Endometriosis is a medical condition that causes endometrial tissue — the tissue that lines the uterus — to grow elsewhere in the body. The endometrial tissue implants outside of the uterus and swells with each menstrual cycle. At the end of the menstrual cycle, this tissue tries to shed, just like normal endometrium in the uterus. However, because the tissue cannot be released through the uterus as normal, it causes pain and inflammation in the area where it has implanted, especially during menstruation. This can lead to a number of symptoms, including pelvic pain and infertility.
The exact cause of endometriosis is not well understood. It is thought the disease may develop as a result of a combination of factors.
Genetics as a cause for endometriosis
Approximately 176 million women worldwide are affected by endometriosis during their reproductive years, and it is thought that genetics is a contributing factor to the development of the disease. Several genes have been linked to the development of endometriosis within families, and it does appear that some women are predisposed to develop the disease.
Hormonal impact on endometriosis
The menstruation cycle is closely linked to circulating hormones, including estrogen and progesterone. The precise link between these hormones and endometriosis is not known, but some women and even some men taking estrogen treatments have developed endometriosis. That indicates there may be some link between these hormones and the disease.
Endometriosis due to retrograde menstruation
Retrograde menstruation occurs when the menstrual blood does not follow its normal course out of the body, and instead makes a backward movement through the fallopian tubes — the tubes connecting the ovaries to the uterus. When this happens, the menstrual fluids flows into the peritoneal cavity, which is the area within the abdomen where the intestines, stomach, and liver are located. The menstrual fluids contain endometrial cells, which in the case of retrograde menstruation may stick to the pelvic walls and surfaces of pelvic organs. Over time, the cells may start to grow, and may continue to thicken and bleed with each menstrual cycle.
Retrograde menstruation has been suggested as a cause of endometriosis, but is no longer considered likely. It has been demonstrated that most women experience some retrograde menstruation every cycle, but only a small percentage of them develop endometriosis.
Endometriosis caused by immune system dysfunction
In some cases, endometriosis may be caused by a dysfunction of the immune system. Some women with endometriosis appear to have a diminished immune response, and it has been suggested this prevents their immune system from recognizing and destroying the endometrial tissue that causes endometriosis. Some researchers have linked environmental factors to immune system dysfunction.
Lymphatic or circulatory spread as a cause for endometriosis
It is possible that endometrial cells may be able to move through the blood circulation or lymphatic system and attach, or implant, in places where they should not. This could explain why these cells sometimes can be found in the brain or the eyes.
Metaplasia as a cause of Endometriosis
Metaplasia has been proposed as a potential cause of endometriosis by some researchers. The term describes a phenomenon in which a cell of a particular type spontaneously transforms into a different type of cell. Some researchers think metaplasia could occur while individuals are still in their mother’s wombs. Other scientists think mature cells may gain the ability to undergo metaplasia, possibly as a result of environmental factors. However, there is not enough current evidence to support either view.
Stem cell theory of endometriosis
Stem cells are cells that have the ability to develop into many different cells types, including skin, muscle and brain. Some types of stem cells — embryonic stem cells — can develop into any type of cell, while others can only give rise to a limited number of cell types.
In the uterus, a type of stem cell called mesenchymal stem cells are responsible for replacing the endometrial layer that is lost every month through menstruation.
The stem cell theory of endometriosis suggests that these stem cells may migrate outside of the uterus, regenerate, and spread, forming endometrial lesions.
Last updated: August 6, 2019
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