Abstract

Eleven gastrointestinal neoplasms from 10 aged horses and 1 pony were examined grossly, his tologically, immunohistochemically, and (in two cases) ultrastructurally. Clinical signs were associated with two neoplasms, and the other nine tumors were incidental findings at laparotomy or necropsy. The neoplasms were solitary (9/11) or multifocal (2/11), well demarcated, serosal or mural masses of stomach (1), jejunum (1), ileum (3), cecum (5), and/or colon (2). Microscopic examination revealed discrete spindle cells arranged in compact patterns with fascicles and whorls or cribriform pattern with fascicles and rare palisades, often with a myxoid interstitial matrix. Three tumors infiltrated between the muscularis interna and the muscularis externa at the myenteric plexi. All neoplasms were vimentin positive, 3/11 were S-100 positive, 2/11 were muscle actin positive, and no neoplasm was positive for glial fibrillary acid protein, desmin, factor VIII, chromogranin, or neuron-specific enolase. Of the two tumors studied ultrastructurally, one contained an admixture of smooth muscle cells and cells resembling Schwann cells, and the second was populated by homogeneous fusiform mesenchymal cells separated by homogeneous matrix. Gastrointestinal stromal tumors (GIST) have been recognized in humans, more recently in dogs and nonhuman primates, and now in equids. Most of these tumors are comprised of a loosely arranged network of spindled cells separated by myxoid matrix. GIST may be composed of myogenic, neurogenic, combined myogenic and neurogenic, and undifferentiated mesenchymal cells.