How Endometriosis Works
Every month when a woman has a period, the cells that line the uterus, known as the endometrium, are shed in the menstrual flow. Some small portion of this combination of blood and endometrial cells may also pass out through the fallopian tubes into the abdominal cavity. Most of the time, the body's natural defense systems attack and destroy these cells before they can begin to grow. However, for reasons that are not clear, this is not always the case.
In certain individuals, these endometrial cells actually implant on structures in the abdominal cavity and begin to grow. This is endometriosis: the presence of viable endometrial cells in places other than the uterine cavity. Then, each month when the normal hormonal changes result in a menstrual period, much the same change occurs in the endometriosis. A small amount of bleeding occurs from the endometriosis cells. This is very irritating to the body, and as a result of this, scarring occurs around the endometriosis. Most often this is a progressive process, with a small additional amount of bleeding and scarring occurring every month.
Once the endometrial cells begin to grow in the abdominal cavity, they are known as implants. Implants can occur on any structure, including the ovaries, fallopian tubes, bladder, bowel, and on the lining of the abdominal cavity (known as the peritoneum). The area behind the uterus, between it and the rectum, is known as the cul-de-sac, and this is the most common site for endometriosis. Implants may appear as small, clear or red, fluid-filled sacs, or most commonly, as dark brown or black areas. The collection of old blood in the implants gives them this appearance. Some scarring is typical around the implants, and can be very localized or, at times, quite severe. When endometriosis develops in the ovaries, large cysts full of old blood, known as chocolate cysts or endometriomas, may result.
There are other theories as to how endometriosis develops, and this scenario (known as retrograde menstruation) certainly cannot explain all cases of endometriosis. It is, however, the most widely held theory and does explain all but the most unusual cases of endometriosis.