Abstract
A granular cell tumour (myoblastoma) occurred in the lung of an aged mare. Multiple greyish-white neoplastic nodules of various sizes almost replaced one lung. Histologically, the neoplasm consisted of small clumps of granular cells in a stroma composed of thick bands of collagen and numerous capillaries. The globoid or stellate neoplastic cells were characterized by a large number of eosinophilic, PAS-positive, diastase-resistant cytoplasmic granules. Ultrastructurally, the granules were of two types: (a) homogeneous, electron-dense granules and (b) multimorphous granules that resembled cell debris. Membrane-bounded packets containing both types of granules were common. It appears that the site of occurrence of granular cell tumours varies between species, with the lung of the horse and the tongue of the dog being major sites.
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