3-year old Female Horse

SIGNALMENT: 3 year old, female, warm blood equine

SOURCE/HISTORY: Right ovary.

Click here for the IDEXX Pathology Report on this case.


Ovary: There is a cystic multiloculated mass comprised of lobules and nests of neoplastic granulosa cells. These cells are polygonal to peripherally more columnar with indistinct cell borders and moderate eosinophilic cytoplasm. There is mild-moderate anisocytosis and anisokaryosis. Nuclei are round with lightly stippled chromatin and small nucleoli. There are 4 mitotic figures per 10 400x fields. There is a second population comprised of sheets and intersecting bundles of neoplastic mesenchymal cells within a minimal fibrovascular stroma. Neoplastic mesenchymal cells are elongate with moderate eosinophilic cytoplasm. There is mild anisocytosis and anisokaryosis. Nuclei are round to oval with lightly stippled chromatin and variably prominent small nucleoli. There is 1 mitotic figure identified within 10 400X fields.


Ovary: Granulosa-thecal cell tumor

Mitotic count: Granulosa component – 4; Thecal component – 1

Margins: Histologic changes extend to the tissue margins.

Vascular invasion: None


Granulosa cell tumors are the most common ovarian tumor in the mare. Steroidogenesis may be associated with granulosa cell tumors. Either estrogens or androgens can be produced, but not all granulosa cell tumors are hormonally active. Inhibin is regularly produced by granulosa cell tumors in the mare and is thought to be the cause of atrophy of the contralateral ovary. Granulosa cell tumors almost never metastasize in mares. Often a combination of patterns exists in the single tumor. Theca cells may also be present in granulosa cell tumors. Additionally, either or both cell types may be luteinized. A theca cell tumor is an infrequent tumor of the ovarian stroma. It is composed of oval or spindle-shaped cells arranged in sheets or interlacing bundles. The cells resemble the cells of the theca interna, but the tumors may contain variable amounts of collagen producing fibroblasts.

Pathology of Domestic Animals, 4th ed., Volume 3, 365-367, 1993


Jamie M. Bush, DVM, MS

Diplomate, American College of Veterinary Pathologists





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