Malignant theca cell tumor of rat.

A large-sized ovarian tumor of theca cell origin was found in a female rat. The mass was located in the right ovary position. Histologically, the tumor was covered by thin fibrous capsule and consisted of a solid area and an abundant necrotic area. Tumor cells were arranged in a storiform or whorled pattern. Connective tissue elements occasionally presented as bundles of dense collagen fibers. Fusiform to elongated cells had oval- to spindle-shaped nuclei with indistinct nucleoli. Large round nuclei and mitotic figures were scattered throughout the tumor cells. These cells were stained positively with S-100 but negatively with vimentin and a-smooth muscle actin. Tumor cells with abundant cytoplasm sometimes contained multiple small-sized lipid vacuoles.