Which histologic findings are characteristic of Merkel cell carcinoma (MCC)?

Updated: Oct 07, 2019
  • Author: Guy J Petruzzelli, MD, PhD, MBA, FACS; Chief Editor: Gregory Gary Caputy, MD, PhD, FICS  more...
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Answer

Pathology

While MCPyV appears to play a role in the pathophysiology of this disease process, the etiology of MCC still requires further exploration. Merkel cells are accessory cells that secrete nerve growth factors to enable cutaneous nerves to develop into nerve endings. The embryologic origin of Merkel cells is also controversial; initially, Merkel cells were hypothesized to have arisen from neural crest cells. However, subsequent evidence suggested that fetal Merkel cells are derived from epidermal keratinocytes. [33]  Others postulate that the cancerous cells in MCC arise from stem cells that develop neuroendocrine properties similar to the wild-type Merkel cells. [34]

Identifying MCC with conventional histologic examinations is problematic, as it resembles other poorly differentiated neoplasms such as cutaneous large cell lymphoma, amelanotic melanoma, small cell carcinoma, and Ewing sarcoma. For a definitive diagnosis, electron microscopy and immunohistochemistry studies are necessary. [35, 36]

Similar to normal Merkel cells, electron microscopy reveals that the cells in MCC contain perinuclear intermediate filaments and electron-dense cytoplasmic secretory granules. In one study, investigators used immunohistochemistry findings to distinguish MCC cells from pulmonary small cell carcinoma. Chan and colleagues identified that the critical difference was the constitutive expression of cytokeratin-20 in Merkel cells and cells from MCC. [37]  In contrast, pulmonary small cell carcinomas, which grossly appear similar to MCC, lacked expression of cytokeratin-20.


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