An ovarian sex cord-stromal tumor with fibromatous and thecomatous differentiation in an Ezo brown bear (Ursus arctos yesoensis).

A mass was found in the left ovary of a 40-year-old captive female Ezo brown bear (Ursus arctos yesoensis) during a necropsy. Histopathology revealed two morphologically distinct stromal cell populations: round to polygonal cells with cytoplasmic microvacuoles, which were positive for inhibin-α and adipose differentiation-related protein (ADRP) and were surrounded by reticulin fibers, and spindle-shaped cells expressing α-smooth muscle actin, which produced collagen and exhibited occasional ADRP-positivity. The ovary also contained cystic and cauliflower-like nodular lesions covered by ovarian surface epithelium. The mass was diagnosed as an ovarian sex cord-stromal tumor with fibromatous and thecomatous differentiation. The cystic lesions were interpreted as surface epithelial cysts, although a subsurface epithelial structures-like origin could not be excluded. The cauliflower-like nodular lesion was described as a stromal protrusion covered by ovarian surface epithelium. This case expands the spectrum of ovarian lesions reported in ursids and contributes to our understanding of reproductive pathology.