Cyst size and cellular metaplasia increased with age in both BR (A-C) and IO (D-F) mice. A. Intraovarian cyst (c) in an ovary from a 6-month BR mouse. Serial sectioning of this ovary showed this cyst originated in the hilus of the ovary. Dilated rete ovarii tubules (RO) are present in the extra-ovarian fat. B. Ovary from a 9-month BR mouse, containing an intraovarian cortical cyst with many papillae (right) and a hilar cyst (lower left). C. Ovary from a 12-month BR mouse containing a large, collapsed hilar cyst, from which cyst fluid has been aspirated. D. Cyst in the medulla of a 6-month IO mouse, which was shown to be connected to the hilus of the ovary on serial sectioning. E. Collapsed cyst of unknown origin in the ovary of a 9-month IO mouse. The original diameter of this ovary was 11 mm before aspiration of 950 μl of cyst fluid. F. Hilar cyst with extensive papillae formation in an ovary from a 12-month IO mouse.