Olfactory neuroblastoma in a horse

Olfactory neuroblastoma is an uncommon malignant neuroectodermal nasal tumor thought to arise from the specialized sensory neuroepithelial olfactory cells normally found in the upper part of the nasal cavity. This report describes the histopathologic, immunohistochemical and ultrastructural features of an olfactory neuroblastoma in a horse. A 25-year-old Argentine gelding horse was examined for progressive masticatory and respiratory dysfunction. A mass protruding from the right nasal cavity was found. Microscopically, the mass showed infiltrative growth, and was composed of packed nests of polygonal to round cells divided by a delicate fibrovascular stroma. At immunohistochemistry, all neoplastic cells were strongly positive for NSE and 80% of neoplastic cells were immunoreactive for vimentin and chromogranin A. Synaptophysin, cytokeratin AE1/AE3, CD3, CD79, S-100 and GFAP were negative. At transmission electron microscopy, the cytoplasm of the cells was clear and scant to abundant and contained dispersed mitochondria, some free ribosomes, and rare dense core vesicles. Sustentacular-like cells were frequently seen at the periphery of the cell nests. The microscopic features of olfactory neuroblastoma may vary and the presence of rosettes and pseudorosettes cannot be considered a distinctive marker. Therefore, diagnosis should be based on a combination of immunohistochemical and transmission electron microscopy investigation.