Spontaneous olfactory neuroepithelioma in a domestic medaka (Oryzias latipes)

Tumors of the central nervous system in fish are rare, and only six cases of spontaneous olfactory neuroepithelioma have been reported. This is the seventh case, found in a medaka, Oryzias latipes. The tumor was noted near the right olfactory orifice and finally measured 1.5 mm in diameter. Histologically the tumor consisted of undifferentiated neuroblasts forming a few true rosettes. Mitosis was frequently observed. Tumor cells stained diffusely for neuron-specific enolase and sporadically for neurofilament proteins by immunohistochemical procedures. Additionally a few large tumor cells were positively stained for S-100 protein. Electron microscopy revealed that the tumor cells had extended cytoplasm in which parallel neurotubules and a few neuroendocrine granules were noted. In the perinuclear region, bundles of intermediate filaments and neuroendocrine granules were seen. Single cilia and a pair of centrioles were occasionally found, but no ciliated cells were found in this tumor. Some large tumor cells contained electron-dense intracytoplasmic inclusions which showed a crystalloid structure by high-magnification electron microscopy; however, this type of crystalloid has never been reported in neuronal tumors.