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Last Updated: Wednesday, 29 December, 2004, 00:11 GMT
How nature keeps eggs 'in check'
Image of an egg in the ovary
Signals between the egg and its surrounding cells are important
Researchers have found a receptor that they believe keeps eggs in arrested development until they are ready to be fertilised.

Although the study involved mice, experts believe a similar receptor will be present in human eggs.

This knowledge could be useful for understanding infertility and birth defects, as well as paving a way for new contraceptives, they say.

The US research is published in the journal Science.

It may be important for understanding unexplained infertility.
Professor Steve Franks, an expert in reproductive endocrinology at Imperial College, London

Unlike men who make sperm on an ongoing basis, every woman is born possessing all of the eggs in her ovary that she will ever use.

These eggs, like sperm, contain half the number of chromosomes as the normal cells in our bodies.

This enables the genetic information between the mother and the father to mix to make a baby.

The process that reduces the chromosome numbers in the eggs in preparation for fertilisation is called meiosis.

It occurs in stages. It starts when the eggs are first formed while the female baby is still in the womb.

Pregnant pause

Meiosis is not completed until each egg is released, usually one each month in response to a surge of hormones, when the woman ovulates and has her period.

Scientists have known that eggs are held in suspended meiosis, and that it has something to do with the cells that surround the eggs in the ovary.

But they have not known exactly how.

The University of Connecticut Health Center team studied what was happening in female mice.

They found one receptor in the egg, called GPR3, was essential for keeping it in this 'paused' state.

When they knocked out the receptor in the mice, the eggs resumed meiosis without being released from the ovary in the usual way.

The researchers still do not know what signal acts on the GPR3 receptor to keep meiosis on pause, but they believe it comes from the surrounding cells in the ovary.

Scientists already know that the hormone surge which triggers ovulation helps to release the egg from the ovary and break its contact with the surrounding cells.

Exciting implications

This would appear to support the researchers' theory.

Professor Steve Franks, an expert in reproductive endocrinology at London's Imperial College, said: "The big mystery has been what keeps the egg in suspended meiosis.

"We have known what resumes meiosis, but what keeps it arrested?

"This is really interesting. It is an excellent piece of research.

"I'd be surprised if we did not find a similar thing exists in humans."

He said the research had important implications.

"It may be important for understanding unexplained infertility, where you have poor fertilisation of eggs and you do not know why.

"It may be a key to understanding chromosomal abnormalities and perhaps even early pregnancy loss.

"We know that chromosomal abnormalities can occur in eggs when they resume meiosis. The timing has to be absolutely right.

"It may be that what is happening is that there is a delay or an acceleration in the resumption of meiosis.

"The other thing is that if you can disrupt the process you could use it for contraceptive purposes without disrupting any of the other functions of the ovary."

He said the next step would be to find the receptor in humans and to look for the signal that activates the receptor.

"It's very exciting. I think there will be a lot of people looking for it."

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