Merkel Cell Carcinoma in a Steer.
Merkel cell carcinoma is a rare and aggressive cutaneous neuroendocrine tumour reported only in man, dogs and cats. A 20-month-old Japanese black fattening steer was presented with necrotic protruding skin masses over the left thoracic area and a 20 × 25 cm subcutaneous mass in the left abdominal area. Microscopical evaluation of the masses revealed cords of small to medium-sized round tumour cells with marked anisocytosis and anisokaryosis and clear and vacuolated cytoplasm, which were separated by a delicate fibrovascular stroma and arranged in a trabecular and nested pattern. Necropsy examination revealed multiple solid white nodular masses in the lungs. Immunohistochemistry (IHC) for cytokeratin (CK) 20 and CKAE1/3 revealed focal perinuclear labelling of tumour cells. IHC for the neuroendocrine markers chromogranin A and neuron specific enolase, the neuroepithelial stem cell marker nestin and the hormonal markers adrenocorticotropic hormone and calcitonin revealed diffuse cytoplasmic labelling of all tumour cells. Ultrastructurally, the tumour cells contained few neurosecretory granules and abundant glycogen pools. The tumours were diagnosed as Merkel cell carcinoma with pulmonary metastases and this case represents the first such diagnosis in cattle.