By Caroline Ryan
BBC News Online health staff in Berlin
Scientists say they have discovered a promising new therapy for endometriosis, a condition which can cause infertility.
Between 10 to 25% of women could be affected
It happens when tissue which normally lines the uterus is found elsewhere in the body.
The US and Dutch told a major fertility conference in Berlin that drugs which affect the development of new blood vessels, prevent these lesions growing.
Existing treatments are not always effective, the researchers say.
Around 10 to 25% of women of reproductive age are estimated to be affected by endometriosis. The condition is difficult to diagnose, and is sometimes missed by doctors.
Endometrial tissue behaves the same way, where-ever it is in the body. It builds up during the menstrual cycle, and then breaks down and sheds off during a period.
This keeps the lining of the uterus healthy and ready to nourish a developing foetus.
However, the when the tissue acts in the same way outside the uterus it can cause problems.
Menstrual blood can be easily shed from the uterus, not from elsewhere in the body.
The researchers, from the University Hospital Maastricht in The Netherlands, and the Children's Hospital in Boston, US, transplanted human endometrial tissue which had been transplanted into 49 mice and allowed it to grow into lesions.
They tested four types of angiostatic treatments, which inhibit the development of new blood vessels.
It was found that the treatments reduced the number of new blood vessels which developed around the tissue.
The number of endometrial lesions was also lower in the mice which received the treatment.
But the treatment did not affect existing endometrial blood vessels, probably because they are protected by smooth muscle cells.
One of the treatments, endostatin, was particularly effective. The researchers say they are not sure why this is the case, but suggest it could hinder the spread of the endothelial cells.
However, the scientists warn their work is still at an early stage, and it could be years before their findings will translate into improved treatments for women with the disease.
Dr Annemiek Nap, of the University Hospital in Maastricht, who worked on the study, said angiostatic therapy could be used as a way of preventing the recurrence of endometriosis after it has first been treated by surgical or hormonal therapy.
"Once endometriosis is diagnosed, a woman could have angiostatic therapy to prevent more lesions forming or the existing ones growing, and then after surgery, the therapy could be continued to prevent a recurrence," she added.
Dr Christian Becker, a researcher in the Department of Vascular Biology at Boston's Children's Hospital, tested endostatin on mouse endometriosis.
It was found the treatment halved the growth and numbers of lesions. In addition, the mice did not experience side-effects, such as changes to hormone levels or menstrual cycles.
He said: "This may be clinically relevant as current therapy for endometriosis is aimed at suppressing the patient's hormones.
"For women who do not want to become pregnant this may not be a big problem, but as infertility is often connected to endometriosis it is problematic to suppress the patient's hormone.
"These patients definitely will not be able to conceive under that medication. With endostatin, however, it would be possible to become pregnant."
He said existing treatments for endometriosis is highly unsatisfactory, due to strong side-effects and a high rate of recurrence so it was "imperative" to find a new way of treating the condition.