Ovarian cysts are a common finding in rats and mice (Figure 1, Figure 2, Figure 3 and Figure 4). Ovarian cysts may be unilateral or bilateral, single or multiple, and may become quite large. Ideally, cysts should be classified by origin, location, and/or morphologic appearance (follicular, luteal, epithelial, rete ovarii, bursal, paraovarian), which may provide information as to pathogenesis in toxicity or carcinogenicity studies. However, origin may be difficult to assign because the size and number of cysts increase with age, and the walls become so thin that identifying features are lost. The pathogenesis of ovarian cysts is often unknown; however, in mice some of them may be of mesonephric duct origin.
Ovary - Cyst should be diagnosed but not assigned a severity grade. Whenever possible, the type of cyst (follicular, luteal, epithelial, rete ovarii, bursal, paraovarian) should be included in the diagnosis as a modifier. The diagnosis of Ovary - Cyst, with no modifier, is reserved for those instances when the origin of the cyst cannot be determined.
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