Endometriosis is a medical condition that causes endometrial tissue (the tissue that lines the uterus) to grow outside of the uterus. 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, and 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 families 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, which indicates there may be some link between these hormones and the disease.

Endometriosis due to retrograde menstruation

Retrograde menstruation occurs when the menstrual blood makes a backward movement through the fallopian tubes or the tubes connecting the ovaries to the uterus, and into the peritoneal cavity (the area within the abdomen where the intestines, stomach, and liver are located), rather than following its normal course out of the body. 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 continue to thicken and bleed with each menstrual cycle. Retrograde menstruation has been suggested as a cause of endometriosis. But is no longer considered to be a likely cause because 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. Metaplasia is the term that describes a phenomenon in which a cell of a particular type spontaneously transforms into a different type of cell. Some researchers think this could occur while a person is still in their mother’s womb. Others think mature cells may gain the ability to undergo metaplasia, possibly as a result of environmental factors. But there is not enough current evidence to support either view. 

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