Endometriosis: Ectopic Endometrial Implantation

Endometriosis is a chronic, estrogen-dependent inflammatory condition characterized by the presence of endometrial-like tissue (glands and stroma) outside the uterine cavity. These ectopic implants most commonly involve the ovaries (endometriomas or 'chocolate cysts'), uterosacral ligaments, posterior cul-de-sac (pouch of Douglas), pelvic peritoneum, fallopian tubes, and rectovaginal septum, but can occur in distant sites (diaphragm, pleura, surgical scars). The ectopic tissue responds to cyclic ovarian hormones just as normal endometrium does: it proliferates under estrogen stimulation during the follicular phase, undergoes secretory changes under progesterone during the luteal phase, and undergoes hemorrhagic breakdown during menstruation. However, unlike normal endometrial shedding that exits through the cervix, the ectopic tissue bleeds into the surrounding tissue with no escape route, causing a chronic inflammatory response. Multiple pathogenetic theories explain endometriosis: (1) Sampson's retrograde menstruation theory (most widely accepted): menstrual blood containing viable endometrial cells flows backward through the fallopian tubes into the peritoneal cavity; approximately 90% of women have retrograde menstruation, but only 6-10% develop endometriosis, suggesting additional factors are required; (2) coelomic metaplasia theory: peritoneal mesothelium transforms into endometrial-like tissue under hormonal or...