HISTOPATHOLOGY OF A SPONTANEOUSLY DEVELOPING MAST-CELL SARCOMA IN A WISTAR RAT

A case report is given of a very rare spontaneous mast cell tumor in t he eyelid of the left eye of a female Wistar rat used in a long-term o ral toxicity study. Metastasis of the tumor had occurred in the mandib ular lymph nodes and in the liver. Clinically, the animal showed bleph arospasm, dacryorrhoea, and exophthalmus. Hematologic findings include d slight eosinophilia and a remarkable basophilia. At necropsy, a bila teral conjunctivitis was diagnosed and a tumorous mass was found in th e left submandibular region. Histologically, the tumor was composed of round to polygonal cells with pale cytoplasm, containing abundant pre dominantly basophilic granules. The intracytoplasmatic granules staine d metachromatically with Toluidine blue and immunostained positively w ith serotonin. Numerous eosinophils were scattered throughout the tumo r and were also present in other organs. Cells with round, oval, or in dented nuclei and abundant cytoplasm, containing pronounced eosinophil ic granules, were found in spleen and bone marrow. They turned out to be immature stages of eosinophilic granulocytes. Characteristics of th e present tumor are compared with observations on mast cell tumors in other species.