Several corpora lutea may develop during certain phases of gestation in the ovary of a pregnant mare. Approximately 16 types of ovarian cysts occur in or around the ovary. These include congenital cysts derived from mesonephric duct remnants. Knowledge of the size of the normal mature follicle is needed in order to make the diagnosis of a pathologic cystic condition. Histologic examination and special staining of the wall lining cells of the cystic structures is mandatory for appropriate classification.
Some controversy exists in the literature relative to the histologic structures of the cyst walls.
These arise from remnants of the mesonephric tubules and ducts. The anterior mesonephric tubules give rise to cysts referred to as epoophoron. The cysts may vary in size from a few millimeters to several centimeters in diameter. They are filled with a clear fluid. They frequently occur in horses and may be already present in the neonate.