Figure 1.

(a) Laparoscopy of the pelvis performed at the time of menstruation. Predictable cyclic ovulatory menses giving rise to repetitious episodes of retrograde travel of endometrial tissue and blood into the dependent portions of the pelvic cavity is the main cause of pelvic endometriosis. Not all women who experience retrograde menstruation, however, develop endometriosis. This suggests that a number of differences between the patients with endometriosis and disease-free women may account for this condition. These include increased quantities of menstrual tissue that reach the abdominal cavity because of outflow track obstruction or deeper separation of the functionalis layer from the basalis layer (see Fig. 6) and cellular and molecular defects in eutopic endometrial or peritoneal tissues of women with endometriosis. (b) Graphic depiction of retrograde flow of endometrial tissue fragments made of spindly stromal and cuboidal epithelial cells. (c and d) Menstrual tissue fragments may survive and grow on peritoneal or subperitoneal locations (peritoneal endometriosis) or may get deposited into the rectovaginal (RV) pouch during repetitious episodes of menstruation and remodel the neighboring vaginal, rectal, and cervical tissues via a chronic inflammatory process to give rise to a deep-infiltrating RV nodule. (e) The endometrial tissue fragments may populate the exposed lining of a follicular or corpus luteum cyst to eventually evolve into an endometrioma. [Adapted with permission from Bulun SE. Endometriosis. In: Strauss J, Barbieri R, eds. Yen & Jaffe’s Reproductive Endocrinology. 8th ed. Philadelphia, PA: Elsevier; 2019:609–642. Copyright © 2019 by Elsevier.]