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Ovarian cyst lining. Coloured scanning electron micrograph (SEM) of irregular cells making up the internal lining of an ovarian cyst. An ovarian cyst is an abnormal fluid-filled cavity that results from a follicle that did not ovulate (produce an egg, the female reproductive cell). The cyst cells are covered in microvilli (white hair-like structures). This cyst resulted from polycystic ovary disease in a patient with Stein- Leventhal syndrome. The cysts may cause pain and menstrual irregularities, and may need to be surgically removed. Magnification unknown. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER)