Malignant Myoepithelioma of the Parotid Gland in a Rat

Myoepitheliomas of the salivary glands have been described in laboratory mice, but not in rats. A 20-week-old Wistar (Han) female rat developed a white to grey firm mass at the left side of the neck. Histologically, the mass was unencapsulated and infiltrated the adjacent tissue. The tumour parenchyma was cell rich without acinar or tubular architecture. The tumour showed a palisading basal cell pattern adjacent to blood vessels. There were areas of necrosis filled with cellular debris. The tumour cells showed strong immunohistochemical labelling for pan-cytokeratin types I and II (AE1/AE3), pan-cytokeratin (cytokeratins 1, 5, 6 and 8), cytokeratin 5, cytokeratin 14, vimentin and podoplanin, and only very slight positivity for cytokeratin 8 in small areas. There was no expression of smooth muscle actin. Based on the histological appearance and the immunohistochemistry, the tumour was diagnosed as a malignant myoepithelioma of the parotid gland originating from the parotid duct.