Endometriosis and Angiogenesis

Whilst the problem of endometriosis may becoming more known by the general public, the cause of the disease remains unclear. Several points seem to be agreed:

• Retrograde menstruation increases the chances of developing the disease
• All women who menstruate can have endometrium in the peritoneal cavity
• It is not known why some of these women go on to develop endometriosis

One suggestion is that either because of the tissue itself or because of other factors in the peritoneal cavity, women who develop the disease have a higher capacity to grow blood vessels around the explants compared to women who do not get the disease.

The process of new blood vessel development is called angiogenesis. Peritoneal fluid taken from women with endometriosis has a greater ability to stimulate endothelial cell proliferation than fluid removed from women without the disease. This fluid contains several agents known to stimulate new blood vessel development. The levels of one of these agents, vascular endothelial cell growth factor (VEGF-A) is increased in the fluid when fresh tissue is deposited at menstruation. Other agents including tumour necrosis factor (TNF) are also increased in the fluid in women with the disease compared to women who do not have the disease.

The source of these agents is interesting. Increased amounts of protein are not found in the explants themselves, rather cells of the immune system (macrophages, t-lymphocytes and natural killer cells) seem to be the principal source. It is not known if these agents are increased before the onset of the disease or as a consequence of the disease.

Several studies have shown an abundant supply of blood vessels to the endometriotic explants, at least those found on the peritoneal surface and on the ovaries. In other situations, pieces of tissue will not survive if they are larger than about 1 m 2. If they are to survive, they need to develop a blood supply. Presumably this basic physiological tenant still applies to endometriotic nodules.

There is considerable interest in the field of cancer for attacking these blood vessels as a means of preventing the growth of tumours. A similar strategy may be successful for endometriosis. Normal angiogenesis occurs in adults in wound healing but otherwise, endothelial cells are quiescent and may last for two to three years. This is not the case in the endometrium. Along with the ovary, this is a privileged site where physiological angiogenesis occurs on a monthly basis. It is this need of the tissue to re-grow on a monthly basis that may be making endometrium so effective in growing at ectopic sites.

Whilst this work is very new (if not the idea), it may be possible to develop treatments that block blood vessel growth and cause the regression of endometriotic nodules. This treatment could be given alone or after surgical removal of significant deposits of endometriosis. The results of studies in our laboratories and those of others may thus lead to new treatment options for women with endometriosis.