The paired ovaries of the rat are grape-like structures that vary in gross appearance and size, depending on the stage of the oestrous cycle. The subgross anatomy of the rodent ovary. Covering its surface is a single layer of modified peritoneal mesothelium, the ovarian surface epithelium (OSE), which is continuous with the broad ligament (mesovarium) that supports the ovary. The OSE of a single ovary can range from squamous to cuboidal, columnar or pseudostratified columnar in type; this regional variation in OSE morphology accompanies the cyclical changes that occur within the underlying ovarian parenchyma during the oestrous cycle.