Ovaries replenish eggs, study suggests

In a finding that could someday revolutionize fertility treatments, researchers yesterday reported evidence that appears to topple a decades-old tenet of reproductive biology: that girls are born with all the eggs they'll ever have, a pool that dwindles and degenerates with age.

Rather, the Massachusetts General Hospital researchers say in the journal Nature, experiments show that -- in mice, at least -- females keep producing eggs through their lives, much as males continue producing sperm.

If the findings are confirmed and hold true for humans, they could eventually lead to previously undreamed-of methods to prolong women's fertility and postpone menopause, said Dr. Jonathan L. Tilly, the paper's senior author.

Dr. Marian Damewood, president of the American Society for Reproductive Medicine, said in a statement that the discovery is potentially "the most significant advance in reproductive medicine since the advent of in-vitro fertilization more than 25 years ago."

Several scientists unconnected to the research called Tilly an outstanding researcher and the findings very exciting, but said they must remain skeptical until more confirmation accumulates.

They emphasized that it will be years before the research could possibly translate into help for would-be mothers.

"This would be a major paradigm shift, so if we're not skeptical, we're not doing our job," said Patricia Hunt, a professor at Case Western Reserve University whose research focuses on reproductive problems that occur with age. "Science is never based on one lab reporting something, especially something as heretical as this."

Tilly's assertions are so radical, she said, that "a lot of us are going to be going, `Whaaat?' "

If it is confirmed that women do indeed produce new eggs, researchers will have to rethink the theory that chromosomal problems such as Down syndrome increase with the mother's age because her eggs have been sitting around and accumulating damage. Instead, they may need to focus on what goes wrong with the stem cells that produce the new eggs.

Tilly also speculates that someday, doctors could collect stem cells from young women, freeze them, and then, many years later, when the woman is ready to have children, defrost the "young" stem cells and let them produce top-quality eggs.

But at this point, major questions remain. One, colleagues say, is whether the findings in mice will translate to humans.

"I've looked at a lot of human ovaries right up to menopause, and I've never seen anything that looked like a stem cell," or heard any report of one, said Roger Gosden, scientific director of the Jones Institute of Reproductive Medicine in Norfolk, Va., and a leading specialist on eggs.

Another question: If Tilly is right, how could so many doctors and researchers have been so wrong for so many years?

Tilly argues that they were overlooking a critical piece of the puzzle: the rate at which egg cells die.

For decades, researchers have examined ovaries of various ages, and focused on the decline in the number of healthy eggs they hold from birth onward.

In humans, baby girls are born with about 1 million eggs, which drops to about 300,000 in adolescence and near zero at menopause.

Scientists concluded, logically enough, that the eggs progressively die off and are not replaced. But for Tilly, who has worked on cell death in ovaries for years, the theory never made sense. Lower species continue making eggs through their lifetimes; why wouldn't mammals? Wouldn't eggs be far better fresh than after 40 years of sitting around?

Almost two years ago, he decided to push for an answer. He set his lab a task: Count how many eggs are dying and how many healthy eggs remain at any given time in mouse ovaries from birth to young adulthood.

The researchers came up with a result so improbable they initially hesitated to tell Tilly: Between 400 and 1,200 eggs were dying at any given time, at a point when mice have a total of only about 4,000 eggs.

That seemed impossibly high. At that rate, the ovaries should have shut down within a couple of weeks; but, in fact, they lasted another 10 months or so.

Perhaps, Tilly speculated, the corpses of dead eggs were lingering in the ovary, inflating the apparent death rate. But two further experiments indicated that the dead eggs were quickly cleared from the ovary.

Tests in two other strains of mice also ruled out the possibility that some quirk of the mouse breed was at work.

Now, Tilly and his team were convinced: Stem cells in the ovary must be producing new eggs. But they worried that they were missing something. After all, they were contradicting every medical textbook around. They had to show that egg stem cells in the ovary were actually replicating to make new eggs.

So they injected the mice with a chemical label that shows up when a cell has been doubling its DNA in order to divide and produce a daughter cell; and, yes, the telltale chemical turned up among apparent egg cells, though in small quantities. That marker also showed up in certain cells that carried a gene called Vasa, which is found only in sperm or eggs.

In other experiments, they found several other signs that new eggs were being produced, including a gene that is turned on only when stem cells produce daughter cells that become eggs or sperm.

At that point, sure that they had proof of new eggs in adult mouse ovaries, Tilly and his team sought outside feedback. Came one response: "The authors dare to assume they can overturn dogma," Tilly recalled.

Clearly, they needed even better evidence. They gathered more. In particular, they took a mouse that had been genetically engineered so that every cell in its body glowed green under fluorescent light, cut its ovary in half, and grafted onto it pieces of a normal mouse's ovary.

In three or four weeks, they found green-glowing eggs that had been surrounded by a layer of nongreen supporting cells to make what is called a follicle. That mixture, Tilly said, indicated that green egg stem cells had migrated into the nongreen pieces of ovary and started to produce new eggs that then attracted the supporting cells they needed to form follicles.

"It was an unbelievable feeling," Tilly said, "because this was it. This was the proof everyone wanted, tracking a marked oocyte" -- or egg -- "and showing it formed a follicle," replete with the support cells. Off went the paper to Nature.

Dr. Frank Bellino of the National Institute of Aging, which helped pay for the research, called the Tilly paper "quite a revolutionary finding," so revolutionary that he hopes another lab will confirm it for greater certainty.

"For the first time in 100 years, the door is open," Tilly said. "We can start to think differently about how to solve these problems." Carey Goldberg can be reached at goldberg@globe.com.