^[13]^[14]^[15] The cytoskeleton of Merkel cells includes intermediate filaments composed of simple epithelial keratins, specifically keratins 8, 18, 19, and 20, which form perinuclear networks and extend peripherally. Unlike keratinocytes, Merkel cells lack extensive tonofilament bundles anchored to desmosomes. They also incorporate melanosomes, often polarized away from nerve contacts, and feature microvilli projecting from the apical surface into the extracellular space above the basal layer.^[10]^[16]^[17]^[18]

Development

Embryonic origin

Merkel cells originate from epidermal progenitor cells during embryonic development, rather than from neural crest precursors, as established through lineage tracing experiments in mice.^[19] These studies employed K14-Cre recombinase to specifically label epidermal lineages, revealing that Merkel cells emerge from this compartment without contribution from neural crest-derived cells.^[19]^[20] In humans, Merkel cells first appear around the 8th to 12th week of gestation.^[21] A long-standing debate regarding the neural crest versus epidermal origin of Merkel cells was resolved in 2009 by independent genetic fate mapping studies, including those by Van Keymeulen et al. and Morrison et al., which confirmed the epidermal derivation through targeted labeling and ablation approaches.^[19]^[5] Differentiation of these progenitors into Merkel cells is critically dependent on the transcription factor Atoh1 and signaling pathways such as Wnt, whose expression initiates around embryonic day 12.5 in mice and drives the specification of postmitotic cells.^[22]^[23] Initial Merkel cells appear in whisker pads and limb buds around embryonic day 14.5, subsequently migrating to the basal epidermis without undergoing further proliferation.^[24]

Adult homeostasis

In adult skin, Merkel cells are maintained by a slow turnover process, with the population primarily sustained by long-lived cells generated during embryogenesis that persist into adulthood, supplemented by infrequent renewal from epidermal progenitors. Lineage tracing experiments in adult mice have identified contributions from stem cells in the hair follicle bulge region and interfollicular epidermis, labeled using Krt17-CreER to target touch dome keratinocytes and Lgr6-CreER to mark interfollicular progenitors, which generate new Merkel cells at a low rate during steady-state homeostasis.^[25]^[26] Merkel cells demonstrate remarkable plasticity in response to perturbations, regenerating rapidly within days following mechanical injury or hair plucking, a process reliant on Atoh1-expressing progenitors that differentiate into new mechanosensory cells. In contrast, sensory denervation triggers a gradual depletion of Merkel cells over several weeks, as the loss of neural signaling disrupts progenitor maintenance and leads to apoptosis without compensatory regeneration under normal conditions.^[26]^[27]^[28] As postmitotic cells lacking significant self-renewal capacity, Merkel cells exhibit a progressive decline in density with aging, contributing to reduced mechanosensory function in elderly skin. This age-related loss occurs without accelerated production from progenitors, reflecting the limited homeostatic replenishment in mature epidermis.^[26]^[29] Studies have revealed distinct progenitor pools supporting Merkel cell maintenance in different skin niches, with touch dome-associated cells deriving from specialized epidermal lineages separate from those in hair follicles, as evidenced by differential expression of markers like Sox9 in primary placode-derived populations.^[30]

Distribution

In skin

Merkel cells exhibit the highest density in glabrous skin regions of the human body, particularly in the fingertips where concentrations reach approximately 100–150 cells/mm², as well as in the lips and oral mucosa, areas critical for fine tactile discrimination.^[7] In contrast, densities are markedly lower in hairy skin, such as the back, with approximately 12 cells/mm², reflecting reduced mechanosensory demands in these sites.^[31] These variations have been quantified through immunohistochemical techniques targeting epidermal sheets from cadaveric samples.^[32] In glabrous skin of the palms and soles, Merkel cells cluster in association with Meissner corpuscles, forming specialized sensory complexes that enhance touch sensitivity.^[7] Within hairy skin, they are predominantly organized into touch domes surrounding hair follicles, with their numbers fluctuating in relation to the hair growth cycle—increasing notably during the anagen phase when follicles are actively growing.^[7] Comparative analyses across species reveal that Merkel cells are more abundant in humans than in rodents, where they constitute a smaller proportion of epidermal cells and are concentrated mainly in whisker pads.^[33] They are absent or present only rarely in certain mammals, including cats, highlighting evolutionary divergences in cutaneous innervation.^[33] Studies from 2022 utilizing CK20 immunostaining have further corroborated these regional density patterns, demonstrating consistent clustering in touch-sensitive glabrous areas and sparser distribution in hairy regions through high-resolution epidermal imaging.^[7] In these cutaneous locales, Merkel cells form intimate complexes with slowly adapting type I (SA1) nerve endings.^[7]

In other tissues

Merkel cells are present in the oral mucosa of humans, where they exhibit a lower density compared to those in the skin. Studies using cytokeratin 20 (CK20) immunohistochemistry have identified these cells in the basal layer of the oral epithelium, with mean densities reported as approximately 1.7 cells/mm² across various sites.^[34] These cells display similar morphological features to their cutaneous counterparts, including oval shapes and association with nerve endings.^[35] Merkel cells are also present in genital mucosa, though at low densities.^[36] In the esophageal epithelium, Merkel cells have been documented in adult humans, primarily in the mid-esophageal region, occurring singly or in small clusters. Identified via CK20 markers, these cells are absent in neonatal esophagus and show a sparse distribution, contributing to epithelial innervation.^[37] Reports of Merkel cells in the human conjunctiva are limited, with some studies noting their presence in the conjunctival stroma or associated structures, though at low densities and often in murine models rather than confirmed extensively in humans.^[38] In rodents, Merkel cells are rare within the hair follicles of vibrissae (whiskers), though they are more commonly associated with the outer root sheath; potential occurrences in the inner root sheath have been suggested in ultrastructural analyses.^[30] These cells are generally absent from most internal organs in mammals. However, in non-mammalian vertebrates, Merkel cell clusters have been reported in analogous sensory structures, such as the beak tip of birds like ducks and quails, where they form a significant portion of sensory receptors in the oral mucosa (up to 65% in quail beak tips), and in the barbels of fish like the carp (Cyprinus carpio), exhibiting ultrastructural features indicative of sensory function.^[39]^[40] Limited data on human non-cutaneous Merkel cells stem from early 2000s immunohistochemical studies, such as those by Moll et al., which utilized CK20 as a reliable marker to confirm their presence and distribution beyond the skin.^[10]

Function

Mechanosensory role

Merkel cells serve as primary mechanotransducers in the epidermis, converting mechanical stimuli into electrical signals via the PIEZO2 ion channel. When indented by sustained touch, PIEZO2 channels open to permit cation influx, including calcium ions, which depolarizes the Merkel cell and evokes action potentials. This process culminates in the release of neurotransmitters, such as serotonin and/or glutamate, from the Merkel cell to its innervating sensory neuron, thereby relaying the tactile signal; however, the exact neurotransmitter(s) remain a subject of debate.^[41]^[42]^[13] These cells associate with slowly adapting type 1 (SA1) Aβ low-threshold mechanoreceptor (LTMR) afferents to form Merkel cell-neurite complexes, which are specialized for detecting low-intensity, prolonged mechanical stimuli. The complexes generate sustained afferent firing that persists through both the initial dynamic phase (onset of indentation) and the subsequent static phase (sustained pressure), encoding stimulus features like duration and magnitude essential for touch perception.^[43] Merkel cells are indispensable for achieving high spatial acuity in touch and discriminating surface textures. Genetic ablation of Merkel cells in mice, such as through Atoh1 knockout, abolishes SA1 afferent responses to light touch and impairs behavioral tasks requiring texture detection, confirming their role in fine discriminatory mechanosensation.^[44] Seminal 2014 investigations by Woo et al. established PIEZO2 as the core mechanotransduction channel in Merkel cells, while Maksimovic et al. showed how Merkel cell activity tunes the sensitivity and spatiotemporal precision of associated LTMR afferents to natural tactile inputs.^[41]^[43]

Multimodal sensory roles

Merkel cells exhibit multimodal sensory capabilities that extend beyond their primary role in mechanosensation, integrating cues such as temperature and itch to contribute to cutaneous sensory processing.^[11] In thermosensation, Merkel cells detect non-noxious cooling through expression of TRPM8 channels, which are cold-sensitive cation channels that trigger intracellular calcium elevations in response to temperatures below 28°C. This function is supported by co-expression of TRPM8 with the mechanosensitive ion channel PIEZO2 in Merkel cells, enabling integrated responses to combined mechanical and thermal stimuli in slowly adapting type I afferents. Studies using genetic deletion of TRPM8 in mice demonstrate reduced neural firing in response to cooling, confirming Merkel cells' contribution to innocuous cold detection independent of free nerve endings.^[45] Merkel cells also mediate mechanical itch via histamine-independent pathways, where light mechanical stroking activates PIEZO2 channels to evoke pruriceptive signaling through low-threshold mechanoreceptors. This process involves Aβ afferent fibers innervating Merkel cell-neurite complexes, distinct from chemical itch circuits, and is physiologically suppressed under normal conditions to prevent excessive scratching. In pruritic states, enhanced activation of these pathways can amplify itch responses to gentle touch, as evidenced by increased scratching behaviors in models with altered Merkel cell function.^[46] Additionally, Merkel cells possess neuroendocrine features, potentially secreting vasoactive intestinal peptide (VIP) and calcitonin gene-related peptide (CGRP) to modulate local vascular tone and inflammation during sensory activation. These peptides, stored in dense-core granules, may be released upon stimulation to influence nearby keratinocytes, immune cells, or blood vessels, thereby fine-tuning sensory feedback loops. Recent single-cell RNA sequencing analyses from 2018 to 2023 have revealed transcriptional profiles supporting this multimodal integration, including co-expression of sensory receptors and neuropeptide machinery in Merkel cells.^[1]^[11]

Clinical significance

Association with Merkel cell polyomavirus

The Merkel cell polyomavirus (MCPyV) was discovered in 2008 using digital transcriptome subtraction on Merkel cell carcinoma (MCC) samples, revealing its clonal integration in tumor genomes.^[47] In normal skin, MCPyV DNA exists primarily as episomes within Merkel cells of most adults, reflecting a widespread latent infection with seroprevalence exceeding 90%.^[48] By contrast, the viral genome is integrated into the host DNA in approximately 80% of MCC cases.^[49] The virus encodes small T antigen and large T antigen, which are expressed at low levels in latently infected normal Merkel cells to maintain viral persistence without productive replication or cell proliferation.^[50] In this state, the antigens interact with host factors like ubiquitin ligases to regulate latency, preventing virion production.^[51] Primary MCPyV infection typically occurs during early childhood, likely through skin-to-skin contact or close interpersonal exchanges such as between siblings, establishing lifelong latency thereafter.^[52] No evidence indicates productive viral replication in normal Merkel cells, which are not the primary site of active infection; instead, dermal fibroblasts support initial replication.^[53] Under conditions of immunosuppression, this latency can be disrupted, potentially leading to MCC development in susceptible individuals.^[54]

Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine malignancy of the skin that typically arises in or differentiates toward Merkel cells, representing less than 1% of all cutaneous cancers.^[55] It is characterized by rapid progression and a high propensity for regional lymph node involvement and distant metastasis, primarily affecting older adults with fair skin.^[56] As of 2025, the annual incidence in the United States is approximately 0.68 cases per 100,000 person-years, equating to about 3,200 new diagnoses yearly; rates have tripled over the past three decades due to improved detection and an aging population.^[56]^[57]^[58] The primary risk factors for MCC include chronic ultraviolet (UV) radiation exposure, advanced age (median diagnosis at 76 years), male sex, and immunosuppression from conditions like HIV, organ transplantation, or chronic lymphocytic leukemia.^[55] A key etiological driver is infection with Merkel cell polyomavirus (MCPyV), present in approximately 80% of cases, distinguishing virus-positive MCC from the UV-driven virus-negative subtype comprising the remaining 20%.^[59] Virus-positive tumors often occur in areas with less sun exposure and are linked to clonal integration of the viral genome, while virus-negative tumors show UV-signature mutations and arise predominantly on sun-exposed sites.^[60] In pathogenesis, MCPyV-positive MCC relies on truncated large T antigen expression, which binds and inactivates retinoblastoma (Rb) family proteins to disrupt cell cycle control, while also indirectly impairing p53 tumor suppressor function through small T antigen-mediated pathways.^[61] Virus-negative MCC exhibits a high tumor mutation burden from UV-induced DNA damage, with frequent inactivating mutations in TP53 and RB1 genes, leading to similar oncogenic deregulation.^[60] Both subtypes commonly overexpress programmed death-ligand 1 (PD-L1) on tumor and immune cells within the microenvironment, fostering T-cell exhaustion and immune evasion that contributes to disease progression.^[62] Clinically, MCC manifests as a rapidly enlarging, painless, firm nodule or plaque, often red to violaceous, measuring 1-5 cm at diagnosis, most frequently on sun-exposed areas such as the head, neck, or extremities in elderly or immunosuppressed individuals.^[63] Regional lymph nodes are involved in up to 30% of cases at presentation, with distant metastases (e.g., to lung, liver, or brain) occurring in 10-20%.^[64] Staging follows the American Joint Committee on Cancer (AJCC) 8th edition system, categorizing disease as stage I (tumor ≤2 cm, node-negative), stage II (tumor >2 cm or with perineural invasion, node-negative), stage III (regional nodal involvement, subdivided by microscopic vs. macroscopic disease), or stage IV (distant metastasis).^[65] Standard treatment for localized MCC involves wide local excision with 1-2 cm margins, confirmed by sentinel lymph node biopsy to guide nodal management, followed by adjuvant radiotherapy to the primary site and draining nodes to reduce locoregional recurrence from 40% to under 10%.^[66] For advanced or metastatic disease, systemic immunotherapy with anti-PD-1/PD-L1 agents—such as pembrolizumab (FDA-approved in 2018) or avelumab (approved in 2017)—has transformed outcomes, yielding objective response rates of 50-60% and durable remissions in responders, irrespective of viral status.^[67] Emerging strategies include clinical trials evaluating hypomethylating agents like decitabine to reverse epigenetic silencing of HLA class I expression, potentially enhancing immunotherapy efficacy, with preclinical data from 2024 showing restored antigen presentation in resistant tumors.^[68] Recent adjuvant trials, such as the 2025 STAMP study, demonstrate that postoperative pembrolizumab reduces recurrence risk by 35% in high-risk stage II-III cases compared to observation alone.^[69] As of 2025, prognosis varies by stage and subtype, with an overall 5-year relative survival rate of 70%; localized disease achieves 82% survival, regional nodal involvement 62%, and distant metastases 27%.^[70] Virus-positive MCC generally portends better outcomes due to lower mutation burden and higher immunogenicity, though virus-negative cases show increased aggressiveness.^[71] Advances in single-cell transcriptomics, including 2025 spatial profiling studies, have unveiled intratumoral heterogeneity and phenotypic plasticity in MCC, highlighting therapeutic targets like epigenetic regulators and immune-modulating pathways to address resistance.^[72]

References

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Immunotherapy after surgery shows potential in preventing the ...
Oct 20, 2025 · “The STAMP trial provides the first evidence that immunotherapy with pembrolizumab after surgery may help people with Merkel cell carcinoma by ...
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Survival Rates for Merkel Cell Carcinoma | American Cancer Society
Jun 27, 2025 · The 5-year relative survival rate for a specific stage of Merkel cell carcinoma (MCC) is 70%, it means that people who have that cancer are, on average, about ...
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Virus-positive Merkel Cell Carcinoma Is an Independent Prognostic ...
Merkel cell carcinoma (MCC) is an aggressive cutaneous neuroendocrine carcinoma that can be divided into two classes: virus-positive (VP) MCC, ...
[65]
High-resolution spatial transcriptomics uncover epidermal-dermal ...
Oct 23, 2025 · Merkel cell carcinoma (MCC) is a rare, aggressive skin cancer with neuroendocrine differentiation usually occurring in the elderly (ref. [1]).