Microscopic Background Changes in Brains of Cynomolgus Monkeys

Brain sections from control cynomolgus monkeys ( Macaca fascicularis) used in toxicology studies were evaluated retrospectively in order to better understand spontaneous background changes in this species. Hematoxylin and eosin–stained slides from 76 animals (38 males and 38 females) of 9 studies were examined. Eleven animals (9 males and 2 females) were each observed to have 1 to 3 findings within the brain sections examined, for a total of 19 findings. No findings were noted in the spinal cord. The most common finding was focal to multifocal perivascular infiltration of mononuclear cells, affecting the parenchyma, the meninges, or the choroid plexus. Additionally, focal gliosis was observed in 6 animals and a single focus of hemosiderin deposition (coincident with focal gliosis and mononuclear cell infiltrate) was noted in 1 animal. Most of the glial foci were composed of cells consistent with microglial cells, with or without admixed lymphocytes. All findings were of slight or minimal severity, lacked an apparent cause, and were considered incidental and of negligible biologic significance. An awareness of the spontaneous incidence of these background findings may facilitate the discernment of toxicologically relevant effects when these findings are observed.