CUTANEOUS MALIGNANT MELANOMAS IN 57 CATS - IDENTIFICATION OF (AMELANOTIC) SIGNET-RING AND BALLOON CELL-TYPES AND VERIFICATION OF THEIR ORIGIN BY IMMUNOHISTOCHEMISTRY, ELECTRON-MICROSCOPY, AND IN-SITU HYBRIDIZATION

Cutaneous malignant melanomas in cats, both melanotic and amelanotic, were diagnosed in 57 of 1,530 skin tumors during the period 1991-1995. All melanomas occurred in domestic shorthaircats of ages 3-19 years ( (X) over bar = 11.5 years). Postmortem examination was performed on 16 cats. All had metastases in the regional lymph node and several organ systems. The average time of survival after surgical removal of the t umor was 4.5 months. Histologically, five types of melanomas could be distinguished: epithelioid, spindle, mixed, signet-ring, and balloon c ell. Whereas all epithelioid, spindle, and mixed epithelioid/spindle c ell types showed pigmentation, signet-ring and balloon cell types were often amelanotic. Immunohistochemical examination of the melanomas re vealed a positive staining for S-100, vimentin, and neuron-specific en olase. The melanomas were negative for muscle cell markers, except in some of the signet-ring cell melanomas; 13 of 21 tumors showed a weak positive staining for polyclonal desmin. Electron microscopic examinat ion of signet-ring cell melanomas revealed an abundance of intermediat e filaments, whereas in some of these tumors a few cells with melanoso mes were found. Nonisotopic in situ hybridization for mRNA encoding fo r tyrosinase verified the melanocytic origin of the amelanotic signet- ring and balloon cell melanomas.