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Endometriosis. Coloured scanning electron micro- graph (SEM) of the surface of an endometriotic ovarian cyst. Endometriosis occurs when fragments of the uterine lining become displaced, attaching to other sites in the lower body. Here the fragment is attached to an ovary. Its surface is similar to that of the normal uterine lining, with secretory cells (brown) possessing tiny microvilli. The displaced fragments respond to the menstrual cycle, and bleed monthly. The blood cannot escape and a cyst forms, which swells and causes heavy menstrual bleeding, pain and maybe infertility. Magnification: x1100 at 6x7cm size. (KEYSTONE/SCIENCE PHOTO LIBRARY/PROFESSOR P. MOTTA & G. FAMILIARI)