Eggs may be made throughout adulthood

Discovery of stem cells in human ovaries overturns dogma

A newly discovered type of stem cell in the ovary could mean big things for women’s health, possibly leading to new fertility treatments and maybe even a way to delay menopause.

Since the 1950s it has been thought that women are born with all of the egg cells they will ever have. But with the discovery of egg-producing stem cells in mice and humans, it now appears that the ovary can replenish its egg supply. Researchers led by Jonathan Tilly, a reproductive biologist at Massachusetts General Hospital in Boston, report the finding online February 26 in Nature Medicine.

Other researchers hail the discovery as a genuine breakthrough with huge implications. “This is like discovering a new planet in our solar system that has a bacterium on it,” says Kutluk Oktay, a reproductive biologist at the New York Medical College in Valhalla. At the very least, he says, the cells offer hope for extending a woman’s reproductive life span.

Tilly didn’t set out to overturn the accepted dogma that women don’t make new eggs. As part of their research into the onset of menopause, he and his colleagues developed ways to track the death of egg cells over time. When the researchers counted the number of healthy egg cells in mouse ovaries, they saw a steady decline with age as expected. But the team also found that dying cells greatly outnumber the starting population of eggs. “What we had was a math problem,” Tilly says. “We refocused all of our efforts on this glaring mathematical dilemma.”

In 2004, Tilly’s group reported the answer to their math problem: There are more dying eggs than healthy ones because stem cells in mouse ovaries are constantly making more eggs, which then die off. The discovery didn’t go over well. “The vast majority of our colleagues were not very receptive,” Tilly says. Many of those who did accept the existence of egg-forming stem cells in mice didn’t think humans would have similar cells.

Tilly and his colleagues isolated stem cells from ovaries that had been removed from six women during sex reassignment surgeries at Saitama Medical Center in Japan. Only about 1.5 percent of cells in the ovaries fit the stem cell profile. The researchers compiled molecular profiles of the cells and demonstrated that the stem cells are able to make precursors to eggs when transplanted into other ovaries. 

Tilly’s group convincingly demonstrates that stem cells in human ovaries can make egg cell precursors. But it remains to be seen if the cells can make mature gametes, says Evelyn Telfer, a reproductive biologist at the University of Edinburgh.

Stopping the depletion of eggs or keeping ovaries functioning could help stave off many of the health problems women experience after menopause, Tilly says. “If we can somehow control this biological clock, to me, the possibilities are endless.”

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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