Peritoneal endometriosis: endometrial tissue implantation as its primary etiologic mechanism.

The mechanisms involved in primary implantation of pelvic endometriosis were investigated in women undergoing laparoscopy as part of an infertility evaluation. Among the women with a diagnosis of endometriosis, those with no observed lesions in the ovaries and no adhesions in the pelvic cavity were classified as being in stage I. These women were further subdivided into two groups: the stage Ia group in which implants were found in one area (25 women) and the stage Ib group in which implants were found in two or more areas (29 women). In both groups the most frequent site was the uterosacral ligaments followed by the posterior uterine serosa, the posterior cul-de-sac, and the posterior broad ligaments in that order. Regardless of the position of the uterus, endometriosis was found significantly more frequently in the posterior compartment. The finding that the most frequent implants on the uterosacral ligaments were near the tubal ostium and, in the stage Ia group, the absence of implants on the anterior uterine serosa but a 33% frequency on the posterior uterine serosa, which is more frequently exposed to menstrual blood, suggest that implantation of endometriosis is the result of retrograde menstruation. However, the possibility that metaplasia of the serosal epithelium is stimulated by menstrual blood cannot be ruled out, inasmuch as the process of implantation in the peritoneum has not yet been proved.