Ovarian mucinous cystadenoma with smooth muscle proliferation in a cynomolgus monkey (Macaca fascicularis)

An ovarian mucinous cystadenoma was found in a 5-year-old female cynomolgus monkey (Macaca fascicularis). The tumor was composed of various sizes of multilocular cystic glands lined by a single layer of mucin-filled epithelium. Each of these cystic glands was surrounded by a large amount of solid fibrous stroma resembling smooth muscle. The ovarian surface epithelium showed partial invagination into the ovarian cortex, and a transition was observed between the surface epithelium and the mucinous cyst-forming epithelium. Immunohistochemically, the stromal cells were positive for alpha-smooth muscle actin and proliferating cell nuclear antigen. Ultrastructurally, the glandular epithelium had numerous mucinous secretory granules and microvilli. The stromal cells had numerous parallel microfibrils with focal density. It is rare to encounter evidence of a transition from the surface epithelium to the mucinous tumor epithelium and to show stromal smooth muscle proliferation in a mucinous cystadenoma.