Canine neuroendocrine carcinoma. A tumor resembling histiocytoma.

The clinical and light- and electron microscopic features of 20 cases of canine neuroendocrine carcinoma, initially classified as atypical histiocytomas, are reported. The locally expansile well-circumscribed dermal tumor nodules were composed of solid masses of cells with high mitotic index and multinucleation, arranged in a trabecular pattern with prominent fibrovascular connective tissue stroma rich in reticulin fibers that outlined compact cell nests. Ultrastructural studies revealed evenly dispersed chromatin, focally indented nuclei and abundant cytoplasm with perinuclear filaments, membrane-bound dense core granules, and prominent interdigitating plasma membrane projections with primitive intercellular junctions. Clinical and pathological comparisons between canine neuroendocrine carcinoma, canine histiocytomas, and human Merkel cell neoplasms are discussed.