Histopathologic, immunohistochemical and ultrastructural features of a granular cell tumour in an Australian parakeet (Melopsittacus undulatus).

An adult male Australian parakeet (Melopsittacus undulatus) presented a firm nodular lesion in the lateral metacarpal region of the right wing. Microscopically, there were neoplastic cells, round and polyhedral in shape, with abundant, slightly eosinophilic granular cytoplasm; they were strongly periodic-acid Schiff-positive and resistant to diastase digestion. Some groups of neoplastic cells were immunopositive for smooth muscle actin and desmin. There was no immunopositivity for S-100 protein, CD68 and cytokeratin. Ultrastructurally, the neoplastic cells were round and polygonal in shape, and they were characterized by abundant cytoplasm with numerous homogeneous osmophilic bodies covered by an electron-dense membrane (lysosomes). The histopathologic, immunohistochemical and ultrastructural features of the neoplastic tissue are consistent with a granular cell tumour, which has been described in different animal species and anatomic locations; however, this seems to be an infrequent neoplasm in Australian parakeets. The immunopositivity of the neoplastic cells for smooth muscle actin and desmin, as well as slight positivity for muscle with Masson's trichrome, suggest that this is a tumour of myogenic origin.