| Summary: | 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.
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