Merkel cell carcinoma is an uncommon malignant skin tumor. The origin of this tumor is controversial. It was first described by Toker in 1972. The tumor was called "trabecular carcinoma," and it was thought to arise from the eccrine cells.
In 1978, Tang and Toker examined these tumors using electron microscopy. They identified cytoplasmic neurosecretory granules in the tumor cells, which led them to suggest that these tumors are derived from the neural crest and are of neuroendocrine nature and differentiation. Thus, they suggested that these tumors arise from the known normal neuroendocrine cells of the skin: Merkel cells. Some authors have proposed the possibility of pluripotent stem cells as the cells of origin.[2,3,4] Currently, these tumors are called "Merkel cell carcinomas" or "primary cutaneous neuroendocrine carcinomas."
Merkel cell carcinoma is an aggressive cancer. It typically presents as a rapidly growing, painless, red-violaceous papule, plaque, or nodule on sun-exposed skin. It is more common in white elderly persons, with a slight male predilection. The skin of the head and neck region is the most commonly involved, followed by the upper extremities. The clinical features can be summarized with the acronym AEIOU (asymptomatic/painless, expanding rapidly, immune suppression, older than 50 years, ultraviolet-exposed skin in a fair-skinned individual).
Merkel cell carcinoma is a high-grade tumor with a relatively high rate of metastasis. The vast majority of tumors involve the dermis. As many as 41% of cases of Merkel cell carcinoma cases can be associated with other skin lesions, including actinic keratosis, squamous cell carcinoma (in situ and invasive), basal cell carcinoma, and melanoma.[2,6,7,8,9,10] These associations, along with the tendency of these tumors to occur in sun-exposed skin, suggest solar irradiation damage as an etiology for Merkel cell carcinoma.
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