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Ovarian cyst lining. Coloured scanning electron micrograph (SEM) of irregular cells (green) making up the internal lining of an ovarian cyst. The red cells may be lymphocytes (a type of white blood cell). 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)