Endometriosis is a chronic inflammatory disease in which the endometrium, the tissue that normally lines the uterus and plays an important part in the menstrual cycle and fertility, also grows in other areas of the reproductive system, and beyond. Three different theories exist about its cause, all predictably named after those who came up with them — Sampson’s theory, Meyer’s theory, and Halban’s theory. These theories still don't offer a comprehensive cause behind all the facets of endometriosis, but they're interesting to read about, anyway.
Retrograde menstruation: Sampson’s theory
Retrograde menstruation cannot fully explain endometriosis for the simple reason that research now suggests that nine in 10 women of reproductive age, with regular periods, have it. Only 10 percent of women have endometriosis, meanwhile. Women with endometriosis do seem to experience more of a retrograde flow than others, on the other hand, and the fact that their reproductive organs are exposed to larger amounts of endometrial cells may explain the increased danger of endometriosis. Future research will hopefully address the question of why retrograde menstruation leads to endometriosis in some women, but not others.
Primate research with baboons and macaques was carried out to further explore this theory. The test subjects' pelvic cavities were exposed to their own menstrual fluids, mimicking the process of retrograde menstruation. Nearly 50 percent of the unlucky primates developed endometriosis lesions after one round of testing, while all of them ended up with endometriosis after two rounds!
Coelomic metaplasia: Meyer’s theory
Meyer's theory does offer an explanation for the fact that endometriosis can strike before puberty even hits, but because they still lack estrogen, this may be a different kind of endometriosis.
Halban’s theory of endometriosis
Halban’s theory of endometriosis instead proposes that endometrial cells are spread through lymphatic or vascular routes. This theory could perhaps make some sense of the fact that endometrial lesions are sometimes — extremely rarely, but nonetheless — found in places like the lungs or brain. It doesn't account for the established patterns of endometrial lesions around only the reproductive system in most patients, however.
Does newer endometriosis research tell us anything new?
These three established theories have been around a while, with Sampson's theory dating back to the 1920s. Does more recent research offer any new insights into the cause of endometriosis?
The observation that endometriosis, which leads to an inflammatory response, seems to behave somewhat like an autoimmune disease has led to research that suggests that immune system abnormalities indeed play a role in the development of endometriosis. The immune system could be responsible for the fact that endometrial tissues outside of the uterus remain in place and grow, rather than being destroyed and reabsorbed.
Scar tissue may also play an important role in the development of endometriosis, more recent research suggests, because endometrial cells may implant at the site of a surgical scar, such as one caused by a cesarean section or other pelvic surgery — in patients who did not previously suffer from endometriosis.
The genetic component of endometriosis remains to be explored further, but we do know there is one — having an immediate female relative who has endometriosis makes it much more likely that you'll develop it, too. A large study made headway into discovering what genes are related to endometriosis, something which will help us understand the cause of endometriosis more deeply in the future.