What happens to the endometrium in normal women?
The lining of the uterus (endometrium) has the most remarkable growth potential of any tissue in the body. Various substances, some which are made within the endometrium itself, and some of which are derived from the circulating blood, promote this growth. The most potent stimulant of endometrial growth is the female hormone called oestrogen. In women, oestrogen is normally produced in the ovary by the egg follicles, and it then circulates in the blood. The endometrial
cells lining the uterus actively and preferentially absorb the oestrogen, which then stimulates the cells to grow and proliferate. As part of the normal function of the endometrium, there is no inflammatory reaction. The endometrium is normally shed during a woman’s period, after the ovarian follicle has died and the oestrogen level has fallen. In nearly all women, some of the endometrium refluxes up the fallopian tubes. It spills out of the tubes, over the ovaries, and into the
pelvis, which is lined with a thin layer of tissue called the peritoneum. The peritoneum normally absorbs this refluxed tissue with no inflammatory response, as with any other part of the body’s natural response mechanism. Consequently, the peritoneal lining stays smooth so that the tubes and intestines can function normally.
What happens to the endometrium in women who have endometriosis?
May have a relatively higher level and longer duration of exposure to oestrogen than other women, or may have higher oestrogen receptor sensitivity. These relatively high levels of oestrogens might also be capable of stimulating a thicker endometrium and more substantial pelvic contamination because of increased menstrual volume and higher incidence of retrograde bleeding.
• The endometrium is more vascular;
• More of the endometrial tissue that is shed stays alive, or vital;
• More of the sloughed tissue refluxes up the fallopian tubes, bathing the peritoneum and pelvic organs in more endometrial reflux that is also more vital.
Whether the refluxed endometrium itself grows, or whether it stimulates the tissues in the pelvis to grow endometrium, is uncertain. In any event, there is growth of endometrium in these tissues outside the uterus. Women who have endometriosis also have an abnormal autoimmune response, because the body’s immune response to the initial spill and growth of the refluxed endometrium is not simply by absorption, but by an inflammatory reaction. Women who have endometriosis may also have other hyperimmune problems – such as asthma, arthritis or colitis – more commonly than other women.