Clinical meaning
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 inflammatory stimulation; (3) lymphovascular dissemination: explains distant endometriosis (lung, brain) via hematogenous or lymphatic spread; (4) stem/progenitor cell theory: bone marrow-derived stem cells may differentiate into endometrial tissue at ectopic sites. The chronic inflammatory microenvironment drives endometriosis progression: ectopic implants produce prostaglandins (PGE2, PGF2-alpha), cytokines (IL-1, IL-6, TNF-alpha), matrix metalloproteinases, and angiogenic factors (VEGF) that promote tissue invasion, neovascularization, adhesion formation, and fibrosis. This inflammation also sensitizes and damages pelvic nerves, creating BOTH nociceptive and neuropathic pain components (explaining why pain severity does not correlate with disease stage). Estrogen is the primary driver: ectopic implants express aromatase (converting androgens to estrogen locally), creating a positive feedback loop where local estrogen production promotes further implant growth and prostaglandin synthesis, which in turn stimulates more aromatase activity. This estrogen dependence forms the basis for all hormonal treatment strategies.