Rewinding the biological clock

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Francis Leroy / Biocosmos / Science Photo Library

Reproductive biologists have been working for decades to solve one of the great mysteries of human fertility: why do women produce all the eggs they will ever have while still in the womb, only to have most of those eggs die off before birth, and many more before the woman reaches puberty?

Jonathan Tilly, chair of the biology department at Northeastern University in Boston, was among those drawn to studying this enduring puzzle. But what he found instead was evidence of a fundamental flaw in the basic tenets that govern our understanding of the female reproductive system. If successful, his research could open the door to the most radical advancement in infertility treatment since in vitro fertilization was invented nearly 40 years ago.

When Tilly examined the ovaries of mice, he noticed that their eggs were dying much faster than the overall egg count would suggest. The mice appeared to be producing new eggs, and were doing so by a process that no one really understood.

Tillys team began researching the role that stem cells play in the process. Chinese researchers later identified a specific type of stem cell located in the ovarian tissue.

Last year, Tillys team published evidence that the same process happening in mice was occurring in humans. He isolated human ovarian stem cells and then implanted them into mice to produce an immature egg called an oocyte.

The implications are huge. If a woman continues to produce eggs into adulthood, then menopause doesnt happen because her body has exhausted its lifetime supply of eggs, but because the process to make new ones has stopped working. It also means that infertility problems in older women arent happening because the eggs she is born with have grown too old, but because the bodys quality-control process has broken down so that it is pumping out poor-quality new eggs.

Theres not a lot you can do about an egg that is 40 years old and has gone bad because of 40 years of accumulated damage, says Tilly. But there is something very much one can do if the egg quality really reflects not eggs going stale, but a production pipeline thats run amiss.

More importantly, if researchers could find a way to somehow kick-start the ovarian stem cells in older women who had stopped producing viable eggs, they could potentially rewind the biological clock.

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Rewinding the biological clock

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