Merkel Cells, Normal and Neoplastic: An Update

Current In Vitro and In Vivo Models to Study MCPyV-Associated MCC

Merkel cell polyomavirus (MCPyV) is the only human polyomavirus currently known to cause human cancer. MCPyV is believed to be an etiological factor in at least 80% of cases of the rare but aggressive skin malignancy Merkel cell carcinoma (MCC). In these MCPyV+ MCC tumors, clonal integration of the viral genome results in the continued expression of two viral proteins: the viral small T antigen (ST) and a truncated form of the viral large T antigen. The oncogenic potential of MCPyV and the functional properties of the viral T antigens that contribute to neoplasia are becoming increasingly well-characterized with the recent development of model systems that recapitulate the biology of MCPyV+ MCC. In this review, we summarize our understanding of MCPyV and its role in MCC, followed by the current state of both in vitro and in vivo model systems used to study MCPyV and its contribution to carcinogenesis. We also highlight the remaining challenges within the field and the major considerations related to the ongoing development of in vitro and in vivo models of MCPyV+ MCC.

Identification of Merkel cells associated with neurons in engineered skin substitutes after grafting to full thickness wounds

Engineered skin substitutes (ESS), prepared using primary human fibroblasts and keratinocytes with a biopolymer scaffold, were shown to provide stable closure of excised burns, but relatively little is known about innervation of ESS after grafting. This study investigated innervation of ESS and, specifically, whether Merkel cells are present in healed grafts. Merkel cells are specialized neuroendocrine cells required for fine touch sensation in skin. We discovered cells positive for keratin 20 (KRT20), a general marker for Merkel cells, in the basal epidermis of ESS after transplantation to mice, suggesting the presence of Merkel cells. Cells expressing KRT20 were not observed in ESS in vitro. However, widely separated KRT20-positive cells were observed in basal epidermis of ESS by 2 weeks after grafting. By 4 weeks, these cells increased in number and expressed keratins 18 and 19, additional Merkel cells markers. Putative Merkel cell numbers increased further between weeks 6 and 14; their densities varied widely and no specific pattern of organization was observed, similar to Merkel cell localization in human skin. KRT20-positive cells co-expressed epidermal markers E-cadherin and keratin 15, suggesting derivation from the epidermal lineage, and neuroendocrine markers synaptophysin and chromogranin A, consistent with their identification as Merkel cells. By 4 weeks after grafting, some Merkel cells in engineered skin were associated with immature afferents expressing neurofilament-medium. By 8 weeks, Merkel cells were complexed with more mature neurons expressing neurofilament-heavy. Positive staining for human leukocyte antigen demonstrated that the Merkel cells in ESS were derived from grafted human cells. The results identify, for the first time, Merkel cell-neurite complexes in engineered skin in vivo. This suggests that fine touch sensation may be restored in ESS after grafting, although this must be confirmed with future functional studies.

Merkel Cells: A Collective Review of Current Concepts

Merkel cells (MCs) constitute a very unique population of postmitotic cells scattered along the dermo-epidermal junction. These cells that have synaptic contacts with somatosensory afferents are regarded to have a pivotal role in sensory discernment. Several concerns exist till date as to their origin, multiplication, and relevance in skin biology. The article, a collective review of literature extracted from PubMed search and dermatology books, provides novel insights into the physiology of MCs and their recent advances.

Rare tumors of esophageal squamous mucosa

In spite of increasing incidence of esophageal adenocarcinoma in the last few decades, esophageal squamous cell carcinoma (SCC) still remains the dominant subtype of esophageal cancer worldwide. Apart from conventional SCC, some rare unconventional tumors of esophageal squamous mucosa are also well known. This study provides an introduction to these and presents a brief review of the literature, including the diagnostic and prognostic importance of each variant.

Fibroepithelioma of Pinkus: Solitary tumor or sign of a complex gastrointestinal syndrome

Fibroepithelioma of Pinkus (FEP), which is considered to be an uncommon variant of basal cell carcinoma, has been described in association with other systemic diseases. However, no specific studies are currently available on this subject. The aim of our study was to evaluate the clinical and morphological characteristics of FEP and investigate whether this rare tumor is a single entity or seen in the context of a more complex syndrome. We retrospectively analyzed 49 cases of FEP diagnosed and excised in a single academic institution from 1995 to 2011. The tumors were mainly located on the trunk (77.55%), followed by the lower extremities (12.20%) and the head and neck (10.20%). In 9 of the 49 cases (18%), FEP was associated with gastrointestinal tumors. The abovementioned cases are presented in an attempt to make clinicians more aware of a possible association between FEP and gastrointestinal cancer. Although a possible underlying common genetic background between FEP and gastrointestinal tumors was not provided, our study suggests that patients with FEP should be screened for the occurrence of gastrointestinal tumors.

Primary retroperitoneal Merkel cell carcinoma: Case report and literature review

BACKGROUND

Merkel cell carcinoma (MCC) is an aggressive cutaneous neuroendocrine carcinoma that affects elderly patients and typically arises in sun-exposed skin. The disease is very rare and only few cases present with no apparent skin lesion. In the retroperitoneum there are only two cases reported in the literature.

CASE PRESENTATION

We report a case of a 54-year-old Mexican male with MCC, which presented as a large retroperitoneal mass. Pathological and immunohistochemical analysis of the transabdominal CT-guided biopsy specimen revealed a MCC. The patient underwent preoperative chemotherapy followed by a laparotomy and the mass was successfully excised.

DISCUSSION

There are two possible explanations for what occurred in our patient. The most plausible theory is the retroperitoneal mass could be a massively enlarged lymph node where precursor cells became neoplastic. This would be consistent with a presumptive diagnosis of primary nodal disease. Moreover, metastasis to the retroperitoneal lymph nodes has been reported as relatively common when compared to other sites such as liver, bone, brain and skin. The less probable theory is the non-described "regression" phenomena of a cutaneous MCC, but we are not found a primary skin lesion.

CONCLUSION

Preoperative chemotherapy and excision of the primary tumor is the surgical treatment of choice for retroperitoneal MCC. We propose that further studies are needed to elucidate the true efficacy of chemotherapy in conventional and unconventional patients with MCC.

Unipotent, Atoh1+ progenitors maintain the Merkel cell population in embryonic and adult mice

Resident progenitor cells in mammalian skin generate new cells as a part of tissue homeostasis. We sought to identify the progenitors of Merkel cells, a unique skin cell type that plays critical roles in mechanosensation. We found that some Atoh1-expressing cells in the hairy skin and whisker follicles are mitotically active at embryonic and postnatal ages. Genetic fate-mapping revealed that these Atoh1-expressing cells give rise solely to Merkel cells. Furthermore, selective ablation of Atoh1(+) skin cells in adult mice led to a permanent reduction in Merkel cell numbers, demonstrating that other stem cell populations are incapable of producing Merkel cells. These data identify a novel, unipotent progenitor population in the skin that gives rise to Merkel cells both during development and adulthood.

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.

Beyond expectations: novel insights into epidermal keratin function and regulation

The epidermis is a stratified epithelium that relies on its cytoskeleton and cell junctions to protect the body against mechanical injury, dehydration, and infections. Keratin intermediate filament proteins are involved in many of these functions by forming cell-specific cytoskeletal scaffolds crucial for the maintenance of cell and tissue integrity. In response to various stresses, the expression and organization of keratins are altered at transcriptional and posttranslational levels to restore tissue homeostasis. Failure to restore tissue homeostasis in the presence of keratin gene mutations results in acute and chronic skin disorders for which currently no rational therapies are available. Here, we review the recent progress on the role of keratins in cytoarchitecture, adhesion, signaling, and inflammation. By focusing on epidermal keratins, we illustrate the contribution of keratin isotypes to differentiated epithelial functions.

Merkel cell polyomavirus: a newly discovered human virus with oncogenic potential

A marked escalation in the rate of discovery of new types of human polyomavirus has occurred over the last five years largely owing to recent technological advances in their detection. Among the newly discovered viruses, Merkel Cell Polyomavirus (MCPyV or MCV) has gained the most attention due to its link with a rare human cancer. Infection with MCPyV is common in the human population, and the virus is detected in several anatomical locations, but most frequently in skin. Study of MCPyV molecular virology has been complicated by the lack of straightforward cell culture models, but recent in vitro studies are making strides towards understanding the virus life cycle, its cellular tropism, and mode of transmission. While MCPyV shares several traditional traits with other human polyomaviruses, the burst of research since its discovery reveals insight into a virus with many unique genetic and mechanistic features. The evidence for a causal link between MCPyV and the rare neuroendocrine cancer, Merkel Cell Carcinoma (MCC), is compelling. A majority of MCCs contain clonally integrated viral DNA, express viral T antigen transcripts and protein, and exhibit an addiction to the viral large T and small t antigen oncoproteins. The MCPyV large T antigen contains MCC tumor-specific mutations that ablate its replication capacity but preserve its oncogenic functions, and the small t antigen promotes an environment favorable for cap-dependent translation. The mechanisms of MCPyV-induced transformation have not been fully elucidated, but the likely etiological role of this new polyomavirus in human cancer provides a strong opportunity to expand knowledge of virus-host interactions and viral oncology.

BMI1 expression identifies subtypes of Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a rare cutaneous neuroendocrine carcinoma. The aims of this study were to investigate the expression of the transcription factors B-lymphoma Moloney murine leukaemia virus insertion (BMI1), myelocytomatosis viral oncogene homologue (c-Myc) and Snail in MCC tumour specimens and to examine the relationship of these markers to Merkel cell polyoma virus (MCV). The study comprised of 133 patients with primary MCC. The expression of BMI1, Snail and c-Myc protein was assessed by immunohistochemistry and compared with clinical parameters, MCV status and patient survival. The presence of MCV was inversely correlated with the expression of BMI1 protein. Tumours expressing BMI1 protein more often presented with lymph node metastases. Snail protein expression was decreased in cases with metastatic dissemination. This study identified two subgroups of MCC: tumours expressing BMI1 but negative for MCV DNA and tumours negative for BMI1 expression but positive for MCV. Importantly, BMI1-positive cases often presented with lymph node metastases. Combined, these results suggest that subtypes of this malignancy exist.

Role of sentinel lymph node biopsy in the management of merkel cell carcinoma

Merkel cell carcinoma (MCC) is a rare and typically aggressive form of skin cancer. It most commonly affects the elderly and has a predilection for the sun-exposed skin of the head and neck region. Other etiological factors include immune suppression, organ transplantation, and polyoma virus infection. MCC has a propensity to spread to regional lymphatics with a high locoregional recurrence rate. Since its discovery in 1972, treatment paradigms have shifted, with no consensus on optimal management strategies. Currently, standard of care includes surgical intervention to the primary and locoregional site with adjuvant radiotherapy for high-risk disease. In this paper, we discuss the history, pathology, and epidemiology of this rare disease with a focus on the evidentiary basis of treatment protocols. The use of sentinel lymph node biopsy as a management option will be the focus of this paper.

Intercellular adhering junctions with an asymmetric molecular composition: desmosomes connecting Merkel cells and keratinocytes

Merkel cells (MCs) are special neuroendocrine epithelial cells that occur as individual cells or as cell groups within the confinements of a major epithelium formed and dominated by other epithelial cells. In the epidermis and some of its appendages MCs are mostly located in the basal cell layer, occasionally also in suprabasal layers and generally occur in linear arrays in outer root sheath cell layers of hair follicles. As MCs are connected to the adjacent keratinocytes by a series of adhering junctions (AJs), of which the desmosomes are the most prominent, these junctions represent heterotypic cell-cell connections, i.e. a kind of structure not yet elucidated in molecular terms. Therefore, we have studied these AJs in order to examine the molecular composition of the desmosomal halves. Using light- and electron-microscopic immunolocalization and keratin 20 as the MC-specific cell type marker we show that the plaques of the MC half of the desmosomes specifically and constitutively contain plakophilin Pkp2. This protein, however, is absent in the keratinocyte half of such heterotypic desmosomes which instead contains Pkp1 and/or Pkp3. We discuss the developmental, tissue-architectonic and functional importance of such asymmetric junctions in normal physiology as well as in diseases, in particular in the formation of distant tumor cell metastasis.

Homo- and heterotypic cell-cell contacts in Merkel cells and Merkel cell carcinomas: heterogeneity and indications for cadherin switching

AIMS

Merkel cell carcinomas (MCCs) are rare but aggressive tumours associated recently with Merkel cell polyomavirus (MCV). As development and progression of several types of carcinomas can be promoted by changes in cell adhesion proteins, the aim of this study was to examine homo- and heterotypic cell contacts of Merkel cells and MCCs.

METHODS AND RESULTS

Merkel cells of healthy glabrous epidermis and 52 MCCs were analysed by double-label immunostaining, immunofluorescence and confocal microscopy. Merkel cells were connected to keratinocytes by E- and P-cadherin, desmoglein 2 and desmocollin 2. In contrast, the vast majority of MCCs (90%) contained N-cadherin, but only 67% and 65% contained E- and P-cadherin, respectively. Interestingly, P-cadherin was absent significantly more frequently in lymph node metastases than in primary tumours and by trend in more advanced clinical stages. Moreover, major subsets of MCCs synthesized desmoglein 2 and, surprisingly, tight junction proteins. No significant differences were observed upon stratification for MCV DNA, detected in 84% of tumours by real-time polymerase chain reaction.

CONCLUSIONS

Assuming that MCCs originate from Merkel cells, our data indicate a switch from E- and P-cadherin to N-cadherin during tumorigenesis. Whether the unexpected heterogeneity of junctional proteins can be exploited for prognostic and therapeutic purposes will need to be examined.

Updated Review of the Pathogenesis and Management of Merkel Cell Carcinoma

Background:

Merkel cell carcinoma is a rare, aggressive, malignant cutaneous tumor of the elderly or immunosuppressed individuals that usually appears on sun-exposed areas of the body. Its pathogenesis is still debated, and, currently, no standardized treatment exists.

Objective:

To provide a current updated review of the most relevant data concerning the pathogenesis and management of Merkel cell carcinoma.

Methods:

Using relevant MeSH terms, we performed a review of the literature on these subjects from 1980 to June 2009.

Results and Conclusion:

The current management of Merkel cell carcinoma is based on surgical excision as the majority of patients present with localized disease, whereas up to 30% have regional lymph node metastases. In these cases, the best outcome is achieved with multidisciplinary management that includes radiotherapy. Chemotherapy is part of the treatment in advanced cases and is mandatory for distant metastatis. Given that a recent work showed the presence of a previously unknown polyomavirus, which the authors called Merkel cell polyomavirus, the therapeutical approach to Merkel cell carcinoma could be reconsidered in the future.

Mammalian Merkel cells are descended from the epidermal lineage

Merkel cells are specialized cells in the skin that are important for proper neural encoding of light touch stimuli. Conflicting evidence suggests that these cells are lineally descended from either the skin or the neural crest. To address this question, we used epidermal (Krt14(Cre)) and neural crest (Wnt1(Cre)) Cre-driver lines to conditionally delete Atoh1 specifically from the skin or neural crest lineages, respectively, of mice. Deletion of Atoh1 from the skin lineage resulted in loss of Merkel cells from all regions of the skin, while deletion from the neural crest lineage had no effect on this cell population. Thus, mammalian Merkel cells are derived from the skin lineage.

Merkel cell carcinoma of the inguinal lymph node with an unknown primary site

Merkel cell carcinoma (MCC) is an uncommon and aggressive primary neuroendocrine malignancy of the skin. Frequent local recurrences and disseminations to regional lymph nodes and distant organs are characteristic. MCC within the lymph nodes in the absence of a primary site is rare and has only been reported sporadically. We report a case of MCC presenting as a painless mass in the left inguinal area for 5 months in a 57-year-old man. The histopathology of the excised lesion revealed a poorly differentiated basophilic small cell tumor. The tumor cells were positive for cytokeratin 20 and CD56, negative for cytokeratin 7, thyroid transcription factor-1 and CDX-2. These immunohistochemical findings were consistent with the diagnosis of a metastatic MCC. Despite extensive clinical and radiological investigation, we failed to identify the origin of the tumor. Our case may represent a lymph node metastasis from an occult or regressed skin primary, but we cannot preclude the possibility of a primary nodal tumor.

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.

Merkel cell carcinoma: review

Merkel cell carcinoma (MCC) is synonymous with primary cutaneous neuroendocrine carcinoma. It tends to affects elderly whites, but there is also an increased incidence among immunosuppressed patients. The recent identification of a novel polyomavirus associated with the tumor has stimulated renewed interest in its pathogenesis. MCC tends to show classic histologic features of a neuroendocrine carcinoma and is often positive for CK20, but nonclassic cytologic findings and unusual immunophenotypes may be observed and can lead to a diagnostic confusion. MCC needs to be distinguished from other primary cutaneous tumors with a small cell appearance and metastatic tumors. Surgical excision is the treatment of choice, but radiation therapy has also found to be effective. Sentinel lymph node biopsy has become an integral part of the staging of patients with MCC.

Nonmelanoma skin cancer of the head and neck I: histopathology and clinical behavior

Non-Melanoma skin cancer (NMSC) is the most commonly encountered malignancy in almost every area of practice, but the cases that present to an Otolaryngology practice will be advanced in nature. The major subtypes of NMSC include basal cell carcinoma, squamous cell carcinoma, dermatofibrosarcoma protuberans, merkel cell carcinoma, and adnexal malignancies. In this review, we present the epidemiology, histology, clinical presentation and management of these major subtypes. Further, we present background on multimodality treatment for NMSC lesions that have become metastatic from their primary site and an introduction to the behavior and treatment of NMSC lesions in patients who have received organ transplants. Understanding the clinical behavior of advanced NMSC is essential knowledge for a general Otolaryngologist.

Keratin expression in endocrine organs and their neoplasms

Keratins are intermediate filaments that provide mechanical support and fulfill a variety of additional functions in epithelial cells. Keratins show outstanding degree of molecular diversity. In humans, 54 functional keratin genes exist. Twenty common types of keratins are expressed in highly specific patterns related to epithelial type and stage of cellular differentiation. In general, keratins are classified as high-molecular-weight keratins (expressed in normal stratified epithelium and tumors derived from it) and low-molecular-weight keratins (expressed in normal simple epithelium and tumors derived from it). Histologically, endocrine organs belong to simple epithelium; thus, endocrine tissues usually express low-molecular-weight keratins. When an endocrine organ undergoes malignant transformation, its keratin profile usually remains constant. However, keratin expression in endocrine organs and endocrine tumors is much more complicated because of their diversified histogenesis. In this review article, we will first briefly review the molecular biology and protein chemistry of the keratins. We will then review the expression patterns of keratins in normal endocrine tissue and endocrine neoplasms.

Merkel cell carcinoma with squamous and sarcomatous differentiation

Merkel cell carcinoma is an aggressive neuroendocrine tumor historically thought to arise from neural crest-derived cutaneous neuroendocrine cells. Recent evidence supports an epidermal origin. We present a case of Merkel cell carcinoma arising on the upper arm of a 94-year-old woman that had multiple morphologic patterns: small cells typical of Merkel cell carcinoma, malignant cells with squamous differentiation and malignant poorly differentiated spindle cells. Subsequent metastatic disease in regional lymph nodes showed only the small cells and the malignant spindle cells. To our knowledge, this is the first case of Merkel cell carcinoma showing these three patterns of differentiation at first presentation. This morphology raises the possibility that Merkel cell carcinomas may arise from epidermal stem cells that can differentiate along different lines.

A comprehensive review of the treatment of Merkel cell carcinoma

Merkel cell carcinoma (MCC) is an uncommon but malignant cutaneous neuroendocrine carcinoma with a high incidence of local recurrence, regional lymph node metastases, and subsequent distant metastases. The etiology of MCC remains unknown. It usually occurs in sun-exposed areas in elderly people, many of whom have a history of other synchronous or metachronous sun-associated skin lesions. The outcome for most patients with MCC is generally poor. Surgery is the mainstay of treatment. The role of adjuvant therapy has been debated. However, data from recent development support a multimodality approach, including surgical excision of primary tumor with adequate margins and sentinel lymph node dissection followed by postoperative radiotherapy in most cases, as current choice of practice with better locoregional control and disease-free survival. Patients with regional nodal involvement or advanced disease should undergo nodal dissection followed by adjuvant radiotherapy and, perhaps, systemic platinum-based chemotherapy in most cases.

Merkel cells

Merkel cells are post-mitotic cells scattered throughout the epidermis of vertebrates. They are particularly interesting because of the close connections that they develop with sensory nerve endings and the number of peptides they can secrete. These features suggest that they may make an important contribution to skin homeostasis and cutaneous nerve development. However, these cells remain mysterious because they are difficult to study. They have not been successfully cultured and cannot be isolated, severely hampering molecular biology and functional analysis. Merkel cells probably originate in the neural crest of avians and mammalians, and their "spontaneous" appearance in the epidermis may be caused by a neuron-independent epidermal differentiation process. Their functions are still unclear: they take part in mechanoreception or at least interact with neurons, but little is known about their interactions with other epidermal cells. This review provides a new look at these least-known cells of the skin. The numerous peptides they synthesize and release may allow them to communicate with many cells other than neurons, and it is plausible that Merkel cells play a key role in skin physiology and physiopathology.

Cytokeratin expression in palatal and marginal mucosa of cleft palate patients

OBJECTIVE

The margin of a palatal cleft is a unique anatomical site since the palatal mucosa is continuous with the nasal or nasopharyngeal mucosa. The aim of this study was to compare the expression patterns of cytokeratins and basal membrane components of the mucosa in the area of the cleft.

DESIGN

Biopsies from the mucosa of the hard palate and from the cleft margin in the soft palate were obtained from five patients during the primary surgical closure of the cleft. The tissues were processed for haematoxylin-eosin staining and for immunohistochemistry. Antibodies against the cytokeratins (CK) 4, 7, 8, 10, 13, 16 and 18, and the basal membrane components heparan sulphate (HS) and collagen type IV (CIV) were used for immunostaining.

RESULTS

The nasopharyngeal epithelium was thinner than the epithelium of the soft palatal mucosa, and showed less interpapillary ridges. The nasopharyngeal epithelium was stratified but expressed the keratins of a simple epithelium (CK 7, 8 and 18). The expression pattern abruptly changed into that of a typical non-keratinized stratified epithelium (CK 4, 13) at the transition to the soft palatal epithelium. The epithelium of the hard palate was a fully differentiated, keratinized and stratified epithelium (CK 10, 16). The basal membrane was thinner in the nasopharyngeal epithelium, which might be related to the presence of abundant inflammatory cells.

CONCLUSION

The area around the palatal cleft showed three different types of epithelium. There was an abrupt transition in phenotype of the epithelium from the oral side to the nasopharyngeal side.

Merkel Cell Tumor in a Trichilemmal Cyst: Collision or Association?

An 86-year-old white male presented with an erythematous, painless, slowly growing, and firm left thigh nodule. Histologic examination revealed a dermal proliferation of monomorphous cells arranged in trabeculae, nests, and sheets with an infiltrative growth pattern. The cells had a high nuclear-cytoplasmic ratio, finely granular nuclear chromatin, and nuclear molding. Numerous mitotic figures, apoptotic cells, and individual cell necrosis were present; lymphovascular invasion was identified. The tumor was attached, demonstrating pagetoid intraepithelial migration, to a follicular cyst lined by squamous epithelium, lacking a granular cell layer and filled with compact keratinous content, diagnostic of trichilemmal cyst. Immunohistochemical study revealed that tumor cells expressed pan-cytokeratin (CK), chromogranin, synaptophysin, neuron-specific enolase, and CK20 (dotlike staining pattern), thus supporting the diagnosis of Merkel cell carcinoma. The association of Merkel cell carcinoma with a cyst is an exceptionally rare occurrence. As a result of the prominent involvement of the cyst wall by tumor cells, we favor that in this case carcinoma arose in the trichilemmal cyst rather than being a collision tumor. This hypothesis is also supported by the recent observation that Merkel cells are frequently present within normal hair follicles, especially in the isthmic portion that corresponds with the area of origin of the trichilemmal cyst.

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.

Poorly differentiated tumours of the anal canal: a diagnostic strategy for the surgical pathologist

Poorly differentiated malignancies affecting the anal canal are uncommon but pose diagnostic difficulties because of the wide range of normal cell types that may occur within a limited anatomical region. The range of lesions that may present as poorly differentiated tumours includes squamous cell carcinoma, adenocarcinoma, small and large cell neuroendocrine carcinoma, neuroendocrine carcinoma expressing epithelial cytokeratins and other patterns of mixed differentiation, undifferentiated carcinoma, malignant melanoma, lymphoma and secondary tumours. This review discusses the differential diagnosis of these neoplasms with the aid of short illustrative case studies.

The immunohistochemical characterization of devil facial tumor disease (DFTD) in the Tasmanian Devil (Sarcophilus harrisii)

Immunohistochemical techniques were used to characterize the disfiguring and debilitating fatal neoplastic disease, devil facial tumor disease (DFTD), which has recently affected a significant proportion of the wild population of Tasmanian Devils (Sarcophilus harrisii). The diagnostic values of a number of immunohistochemical stains were employed to further characterize 50 representative cases. The neoplasms were negative for cytokeratin (0/48), epithelial membrane antigen (0/42), von Willebrand factor (vWF) (0/11), smooth muscle actin (SMA) (0/26), desmin (0/47), glial fibrillary acid protein (0/13), CD16 (0/13), CD57 (0/43), CD3 (0/18), and LSP1 (0/16). DFTD cells were positive for vimentin (50/50), S-100 (41/48), melan A (11/39), neuron specific enolase (35/35), chromogranin A (12/12) and synaptophysin (29/30). The cells were negative for amyloid (0/30) and stained negatively with Singh's silver (0/34) but were weakly argyrophilic (3/40) using Grimelius histochemical stain. These staining characteristics are consistent with cells of neuroectodermal origin.

Prognostic value of tumor thickness in patients with merkel cell carcinoma

BACKGROUND AND OBJECTIVES

Merkel cell carcinoma is an aggressive skin malignancy that often presents with tumor metastases. We hypothesized that tumor thickness might correlate with both regional and metastatic tumor spread and could, therefore, be used as an independent prognostic variable. The purpose of this study was to see if depth of tumor invasion would predict prognosis independent of tumor stage.

METHODS

Data pertaining to clinical presentation, pathology, treatment, and survival were collected for patients diagnosed with Merkel cell carcinoma from 1972 to 2005. Patients were staged according to AJCC guidelines. Pathologic specimens were evaluated for tumor thickness. The relationship between tumor thickness and disease-free survival or overall survival was analyzed using Kaplan-Meier survival analyses.

RESULTS

Sixty patients were identified. Five-year disease-free survivals for Stages 1, 2, and 3 patients were 20%, 33%, and 0%, respectively. Five-year overall survivals for Stages 1, 2, and 3 patients were 33.3%, 60%, and 16.7%, respectively. There was no correlation between tumor thickness and either disease-free survival or overall survival.

CONCLUSIONS

This study suggests that tumor thickness is not an independent risk factor for survival. Mean tumor thickness did increase with the AJCC stages, but this most likely represents more advanced stage of disease.

Hair—the most sophisticated biological composite material

Hair is a proteinaceous fibre with a strongly hierarchical organization of subunits, from the alpha-keratin chains, via intermediate filaments, to the fibre. The chemistry of the different morphological compartments results in exciting physical properties, including the hydrophilic/hydrophobic paradox. The present tutorial review will be of interest for protein- as well as polymer chemists, who want to learn from nature, and also for biochemists interested in the cytoskeleton and particularly in intermediate filaments; it also presents a scientific basis for hair cosmetics.

Expression profiles associated with aggressive behavior in Merkel cell carcinoma

Primary neuroendocrine carcinoma of the skin, or Merkel cell carcinoma, is the most aggressive cutaneous neoplasm. In spite of its similarities to small cell carcinomas from other locations, Merkel cell carcinoma shows many peculiarities probably related to its epidermal origin and the etiologic role of UV radiation. We have immunohistochemically investigated 43 markers on a tissue microarray in which 31 surgically resected Merkel cell carcinomas were represented. Of these, 15 patients remained free of disease after removal, whereas 16 developed metastases. Immunoreactivity was scored according to staining intensity and the percentage of positive cells. We found statistically significant correlations between metastatic tumor spread and overexpression of matrix metalloproteinase (MMP) 7, MMP10/2, tissue inhibitor of metalloproteinase 3, vascular endothelial growth factor (VEGF), P38, stromal NF-kappaB, and synaptophysin. Also detected were statistically significant correlations between the expression levels of MMP7 and VEGF, MMP7 and P21, MMP7 and P38, MMP10/2 and VEGF, P38 and synaptophysin, P38 and P53, and P21 and stromal NF-kappaB. These findings may be helpful in predicting the clinical course of Merkel cell carcinoma and are potentially useful for the development of targeted therapies.

Immunohistochemical and ultrastructural investigation on cutaneous neuroendocrine carcinoma: report of a case and review of the literature

We report a tumor in an 80-year-old man that was difficult to distinguish from other tumors, i.e., small cell carcinoma of the lung, PNET/Ewing tumor, malignant lymphoma, or malignant melanoma (amelanotic), and which was finally identified as cutaneous neuroendocrine carcinoma using immunohistochemical and ultrastructural methods. Autopsy did not show any tumors in the lungs, excluding the possibility of small cell carcinoma of the lung. Immunohistochemistry tests gave negative results for LCA, UCHL-1, CD3, and CD20, thereby excluding malignant lymphoma, and the negative results for S-100 protein and HMB-45 ruled out malignant melanoma. The possibility of PNET/Ewing sarcoma was also excluded because of negativity for CD99. In addition, the ultramicrostructure showed intercellular junctional complexes and neuroendocrine granules, indicating that the tumor had characteristics of both epithelial and neuroendocrine tissues. We therefore diagnosed the primary carcinoma of the skin as cutaneous neuroendocrine carcinoma.

Merkel Cell Carcinoma: Is there a Role for 2-Deoxy-2-[F-18]fluoro-d-glucose-Positron Emission Tomography/Computed Tomography?

PURPOSE

2-Deoxy-2-[F-18]fluoro-D-glucose (FDG)-positron emission tomography (PET)/computed tomography (CT) is becoming widely available as a powerful imaging modality, combining the ability to detect active metabolic processes and their morphologic features in a single study. The role of FDG-PET/CT is proven in lymphoma, melanoma, colorectal carcinoma, and other cancers. However, there are rare malignancies such as Merkel cell carcinoma that can potentially be evaluated with PET/CT. We were therefore prompted to review our experience with FDG-PET/CT in the management of patients with Merkel cell carcinoma.

PROCEDURES

This is a retrospective case series of six patients with Merkel cell carcinoma, 58-81 years old (average 69 +/- 8.3), who had whole-body PET/CT at our institution from January 1st, 2003 to August 31st, 2005. Two patients were women and four were men. Reinterpretation of the imaging studies for accuracy and data analysis from medical records were performed.

RESULTS

Twelve examinations were acquired for the six patients (one patient had six PET/CT, one patient had two PET/CT, and four patients had one PET/CT). The injected FDG doses ranged 381.1-669.7 MBq (average 573.5 +/- 70.3). Four patients had the PET/CT as part of initial staging, and two patients had the exam for restaging (after surgery and XRT). A total of six Merkel lesions (pancreas, adrenal, lip, submandibular lymph nodes, cervical lymph nodes, and parapharyngeal soft tissue) were identified in three patients and confirmed on histopathological examination. The FDG uptake in these areas was intense, with maximum standardized uptake value (SUVmax) values of 5-14 (average 10.4 +/- 3.8). In one patient, the PET/CT scan identified abnormal focal distal sigmoid uptake that was biopsied and diagnosed as adenocarcinoma. Two patients had negative scans and had no clinical evidence of disease on follow-up office visits (up to one year after PET/CT).

CONCLUSIONS

This case series suggests that FDG-PET/CT may have a promising role in the management of patients with Merkel cell carcinoma.

Biochemistry of human skin--our brain on the outside

The skin, our body's largest organ, is located at the interface between the external and internal environments, and so is strategically placed to provide not only a barrier against a range of noxious stressors (UV radiation, mechanical, chemical and biological insults) but also to act as the periphery's 'sensing' system. Recent developments suggest that this organ is much more critical to maintaining body homeostasis than previously thought. This tutorial review introduces the reader to some of the biochemistry that underpins the skin's enormous multi-functionality.