Histologic and ultrastructural studies of dermal sarcoma of walleye (Pisces: Stizostedion vitreum).

Sixty-seven adult walleye fish were examined by light and transmission electron microscopy. The fish were affected by a mesenchymal tumor previously termed Walleye Dermal Sarcoma that commonly affects up to 27% of the population seasonally. Biopsies from 24 fish were collected, and complete postmortem examinations were performed on 43 fish. Grossly, the tumors had the appearance of randomly distributed, often clustered, spherical nodules, 2-5 mm in diameter with a smooth and often ulcerated surface. The tumors arose from the superficial surface of scales and consisted of fibroblast-like cells separated by a moderate amount of collagen (43/67) or osteoid material (24/67). Lymphocytic infiltration (28/67) associated with vacuolar degeneration of tumor cells (28/67) and centrally located coagulation necrosis (30/67) were observed. Although tumor cells were often highly anaplastic, no local invasions or metastases were present. In contrast with previous descriptions of this tumor, no viral particles could be observed electron microscopically. The variably anaplastic appearance of the tumor, its biological behavior, and its restriction to dermis are features in common with canine cutaneous histiocytoma and equine sarcoid. The multicentric origin, the restriction to the dermis, and the absence of invasion or metastases of Walleye Dermal Sarcoma differ from retrovirus-induced avian and murine sarcomas that arise locally, that invade, and that often metastasize.