麻豆传媒

IT鈥橲 A BURNOUT process that throws the Bridget Joneses of the world into a flat spin.

Of the millions of eggs a woman is born with, a mere few hundred stand the slimmest chance of making a baby. The rest languish in the ovaries getting steadily stale, or commit suicide-mostly before a woman reaches the grand old age of forty. It makes for some grim choices: either find a partner and establish your career in double-quick time so that you can have your babies young, or risk leaving children until your eggs are no longer up to the job.

That, however, is all about to change. Fertility researchers are powering ahead with their bid to rewind the hands of the dreaded biological clock. That means girls being born today can probably safely leave child rearing until their 40s, 50s or even 60s. But while the coming revolution promises to be more liberating than the contraceptive pill, paternity leave and emancipation combined, it might just come with a nasty sting in its tail.

鈥淭he only thing in life that is so cataclysmic is this breeding business. You have to cram it in by your late 30s,鈥� says Alan Trounson, director of the Monash Institute for Reproduction and Development in Melbourne. 鈥淲e鈥檒l find a way to stop menopause being such a dramatic thing for women, but we have to make sure we do it safely.鈥�

Make no mistake, doctors face immense challenges in trying to understand the human egg-the key to delaying menopause. The human egg is impossible to grow for any length of time in a lab dish, and while tucked away inside a woman鈥檚 body, in its earliest stages it is invisible even with the help of the most sophisticated imaging equipment. Reproductive physiologists like to claim-with some justification-that we know more about outer space than we do about the inner workings of the human egg.

Even before a woman realises that she is pregnant, the cells that will form her grandchildren have been carefully set aside in her embryo, still less than the size of a rice grain. If the fetus is female, by the seventh month of pregnancy, those cells will have spawned a mind-boggling 7 million immature eggs.

From there on, though, it鈥檚 all downhill. By the time a girl is born, numbers have plummeted to about 2 million-a mass slaughter whose purpose remains a mystery. Even then the carnage is far from complete. Over the next five decades, the egg population steadily diminishes as cells commit suicide in a series of defined steps-a process called apoptosis.

Of course, a few eggs escape that fate. When the girl hits puberty hormonal signals from the pituitary gland ensure that every few weeks groups of eggs start to mature. Nourished by a layer of granulosa cells, the chosen few take around six months to balloon up to 60 times their original size. Eventually, one egg outstrips the others, and ovulation occurs following another hormonal surge from the pituitary gland. The egg escapes from the ovary, and jets off into the fast lane of the Fallopian tube, and the possibility of a brief encounter with a sperm.

That onerous selection process means that by the time a woman has reached her late thirties, she鈥檚 scraping the bottom of the ovarian barrel. By 45, her chances of getting pregnant the old-fashioned way 鈥渁re very, very small,鈥� says Robert Winston, a leading fertility expert at Imperial College, London. 鈥淓ven by IVF, it is only about 3 or 4 per cent.鈥�

According to current thinking those odds could be much improved if reproductive physiologists could find some way to stop the untimely deaths of immature eggs.

鈥淢y gut feeling is that menopause is determined by how many of the resting follicles you have feeding the pipeline to make more maturing follicles,鈥� says Jonathan Tilly, a reproductive biologist at the Vincent Center for Reproductive Biology at Harvard. Once that stockpile dries up, the pipeline shuts down, and the woman鈥檚 reproductive system throws in the sanitary towel.

One way to stop that happening is to put your eggs on ice until you are ready to use them. Although human eggs are notoriously sensitive to freezing and thawing, these techniques are slowly edging into the mainstream-nowadays, clinics on at least three continents offer egg freezing to young women, and up to 60 babies worldwide have been born from frozen eggs.

Still, for now at least, the success rate remains low and none of the clinics 麻豆传媒 contacted is prepared to guarantee that its frozen eggs will be up to the job when they are defrosted. But if the techniques continue to improve, it鈥檚 likely that babies born today won鈥檛 bat an eyelid about freezing their eggs while they build a career or search for Mr Right. 鈥淚t鈥檚 an insurance policy,鈥� says Mohammed Taranissi, medical director of the Assisted Reproduction and Gynaecological Centre in London.

But it鈥檚 a costly and inconvenient one. The eggs can only be removed under sedation, after a series of unpleasant hormone injections. Far more convenient to find a way to keep immature eggs alive inside a woman鈥檚 body-which is exactly what Tilly and his team are working towards.

First they examined mice genetically engineered so they couldn鈥檛 make a protein called Bax, which plays a key role in cell suicide. They found that young adult Bax-less females had three times as many eggs as normal, but could still produce pups-proof that blocking cell suicide improves egg survival and doesn鈥檛 affect mouse fertility.

Two-year-old females-who鈥檇 be entitled to a telegram from the Queen if they were human-had a little more trouble making pups, but only because their bodies were too clapped out to carry a pregnancy. Eggs from the old Bax-less mice still formed normal embryos when fertilised in a test tube.

Artificial wombs are unlikely ever to be an option (see 鈥淎 bun in the husband鈥�), so that raises the question of how far into her dotage a woman could safely carry a pregnancy. No one knows for sure, but with the help of reproductive technology several women have given birth in their 60s. Many obstetricians suspect that the ability to carry a baby safely has more to do with a woman鈥檚 overall fitness and health than her chronological age.

Good egg

Of course, tinkering with a women鈥檚 genes to ward off the menopause is pie in the sky, but Tilly鈥檚 team is also working on a drug that will block cell suicide. Young women undergoing treatment for cancers like leukaemia and lymphoma urgently need this kind of research. Chemotherapy and radiation mean that over 80 per cent of them go through an early menopause, sometimes while still in their twenties.

One way cancer therapy seems to work is by triggering the cascade of signals that instruct a cell to commit suicide-the immature egg cells simply get caught in the crossfire. Reasoning that there鈥檚 absolutely no point in keeping damaged eggs hanging on by a thread-especially if they are to form the next generation, Tilly鈥檚 team has found a way of blocking the very earliest stages of egg suicide. 鈥淲e want to have a population of good oocytes, not the Night of the Living Dead oocytes,鈥� he says.

To do this, they tried a drug called sphingosine-1-phosphate (S1P), which has already been shown to halt cell suicide in human white blood cells. When Tilly鈥檚 team injected the drug directly into the ovaries of mice and treated them with radiotherapy, the mice鈥檚 immature eggs survived intact. They even went on to form normal embryos in the test tube, and the pregnancy rate of irradiated mice treated with S1P was double that of the mice that didn鈥檛 get the drug.

鈥淚t鈥檚 taken us 10 years to get to this point in mice, so it鈥檚 going to take a while,鈥� says Tilly. 鈥淸But] the technology could be useful for giving women an additional four or five years of fertile lifespan.鈥�

Another way to keep the biological clock ticking would be to stop eggs maturing in the first place, allowing them to remain in ovarian storage until women were ready to use them. 鈥淲hen you found a partner you wanted to have children with, you could concentrate all your reproductive [resources] at that time,鈥� says Trounson.

One factor that plays a role is anti-Mullerian hormone, or AMH. In mice, AMH is made by granulosa cells in the ovary, and puts a brake on the number of eggs selected to mature (麻豆传媒, 11 November 2000, p 20). Axel Themmen and colleagues at Erasmus University in Rotterdam studied mice that lacked AMH. At four months old, the mice had three times as many growing follicles as normal mice. By the time they were 13 months old, the mutant mice had completely run out of eggs.

Themmen is now investigating how AMH works in humans. A drug that enhances its effects might one day become the basis for a new 鈥渃areer pill鈥�.

But while research into career pills and anti-menopausal drugs is moving at a comfortable pace, techniques to improve pregnancy rates among women who are already running out of eggs are being unleashed at a speed that is causing some consternation, even in fertility circles.

This is of particular concern in countries such as the US, where there is no direct federal government regulation of fertility treatments. There, the Food and Drug Administration strictly controls the testing of new drugs, while new medical technologies tend to be left to individual states to regulate. All too often, that means fertility doctors need only get consent from their patients and their clinic鈥檚 own review board before they try out new techniques.

Take cytoplasmic transfer, the highly controversial technique that creates children that carry the genetic material of three people. Jacques Cohen and Jason Barritt of the St Barnabas Medical Center in New Jersey attracted international criticism for their work. Among fertility experts, the major worry is not about the few extra genes carried in the mitochondria-the cell鈥檚 powerhouses that make up a fraction of the jelly-like cytoplasm-but about the wisdom of using the new technique in humans without testing whether it works.

One of the main problems older women face when trying to conceive is that their eggs often end up with the wrong number of chromosomes, a condition called aneuploidy. If an aneuploid egg is fertilised, the resulting embryo either miscarries-sometimes before the pregnancy is detected-or the child is born with a handicap such as Down鈥檚 syndrome. One theory is that the cellular machinery in the cytoplasm that is needed to deal the right number of chromosomes into the two halves of the dividing egg is too old to do its job properly. That has led to the controversial notion that a shot of fresh cytoplasm from a younger woman鈥檚 egg could also help when an older woman鈥檚 eggs fail to produce babies for unknown reasons.

The trouble is, say critics like Winston and Trounson, there鈥檚 no hard evidence that the technology helps women get pregnant. True, the women that Cohen and Barritt treated had had many failed IVF attempts before they got pregnant following the new procedure. And, true, their embryos looked better under a microscope than those they produced without the procedure-but who鈥檚 to say that the same wouldn鈥檛 have happened with just one more round of IVF? To find out, you would need to run trials that compared cytoplasmic transfer or IVF alone in women with the same history of infertility.

What鈥檚 more, the few experiments testing similar techniques in animals have been contradictory. Even worse, the cytoplasm contains substances that are vital to the normal development of the embryo, so there鈥檚 even a slim chance that cytoplasmic transfer may threaten the long-term health of the baby.

Nothing is really known about the consequences of techniques like cytoplasmic transfer, says Tilly, who believes patient demand and doctors鈥� willingness to comply with those demands is rushing new treatments into the clinics too quickly. After all, one in six couples have fertility problems and for them the treadmill of fertility treatments can seem like their only option (see 鈥淩ight to choose鈥�) 鈥淧atients are driving a whole load of technologies that haven鈥檛 been validated,鈥� he says.

In fact, the babies born after cytoplasmic transfer are all reportedly doing fine. But two of the 17 fetuses created by Cohen and Barritt had a chromosomal abnormality that causes a disorder called Turner syndrome and did not survive, although no one can say whether the technique is to blame.

Eggbert the mouse

What we do know is that animal experiments that involve manipulating eggs paint a disturbing picture. Take Eggbert the mouse, the only mammal created so far from an egg grown mainly in a lab-a feat that fertility doctors would dearly like to repeat with human eggs to reduce the need for egg donors, and slash the costs of IVF.

At birth Eggbert appeared normal enough, and even went on to father a few pups. But at six months, disaster struck. Eggbert became obese, developed neurological problems and eventually died. A post-mortem revealed that Eggbert suffered from some of the same defects that plague cloned animals (麻豆传媒, 19 May, p 14).

鈥淭hat鈥檚 why we are so concerned. People should be very, very careful,鈥� says John Eppig, a reproductive biologist at The Jackson Laboratory in Maine who led the team that created Eggbert. 鈥淪imply having a baby鈥檚 footprints at birth doesn鈥檛 [necessarily] mean success.鈥�

Of course, the egg that became Eggbert was manipulated in a different way from the human eggs undergoing cytoplasmic transfer, but it illustrates a point. The egg 鈥渋s not just a quiescent little blob鈥�, says Kate Hardy, a reproductive biologist at Imperial College, London. 鈥淭here鈥檚 an incredible amount going on,鈥� most of which fertility researchers are only just beginning to understand.

Still not everyone is so quick to condemn the aggressive approach some infertility clinics take. 鈥淵ou鈥檒l hear that there鈥檚 no regulation in the States-and that鈥檚 not an accurate statement at all,鈥� says Sean Tipton, a spokesman for the American Society for Reproductive Medicine. 鈥淲hat we don鈥檛 regulate is people鈥檚 reproductive choices.鈥� (The St Barnabas clinic wouldn鈥檛 let Barritt talk to 麻豆传媒, but sent a written statement saying that its research complied with the hospital鈥檚 鈥渟tringent medical guidelines鈥�.)

What鈥檚 more, points out Tipton, similar criticisms were once levelled at IVF, an innovation that society now embraces. 鈥淲e don鈥檛 know for a certain fact that every human being conceived through an IVF process is not going to drop dead at the age of 30. We don鈥檛 know that because the first IVF baby is not that old yet. But there is certainly nothing to indicate that that鈥檚 going to be a problem. If you insist on that sort of long-term outcome, you鈥檙e never going to get anywhere.鈥�

But while fertility researchers struggle to balance the demands of science and their patients, one thing is clear: the number of patients is only going to grow. Women are delaying childbirth like never before-the average age for having a first child in Britain is now nearly 30-and without medical intervention a portion of those women are destined to swell the ranks of the infertile.

Bridget Jones aside, the need for a safe way to slow down the biological clock has never been more urgent.