Gastrointestinal stromal tumors of the equine cecum

Ten cecal tumors were identified during the postmortem examination of seven horse carcasses at slaughter tone horse had three tumors). The multinodula r and hemorrhagic tumors ranged from 1 to 10 cm in diameter and consisted o f spindle cells arranged in thin, interconnected trabeculae that were often separated by sinuses filled with mucinous fluid, erythrocytes, and siderop hages. Spindle cells of all tumors were immunopositive for vimentin, neuron -specific enolase, and c-kit protein but lacked reactivity with antibodies to glial fibrillary acidic protein, S100 protein, and desmin. In one tumor, spindle cells diffusely bound antibodies to synaptophysin. Most tumors con tained focal reactivity to smooth muscle actin antibodies; one tumor reacte d diffusely. Ultrastructurally, tumor cells were connected by desmosome-lik e structures and exhibited extended cell processes; some contained dense co re neurosecretory granules. These equine stromal tumors appeared to share s ome characteristics with human gastrointestinal stromal tumors.