For nearly a century, scientists have firmly believed that whereas men can produce sperm throughout their lives, women are born with all the eggs they will ever have. But new research suggests that this basic tenet of reproductive biology is wrong, a discovery that could have enormous repercussions for fertility treatment.

Egg cells, or oocytes, that are not fertilized are known to undergo a natural process of cell death. While investigating the effects of chemotherapy drugs on fertility in mice, Jonathan L. Tilly of Massachusetts General Hospital and his colleagues began to count the normal rate at which oocytes die. "We certainly didnt set out two years ago to overturn dogma," says Tilly, lead author of the report detailing the findings, published today in Nature. But to their surprise, the scientists found that the number of oocytes dying over a period of several days was far greater than would be sustainable over the long term if the egg supply was not being replenished. In fact, at that rate, mice would be fertile for just two weeks following birth, as opposed to more than a year.

The team subsequently conducted a series of experiments to verify the observation. Careful examination of the lining of adult mouse ovaries revealed cells that closely resemble germline stem cells, those continuously dividing cells that give birth to oocytes. The researchers determined that these ovarian cells express a protein that is associated with meiosis, the process by which sex cells divide. When they transplanted the putative germline stem cells into a strain of transgenic mice whose cells all express a green fluorescent marker, they found that the transplanted cells had divided and produced oocyte follicles in the host tissue. Tilly and his collaborators also analyzed the effects of a chemotherapy agent called Busulfan on the mouse ovaries. The drug, which has been used to study sperm proliferation, destroys the ability of male germline stem cells to divide into new sperm, but does not harm existing sperm. Several weeks after injecting the ovaries with Busulfan, the researchers found that the number of oocytes had decreased dramatically. There was no sign of the cell death that marks oocyte degeneration, however. The drug apparently targeted the female stem cells, preventing their ability to produce oocytes.

Early in the 20th century, some scholars had suggested that eggs could in fact be replenished in adult mammals. But a 1951 study definitively argued that egg numbers are determined at birth, shutting the door to further work for the next half century. "People were viewing ovaries very differently than we do now," Tilly reflects. "The technology was just based on histological analysis." Although biological markers for germline stem cells have since been developed, conclusions about mammalian ovaries were never reexamined because they were believed to be "as sound as telling people that the sun sets in the west."

The team is now working to demonstrate the existence of germline stem cells in human ovaries, and it is confident that the finding will carry over. The next step is to figure out which genes instruct a germ stem cell to differentiate into an egg. Ultimately, Tilly says, transplanting these cells into the ovaries of menopausal women or women whose ovaries have been prematurely damaged by cancer treatment could restore their fertility. --Alla Katsnelson