Abstract
In a 6.5-year-old cynomolgus monkey (Macaca fascicularis), a tumor mass was macroscopically located near the right ovary, connected to the oviduct, and completely separated from the uterus. The mass was an elongated spherical shape with a smooth surface and milky-white color. It was approximately 3.5 cm across its major axis, and the sagittal section was composed of cystic walls and a multi-lobular luminal nodule. Light-microscopically, the polypoid mass consisted of admixtures of neoplastic mesenchymal and epithelial elements. Lipid-rich foamy cells scattered within the tumor mass formed nest-like/aggregated populations. Immunohistochemically, mesenchymal tumor cells stained diffusely positive for vimentin, desmin, and alpha (α)-smooth muscle actin, demonstrating a smooth muscle origin. Mesenchymal tumor cells contained mitotic figures, and tumor elements including mesenchymal, epithelial, and lipid-rich foamy cells stained strongly positive for proliferating cell nuclear antigen (PCNA). Moreover, lipid-rich foamy cells elicited positive reactions for testosterone, suggesting sex-cord element differentiation. Electron-microscopically, actin filaments, basement membranes, and electron-dense cytoplasmic bodies were noted in the spindle cells, and invaginated nuclei were observed in adenomatous cells. In contrast, foamy cells contained numerous lipid vacuoles in the cytoplasm. From these findings, the tumor was diagnosed as an atypical polypoid adenomyoma (benign mixed müllerian tumor) with sex-cord differentiation arising from the oviduct. This tumor was considered to be an exceedingly rare finding in the adolescent cynomolgus monkey.