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 (mean = 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 tumor was 4.5 months. Histologically, five types of melanomas could be distinguished: epithelioid, spindle, mixed, signet-ring, and balloon cell. Whereas all epithelioid, spindle, and mixed epithelioid/spindle cell types showed pigmentation, signet-ring and balloon cell types were often amelanotic. Immunohistochemical examination of the melanomas revealed a positive staining for S-100, vimentin, and neuron-specific enolase. 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 examination of signet-ring cell melanomas revealed an abundance of intermediate filaments, whereas in some of these tumors a few cells with melanosomes were found. Nonisotopic in situ hybridization for mRNA encoding for tyrosinase verified the melanocytic origin of the amelanotic signet-ring and balloon cell melanomas.