Is the neuroendocrine carcinoma of the skin a Merkel cell tumor? What we can learn from immunohistochemical and ultrastructural studies

Merkel Cell Polyomavirus: Infection, Genome, Transcripts and Its Role in Development of Merkel Cell Carcinoma

The best characterized polyomavirus family member, i.e., simian virus 40 (SV40), can cause different tumors in hamsters and can transform murine and human cells in vitro. Hence, the SV40 contamination of millions of polio vaccine doses administered from 1955-1963 raised fears that this may cause increased tumor incidence in the vaccinated population. This is, however, not the case. Indeed, up to now, the only polyomavirus family member known to be the most important cause of a specific human tumor entity is Merkel cell polyomavirus (MCPyV) in Merkel cell carcinoma (MCC). MCC is a highly deadly form of skin cancer for which the cellular origin is still uncertain, and which appears as two clinically very similar but molecularly highly different variants. While approximately 80% of cases are found to be associated with MCPyV the remaining MCCs carry a high mutational load. Here, we present an overview of the multitude of molecular functions described for the MCPyV encoded oncoproteins and non-coding RNAs, present the available MCC mouse models and discuss the increasing evidence that both, virus-negative and -positive MCC constitute epithelial tumors.

Evidence of proliferative activity in human Merkel cells: implications in the histogenesis of Merkel cell carcinoma

The cellular origin of Merkel cell carcinoma (MCC) is controversial. We previously hypothesized that MCC originates from hair follicle stem cells or Merkel cell (MC) progenitors residing within the hair follicle bulge. Examination of three cases of combined MCC led to the unexpected discovery that numerous keratin 20 (CK20)-positive MCs within the squamous cell carcinoma (SCC) component of combined MCC appeared morphologically normal with dendritic and oval shapes. Moreover, one extremely rare case of combined SCC and MCC showed both intra-epidermal and dermal MCCs. These three cases represent the first documentation of MC hyperplasia in MCC, besides various benign follicular neoplasms associated with MC hyperplasia. Therefore, to elucidate the proliferating potential of MCs and their histogenetic relationship with MCCs, we further investigated these cases based on pathological observations. We identified numerous cells co-expressing CK20 and the proliferation marker Ki-67, identical to the morphological and immunohistochemical features of normal MCs. This finding indicated that MCs can no longer be considered as pure post-mitotic cells. Instead, they have proliferative potential under specific conditions in the diseased or wounded skin, or adjacent to various skin tumors, including MCC. Intimate co-existence of two malignant cell components composed of intradermal and intra-epidermal MCCs, with the proliferation of normal-appearing MCs in the same lesion, lends support to the hypothesis that MCs and MCC cells are derived from MC progenitors residing within the hair follicle bulge. Specifically, MCCs are derived from transformed MC progenitors with potential for dual-directional differentiation towards neuroendocrine and epithelial lineages.

Combined Merkel Cell Carcinoma and Atypical Fibroxanthoma

Background:

Although there are reports of squamous cell carcinoma arising within and adjacent to Merkel cell carcinoma, and one report of an atypical fibroxanthoma-like tumor arising in an irradiated recurrent Merkel cell carcinoma, there have previously been no reports of an immunohistochemically verified atypical fibroxanthoma occurring in conjunction with a Merkel cell carcinoma.

Objective:

We report on a neoplasm with distinct features of both Merkel cell carcinoma and atypical fibroxanthoma.

Methods:

Histologic and immunohistochemical evaluations were performed.

Results:

Our results verify the finding of a combined Merkel cell carcinoma and atypical fibroxanthoma.

Conclusion:

This case is an interesting and unusual combination of tumors. The origin of the Merkel cell carcinoma is revisited. Debated origins include neuroendocrine, epithelial, and pleuripotent stem cell.

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.

Combined Merkel Cell Carcinoma and Atypical Fibroxanthoma

BACKGROUND

Although there are reports of squamous cell carcinoma arising within and adjacent to Merkel cell carcinoma, and one report of an atypical fibroxanthoma-like tumor arising in an irradiated recurrent Merkel cell carcinoma, there have previously been no reports of an immunohistochemically verified atypical fibroxanthoma occurring in conjunction with a Merkel cell carcinoma.

OBJECTIVE

We report on a neoplasm with distinct features of both Merkel cell carcinoma and atypical fibroxanthoma.

METHODS

Histologic and immunohistochemical evaluations were performed.

RESULTS

Our results verify the finding of a combined Merkel cell carcinoma and atypical fibroxanthoma.

CONCLUSION

This case is an interesting and unusual combination of tumors. The origin of the Merkel cell carcinoma is revisited. Debated origins include neuroendocrine, epithelial, and pleuripotent stem cell.

Merkel cell carcinoma: squamous and atypical fibroxanthoma-like differentiation in successive local tumor recurrences

Merkel cell carcinoma (MCC) is a rare, frequently lethal, primary neuroendocrine carcinoma of the skin. Histopathologically, it appears as a dermal nodule of small, undifferentiated malignant cells. Historically, MCC was considered to be an eccrine carcinoma. Recognition of its neuroendocrine features later led to the hypothesis that it arose from Merkel cells in the skin, although recent evidence revisits the question of an epithelial origin. We present a case of MCC arising on the temple of a 78-year-old male, in association with an actinic keratosis. Three years later, a local tumor recurrence showed a mixed malignancy comprising small cell neuroendocrine and large cell squamoid components. A further recurrence at the site two years later, after local radiotherapy, revealed a bizarre pleomorphic large cell morphology with retention of immunohistochemical features of a neuroendocrine carcinoma. Evolution to a bizarre pleomorphic large cell neoplasm has been recorded in malignant tumors treated by radiotherapy, but is unique for MCC. The association of this MCC with an actinic keratosis and the development of squamoid differentiation in a local recurrence support the link between MCC and epithelial neoplasia. In addition, its evolution to an atypical fibroxanthoma-like morphology is of interest, because some view atypical fibroxanthomas as bizarre variants of squamous cell carcinoma.

Pathology of APUDomas

The pathology of APUDomas involves two main areas. The first is the biochemical aspects of these tumours and the use of biochemical tests to investigate a patient suspected of having an APUDoma, and the other is the histopathology of these tumours. This review is devoted to the histopathological considerations.