The Facts
The endometrium is the tissue that lines the inside of the uterus
(the womb). It is the tissue that is shed each month when women menstruate.
Each month it builds up rapidly in case of pregnancy, and each month the
excess endometrial tissue is sloughed off during menstruation if pregnancy does
not occur.
In endometriosis, endometrial cells are found outside the uterus, usually
in other parts of the abdomen. These cells respond to female hormones in
the same way as the lining of the uterus does. Each month, tissue builds up,
breaks down, and sheds. Unlike the cells inside the uterus, the blood and tissue
that are shed in the abdomen have no way of leaving the body. They stick to
other tissue and sometimes start to divide and multiply. They may grow into
other tissue, or form strands that bind organs together. They may create scar
tissue, which can be painful. Sometimes the endometrial cells create cysts that
can rupture and bleed.
The process sounds a bit like cancer, but endometriosis isn't cancer. However,
it may very slightly increase the risk of getting certain cancers. Endometriosis
isn't a fatal disease, but can be widespread. About 1 in 10 actively menstruating
women between the ages of 25 and 44 has endometriosis. About one third to one
half of infertile women have the disease.
Causes
Various theories have tried to explain endometriosis. Most involve the
idea of retrograde flow. During menstruation, the endometrium sheds its
top layers. These layers normally leave the body. The tissue and blood is forced
out by muscle contractions. Sometimes, however, it flows backwards, going up
the fallopian tubes towards the ovaries. Endometrial cells could reach both
the ovaries and the pelvic cavity in this way.
Retrograde flow doesn't explain everything though, because it's often seen
in women without endometriosis. For endometriosis to develop, something
else such as an immune disorder is usually needed. There are measurable differences
in the immune systems of women with endometriosis, but we don't know yet what
the significance is.
Endometriosis may have a genetic component. The daughters and sisters of women
with endometriosis are at a slightly higher risk of getting the disease.
Another theory of endometriosis suggests that it spreads through the blood
or lymphatic vessels. There is also the possibility that normal tissue inside
the abdominal cavity may change and become endometriosis.
