STI: Scientists test boundaries of fertility

March 15, 2004
Scientists test boundaries of fertility

PARIS  -  Once upon a time, women faced a biological clock, which tick-tick-ticked away the years of their fertility until it rang, with a dull and often dreaded clang, in their forties.

That deadline is the force behind innumerable decisions made by women, ranging from when to have a family, how to approach the dating market and how to manage their careers.

Men are far luckier in this respect, for they produce sperm from germline cells in their testes throughout their lives.

But the latest research suggests that, one day, women may be able to put the clock on hold for years  -  and if that happens, the social impact will echo just as loudly as the introduction of the contraceptive pill in 1960.

One plank of the 'clock' theory is that women are born with a given number of eggs in their ovaries and cannot produce any more during their lifespan.

But, 83 years after it was born, this dogma has been hammered by Harvard Medical School scientists.

They gave pre-pubertal mice a chemical that kills egg cells and were astonished to find the rodents continued to produce eggs in adulthood, proving an ability to generate fresh eggs to replace damaged ones.

'If these findings hold up in humans, all theories about the ageing of the female reproductive system will have to be revisited,' says lead researcher Jonathan Tilly, a Harvard professor of obstetrics and reproductive biology.

'We also may need to revisit the mechanisms underlying such environmental effects on fertility as smoking, chemotherapy and radiation. Eventually, this could lead to totally new approaches to combating infertility in cancer patients and others.'

In his mice, the new eggs were replaced thanks to stem cells  -  the immature master cells that grow, or differentiate, into specialised cells  -  in the ovaries.

If the same germline stem cells can be found in women, and a way found to make them grow into the egg precursors called follicles, the menopause could be postponed.

Says Mr Allan Sprading of the Carnegie Institution in Washington: 'Germline stem cells in humans might easily have been missed for the same reasons that they escaped detection in mice for so long.'

He speculates that depletion of these germ cells may be a cause in the sharp sudden decline in the egg quality when women reach their thirties. Flaws in these older eggs make it harder to become pregnant and avoid a miscarriage.

Another assault on fertility doctrine is being led from another direction  -  transplants of ovarian tissue.

Studies published by Nature this week, where Dr Tilly's work also appears, report on two remarkable experiments in which mammalian ovarian tissue was transplanted to another part of the body, where it grew and was coaxed with hormone treatment into yielding eggs.

In the first case, a transplant was carried out on a 36-year-old woman who had had an ovary removed and frozen ahead of cancer treatment six years earlier.

Thawed and inserted under her skin, the tissue's follicles yielded more than 20 eggs, which were gently sucked out and fertilised in vitro.

Only one developed normally; when it had reached the four-cell stage it was transplanted into her uterus, but she did not become pregnant.

In the second experiment, transplanted ovarian tissue in seven rhesus monkeys led to the birth of a healthy female. However, the tissue was fresh, and had not been frozen and thawed.

The work  -  still in its earliest stages  -  mainly targets women who urgently need chemotherapy or radiotherapy for cancer which will make them sterile.

They do not have the time, or cannot take the hormones, to coax their ovaries into producing eggs that are then harvested, fertilised and stored.

But the potential market is vast.

The idea of storing away fertility and reviving it years later, perhaps using IVF and surrogate mothers, will interest many thirtysomething women, oppressed by that ticking clock.  --  AFP