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While no comprehensive theory definitively explains all facets of endometriosis yet, three established theories shed some light — and newer research offers insights, too.

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

Sampson's theory of retrograde menstruation is the most well-known and oldest theory on the cause of endometriosis, as well as the one most heavily supported by scientific evidence so far. The term "retrograde" will be familiar to almost anyone thanks to astrology, and it simply means "moving backward". The theory is, then, that endometriosis can result when some menstrual fluids flow back to the fallopian tubes and beyond, rather than all being expelled from the body. Once these fluids, which contain endometrial cells, reach other areas of the reproductive tract, they can then implant and proliferate there. 

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 proposes that endometrial lesions come not from retrograde menstruation — in which endometrial cells are transported from their original site — but that other cells transform into endometrial cells. This is also called the coelomic metaplasia theory, metaplasia referring to abnormal cell development. Because endometrial tissues can be present outside of the uterus even in female fetuses, the theory is that endometriosis is caused by abnormal fetal development. The "faulty endometrial cells" then activate and start causing symptoms once the female reaches puberty and has estrogen circulating in her system. 

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.

  • T. M. D'Hooghe, C. S. Bambra, M. A. Suleman, G. A. Dunselman, H. L. Evers, and P. R. Koninckx, “Development of a model of retrograde menstruation in baboons (Papio anubis),” Fertility and Sterility, vol. 62, no. 3, pp. 635–638, 1994.
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