Merkel cell carcinoma: is this a true carcinoma?

Recent years have brought an enhanced understanding of Merkel cell carcinoma (MCC) biology, especially with regard to the Merkel cell polyoma virus as a causative agent. Differences between Merkel cell polyomavirus-positive and Merkel cell polyomavirus-negative MCC in morphology; gene expression, miRNA profiles and prognosis have been reported. Origin of MCC is controversial. Presence of neurosecretory granules has suggested that these carcinomas originate from one of the neurocrest derivatives, most probably Merkel cells; the name Merkel cell carcinoma is now widely accepted. Expression of PGP 9.5, chromogranin A and several neuropeptides, initially regarded as specific markers for neural and neuroendocrine cells, has recently been shown in a subset of lymphomas. MCC commonly expresses terminal deoxynucleotidyl transferase and PAX5. Their co-expression under physiologic circumstances is restricted to pro/pre-B cells and pre-B cells. These findings lead to the hypothesis by zur Hausen et al. that MCC originates from early B cells. This review was intended to critically appraise zur Hausen's hypothesis and discuss the possibility that MCC is a heterogenous entity with distinct subtypes.