Light micrograph human tissue section showing adenomyosis. This condition occurs when the tissue that normally lines the uterus (endometrial tissue) grows into the muscular wall of the uterus (myometrium). The displaced tissue continues to act normally, thickening, breaking down and bleeding, during each menstrual cycle. An enlarged uterus and painful, heavy periods can result. Removal of the uterus (hysterectomy) cures adenomyosis.