The long-held biological dogma that females are born with all the eggs they will ever have is wrong, according to a study published Thursday in the journal Nature. Instead -- at least in mice -- eggs are renewed throughout life, probably from a store of stem cells in the ovary.
If the finding holds up for humans also, researchers envision a range of possible applications -- including better, fertility-sparing therapies for cancer patients, prolonged fertility and improved health for women after menopause.
"I think that all of reproductive biology would have to be re-evaluated in the light of this," said Frank Bellino, a deputy associate director at the National Institute on Aging.
Every biology student learns that while men can produce fresh sperm into their dotage, women are born with a fixed supply of 1 million to 2 million developing eggs that slowly die off as they age -- leading inexorably to menopause and infertility.
"It's in every textbook that you pick up -- it's probably one of the most basic doctrines of our field," said Jonathan Tilly, director of the Vincent Center for Reproductive Biology at Massachusetts General Hospital and principal author of the Nature study.
The belief dates at least to the early 20th century and was set solidly in stone by 1951, Tilly said.
Tilly's team first suspected the dogma might be false after discovering that eggs in mouse ovaries die at such a rapid clip that their ovaries would be stripped of eggs in a mere two weeks unless the supply was somehow replenished.
Yet mouse ovaries remain well-stocked with eggs for an entire year.
"We just couldn't reconcile the fact that there was so much death going on, yet the ovaries continued to function," Tilly said.
In subsequent experiments, the scientists found clear signs that eggs were being renewed in ovaries in much the same way that sperm is renewed in testes.
Experts in reproductive medicine said the results, if confirmed in humans, might one day be used to extend fertility.
Tilly said that if ovarian stem cells were frozen when a woman was young, years later they could be implanted back into her ovaries and render her fertile after toxic, ovary-damaging chemotherapy treatments.