Where might endometriosis tissue grow?

In endometriosis, the kind of tissue that normally lines the womb also grows elsewhere. Medically speaking, there are different types of endometriosis, depending on where the areas of endometriosis tissue (endometrial implants) are:

• In the peritoneum (common): Here the endometriosis tissue is attached to the lining of the abdominal cavity.
• In the muscles of the womb (common): The tissue grows into the muscular wall of the womb. The medical term for this is adenomyosis.
• In the lower part of the pelvis (common): Here the endometriosis tissue grows in the ovaries, fallopian tubes and/or the “Pouch of Douglas” (found between the womb and rectum at the end of the bowel).
• Outside this area of the pelvis (rare): For instance, in the bladder or bowel, or very rarely in parts of the body that are further away, like the lungs.

Every month during the menstrual cycle, the endometriosis tissue is also built up and shed outside of the womb. But because the blood and shed tissue can't leave the woman’s body through her vagina, they stay near the endometriosis tissue. Here they may lead to inflammations, which in turn can cause scarring and adhesions (when tissue sticks together).

Endometriosis tissue (side view of abdomen)

What causes endometriosis?

There are various theories about this. According to one of them, cells from the lining of the womb get into other parts of the body and settle there. It is also thought that a hormonal imbalance or a problem with the immune system plays a role. Our immune system usually makes sure that cells from a particular organ don't grow elsewhere in the body.

The hormones estrogen and progesterone aren't regulated properly in women with endometriosis, either: Estrogen in particular causes endometriosis tissue to grow, whereas progesterone stops it from growing. But in endometriosis, this effect of progesterone is inhibited.

According to other theories, certain cells outside of the womb can turn into endometriosis cells for no known reason. Endometriosis seems to be more common in certain families, so genetic factors might play a role too.

Sources

• Deutsche Gesellschaft für Gynäkologie und Geburtshilfe (DGGG). Diagnostik und Therapie der Endometriose (S2k-Leitlinie, in Überarbeitung). AWMF-Registernr.: 015-045. 2020.
• Horne AW, Missmer SA. Pathophysiology, diagnosis, and management of endometriosis. BMJ 2022; 379: e070750. [PubMed: 36375827]

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