Contrary to the belief that women are born with a finite number of eggs, there may in fact be a way to replenish the supply, a new study suggests.
Researchers have isolated stem cells from adult human ovaries that appear to be capable of producing eggs.
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The new findings follow a number of recent studies that have suggested such stem cells exist in adult mice, and can give rise to healthy offspring in animals that have had their fertility destroyed by chemotherapy. However, these studies have been controversial, because they go against years of research suggesting otherwise, experts say.
In the new study, the researchers devised a more rigorous way to isolate these cells, and for the first time, suggested their existence in people.
If true, the findings could have implications for women's fertility treatments. Currently, women who choose to undergo in vitro fertilization (IVF) for infertility must endure hormone injections so doctors can retrieve eggs for fertilization, said study researcher Jonathan Tilly, director of the Vincent Center for Reproductive Biology at Massachusetts General Hospital. But if researchers could isolate egg-producing stem cells from ovaries, it might be possible to conduct that whole process outside the body, Tilly said.
"That whole program of IVF… becomes a non-necessity," Tilly said.
The study is published online Feb. 26 in the journal Nature Medicine.
Egg Stem Cells
In the new study, Tilly and colleagues isolated egg-producing stem cells from human ovary tissue by targeting a protein found on the surface of only these cells. In dishes, the cells grew into cells that had properties of human eggs. For instance, they had half the genetic material of other cells in the body.
Next, to show the stem cells could produce eggs, the researchers placed a gene into the stem cells that made them glow green, placed the stem cells into human ovarian tissue (taken during a biopsy), and grafted this tissue into mice. One to two weeks later, this tissue contained egg cells glowing green, showing they had formed from the stem cells, the researchers said.
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The researchers don't yet know if these egg cells could be fertilized to produce children. The United States does not allow human eggs to be fertilized for research purposes. The researchers also don't know whether these egg-producing stem cells are active throughout a woman's life, or only when they receive a particular signal, Tilly said, although the researchers have a follow-up study planned to address this question.
The number of egg-producing stem cells appear to be quite minute. In mice, they make up about 0.014 percent of all cells in the ovary, Tilly said.
Still a Controversy
"It's very novel and it's very exciting," said Dr. Sandra Carson, professor of obstetrics and gynecology, at Brown University's Women & Infants Hospital, who was not involved in the study.
"It certainly makes sense that there would be those stem cells still there," said Carson, noting men have stem cells that produce sperm throughout life.
However, other researchers say the new paper does not resolve the controversy of whether egg-producing cells exist in adult ovaries.
"I would like to see better characterization of this very small pool of cells that may be present in the ovary," said Dr. Marco Conti, professor and director of the Center for Reproductive Sciences at the University of California, San Francisco. Conti noted that some properties of the egg-producing cells described in this study do not match descriptions from previous studies.
And the paper still does not address whether these cells have any role in adult humans.
"There is no real functional evidence that this pool of cells indeed contributes to [egg formation] in the adult," Conti said.
But if these cells do in fact work in the way the researchers suspect, it might be possible to grow and mature them in an environment that resembles an ovary, Carson said.
In addition, unlike human eggs, these stem cells can be frozen without damage, Tilly said, so it may be possible to store them for future use.
Tilly is a co-founder of OvaScience, Inc, which has licensed the commercial potential of these findings for development of new fertility-enhancing procedures.
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Stem Cells in Ovaries May Give Women More Eggs