Dermal melanoma with schwannoma-like differentiation in a brown bullhead catfish (Ictalurus nebulosus)

Animal Disease Diagnostic Laboratory, Purdue University, West Lafayette, IN 47907, USA.

A dermal melanoma with histologic similarities to mammalian melanocytic schwannomas was diagnosed in a wild-caught brown bullhead catfish (Ictalurus nebulosus). The diagnosis was based on cytologic, histopathologic, immunohistochemical, and ultrastructural findings. The neoplasm protruded from the cutaneous surface of the dorsal midline immediately caudal to the dorsal fin, was diffusely black and focally ulcerated, and was attached to the underlying skin by a broad-based stalk. Cytologically, the tumor was composed of a monomorphic population of melanophores, the melanin-producing and storing cells within the dermis and epidermis of fish that are partially responsible for rapid color changes. Histopathologic examination of the neoplasm revealed an unencapsulated, well-circumscribed, moderately cellular neoplasm composed of 2 distinct cell populations: spindle cells arranged in a "herringbone" pattern and numerous melanophores. The spindle cells had scant bipolar eosinophilic cytoplasm, with small centrally located vesicular nuclei; nucleoli were not seen. The histologic appearance was similar to that described for mammalian melanocytic schwannomas. Immunohistochemically, the spindle cells exhibited positive cytoplasmic staining for S-100 protein but were negative for vimentin, neurofilament protein, and glial fibrillary acidic protein. Ultrastructurally, the tumor consisted of neoplastic spindle cells with rudimentary cell junctions, actin bundles, and few melanosomes and melanophores with abundant intracytoplasmic melanosomes.