Endometriosis. Coloured scanning electron micro- graph (SEM) of collagen fibres (green) underlying the surface of an endometriotic ovarian cyst. The fibres are immersed in stroma cells (purple), those which support the functional areas of an organ. Endometriosis occurs when fragments of the uterine lining become displaced, attaching to other sites in the body. Here the fragment is attached to an ovary. The fragments respond to the menstrual cycle, and bleed monthly. The blood cannot escape, and a cyst forms. The cyst causes heavy menstrual bleeding, pain and sometimes infertility. Magnification: x3000 at 6x7cm size.