Neuroblastoma at the trigeminal nerve in a cynomolgus monkey (Macaca fascicularis)

A male cynomolgus monkey (Macaca fascicularis) of 5 years and 11 months of age from the vehicle control group of a 4-week repeated oral dose toxicity study had a spontaneously occurring mass lesion directly attached to the proximal part of the left trigeminal nerve. Histologically, the mass was characterized by a multifocal nodular appearance. Nodular zones showed low to moderate cellularity and were composed of small round cells exhibiting nuclear uniformity. On the other hand, inter-nodular zones were composed of nerve fiber containing septa and closely aggregated highly pleomorphic cells. Immunohistochemically, the small round cells were strongly immunopositive for synaptophysin, neuN, and class III beta-tubulin, while the highly pleomorphic cells were weakly immunopositive for neuN and occasionally immunopositive for class III beta-tubulin and doublecortin, suggesting that the tumor had originated from a neuronal lineage cell. Based on these findings, the mass was diagnosed as a neuroblastoma at the trigeminal nerve.