Granular cell tumour affecting the left vocal cord in a dog.

Granular cell tumours (GCTs) have been described in human vocal cords, but not in those of the dog. A single polyp in the left vocal cord, associated with laryngeal oedema and monolateral hyperplasia of the tonsil, was observed endoscopically in a 6-year-old male dog. Cytologically, the mass was characterized by well-defined polygonal cells, with granular cytoplasm and an eccentric nucleus. Histologically, the mass showed sheets of globoid to polygonal cells, filled with numerous diastase-resistant, periodic acid-Schiff-positive granules, and scanty stroma. Ultrastructurally, the granules were represented by heterogeneous lysosomes of variable size, suggesting a diagnosis of GCT. As reported in man, hyperplasia of the laryngeal and tonsillar epithelium and the presence of foci of pearl-like epithelial cells, mimicking invasive squamous cell carcinoma, were observed. Immunohistochemical examination revealed labelling of neoplastic cells for S-100 protein, CD68, collagen IV, and, focally, for glial fibrillary acidic protein, neuron-specific enolase and vimentin. These findings suggested a Schwann cell origin of the GCT.