Malignant melanoma in a zebra finch (Taeniopygia guttata): cytologic, histologic, and ultrastructural characteristics.

An approximately 3-year-old adult male zebra finch (Taeniopygia guttata) was diagnosed with malignant melanoma. The large darkly pigmented tumor was located in the coelom, extended from the apex of the heart to the cloaca, and was adherent to the intestines and the ventriculus. Dark small masses (likely metastases) were observed in the lungs. Cytologically, the neoplasm consisted mainly of round to oval cells with brown or pale blue to blue-brown pigment. Lesser numbers of cells were stellate to dendritic with abundant amounts of brown pigment granules or were markedly pleomorphic with variable amounts of pigment. Histologically, the tumor consisted of dense sheets and aggregates of infiltrative melanocytes that were negative for S-100 and Melan A. A few cells were consistent with "signet-ring" melanocytes. Melanocytes examined by electron microscopy contained typical structures, mainly premelanosomes and melanosomes, of this cell type. However, melanocytes with marked pleomorphism also contained intracytoplasmic aggregates of filaments, consistent with previously reported ultrastructural findings in signet-ring or rhabdoid melanoma of nonavian species.