Feline and canine Merkel cell carcinoma: A case series and discussion on cellular origin.

Merkel cell carcinoma (MCC) is in humans and cats a malignant cutaneous neuroendocrine carcinoma, whereas in dogs it possibly has a more benign behaviour. It may be cytologically confused with round cell tumours such as lymphoma because of its striking cytomorphologic similarity. Although MCC is considered to arise from Merkel cells, recent findings indicated that primitive (epi-)dermal stem cells, early B-cells or dermal fibroblasts were the origin of human MCC. The aim of our study was to evaluate a possible lymphoid origin in feline and canine MCCs. Specific analysis of CD3, PAX-5, KIT and PARR assay were performed in 3 feline and 3 canine MCCs. All MCCs (6/6) were negative for CD3 and PAX-5. KIT was expressed in all MCCs (6/6). Assessment of clonality by PARR assay exhibited a polyclonal B- and T-cell receptor rearrangement in all five cases tested. In conclusion, a lymphoid origin of feline and canine MCCs could not be demonstrated. This is in contrast with human MCCs, that often express early B-cell lineage markers.