Transplantation Theory The transplantation theory, originally proposed by Sampson in the mid-1920s, is based on the assumption that endometriosis is caused by the seeding or implantation of endometrial cells by transtubal regurgitation during menstruation (30). Substantial clinical and experimental data support this hypothesis (5,31). Retrograde menstruation occurs in 70% to 90% of women, and it may be more common in women with endometriosis than in those without the disease (8,32). The presence of endometrial cells in the peritoneal ﬂuid, indicating retrograde menstruation, is reported in 59% to 79% of women during menses or in the early follicular phase, and these cells can be cultured in vitro (33,34). The presence of endometrial cells in the dialysate of women undergoing peritoneal dialysis during menses supports the theory of retrograde menstruation (35). Endometriosis is most often found in dependent portions of the pelvis—the ovaries, the anterior and posterior cul-de-sac, the uterosacral ligaments, the posterior uterus, and the posterior broad ligaments (36). The menstrual reﬂux theory combined with the clockwise peritoneal ﬂuid current explains why endometriosis is predominantly located on the left side of the pelvis (reﬂuxed endometrial cells implant more easily in the rectosigmoidal area) and why diaphragmatic endometriosis is found more frequently on the right side (reﬂuxed endometrial cells implant there by the falciform ligament) (37,38).
Endometrium obtained during menses can grow when injected beneath abdominal skin or into the pelvic cavity of animals (39,40). Endometriosis was found in 50% of Rhesus monkeys after surgical transposition of the cervix to allow intra-abdominal menstruation (41). Increased retrograde menstruation by obstruction of the outﬂow of menstrual ﬂuid from the uterus is associated with a higher incidence of endometriosis in women and in baboons (42–44). Women with shorter intervals between menstruation and longer duration of menses are more likely to have retrograde menstruation and are at higher risk for endometriosis (45). Menstruation is associated with intraperitoneal inﬂammation in both women and baboons, but a limited quantity of endometrial cells can be identified in peritoneal ﬂuid during menstruation in women, possibly because endometrial–peritoneal attachment is reported to occur within 24 hours (46–48). Ovarian endometriosis may be caused by either retrograde menstruation or by lymphatic ﬂow from the uterus to the ovary; metaplasia and bleeding from a corpus luteum may be a critical event in the development of some endometriomas (49–51).