Authorities who do not accept the Merkel cell as the origin of MCC postulate that the cell of origin may be an immature pluripotential stem cell of the dermis that acquires neuroendocrine features during malignant transformation. Ultrastructural and immunohistochemical studies provide support for each hypothesis. Both Merkel cells and MCCs possess similar neuroendocrine markers such as neuron-specific enolase, chromogranin, and synaptophysin. However, MCC expresses neurofilament proteins not observed in normal Merkel cells. The overlying epidermis usually is intact, although advanced lesions may become ulcerated.
Diagnosis can be difficult.
Because of its nonspecific appearance, the true nature of the tumor generally is not established except by biopsy. MCC can be mistaken for basal cell carcinoma, squamous cell carcinoma, malignant melanoma, lymphoma, or small cell carcinoma of the skin. The majority of MCCs provisionally are diagnosed as basal cell carcinomas prior to biopsy. As a result, the margin of excision often is inadequate and reexcision is necessary to prevent local recurrence. These tumors have a tendency to grow rapidly.