Canine laryngeal rhabdomyoma.

Three canine laryngeal tumors were diagnosed as oncocytomas by light microscopy, but were determined to be rhabdomyomas following ultrastructural and immunocytochemical examination. Tumors consisted of large eosinophilic cells interspersed with smaller dark cells. Large tumor cells had a granular, intensely eosinophilic cytoplasm. Scattered through the tumors were a few elongated cells with cytoplasmic cross striations and multiple nuclei. Tumor cells from all three dogs contained numerous mitochondria and bundles of myofibrils with electron-dense Z-lines typical of striated muscle cells. Intracellular myoglobin and desmin were detected in the tumors by immunocytochemistry. Comparisons are made with a previous report of canine laryngeal oncocytomas.