Spontaneous extracutaneous systemic mastocytosis in a cynomolgus macaque (Macaca fascicularis).

A uniform cell population of proliferating mast cells with poor cytoplasmic granularity and a few eosinophilic infiltrates was observed in hepatic portal tracts and the cecal submucosa of an adult male cynomolgus macaque (Macaca fascicularis) that was part of a drug safety assessment toxicity study. The proliferating mast cells were positive for Giemsa and toluidine blue staining and had strong immunoreactivity for mast cell tryptase and CD68. Considering size, morphology, immunoreactivity, and distribution of the lesions a benign proliferative disorder of connective tissue mast cells was diagnosed comparable to that seen in indolent systemic mastocytosis in humans, but lacking associated skin lesions. The finding of visceral mast cell proliferation has to be included in the spontaneous background pathology spectrum of cynomolgus macaques used in toxicological studies.