In endometriosis one finds ectopic endometrial tissue outside the uterus, either on the ovaries, the fallopian tube, the uterine ligaments, the recto-vaginal septum, the pelvic peritoneum or possibly in a scar following a laparotomy. Just like the uterine endometrium, this tissue takes part in the menstrual cycle.
There are various theories concerning the origin of this tissue:
- · The metaplasia theory maintains that the coelomic epithelium, out of which the paramesonephric duct through invagination has arisen, and formed the fallopian tube, the uterus and parts of the vagina, retained its ability to differentiate and become endometrium.
- The reflux theory holds that scaly endometrial material gets into the abdominal cavity retrogradely and primarily grows into the small pelvic area and only secondarily into the peritoneum.
- The implantation theory says that possibly through an inadvertant transplantation during a surgical intervention endometrial tissue gets outside the uterus and starts to grow there.
- In the engendering of endometriosis genetic, immunologic and hereditary factors probably also play a role. This explains why not all women in whom endometrial cells get into the abdominal cavity are susceptible to the same extent for the formation of an endometriosis.