A new stem-cell study on mice by a team of Chinese scientists may have paved the way to a cure for infertility in women. The research challenges the long-held belief that most female mammals, including humans, are born with a fixed number of eggs and are unable increase their number throughout their lives.

The study by scientists at Shanghai Jiao Tong University, led by Ji Wu, pointed at the possibility that it is possible to prolong the working life of ovaries by transplanting female stem cells that develop into mature eggs. Thus, stem-cell injections may one day make it possible for women to someday be able to delay menopause, or even create new ova after this inevitable event.

The findings raise the prospect of treating some forms of female infertility where the ovaries do not produce eggs. The hope is that one day stem cell transplants could replenish the supply of fresh eggs in infertile women.

As long as four years ago, US scientists showed it was possible to obtain stem cells from the ovaries of adult women and grow them into mature egg cells. Scientists have been following this train of research ever since.

Now the Chinese scientists have shown that it is possible to isolate stem cells from both immature and mature ovaries of mice, store the cells in the laboratory, and then transplant them back into sterile females to enable them to give birth to healthy offspring.

The scientists isolated female germ-line stem cells of newborn mice and adult females. They cultured them for up to 15 months and six months respectively before transplanting them into the ovaries of sterile mice, which gave birth to healthy offspring.

The study, which appears in the journal Nature Cell Biology, was received with caution, with many experts emphasising that the findings are tentative and need to be confirmed by other researchers.

Professor Azim Surani, of the Gurdon Institute at Cambridge University, said the results of the study have important implications for women who do not produce mature eggs. "Sperm are produced continuously in men but the number of eggs in women is fixed at birth," he said. "This study ... suggests there are also stem cells present in ovaries that can be cultured in a dish, which can develop into viable eggs."

But he added the study in Nature Cell Biology has failed to answer important questions. "Extraordinary claims require extraordinary evidence ... to me this is a very incomplete piece of work..." he said.

"This [study] will stimulate lots of activity in the scientific community. But what would be unfortunate is if this is hyped as a cure for female infertility … a lot more study is needed," he added.

Of mice and women
Germline cells are the only cells in the body that grow into sperm or eggs. While sperm-making stem cells have been found in male testes, no one has ever found incontrovertible evidence for similar cells that make eggs in ovaries.

To test whether the extracted cells could make eggs, the researchers sterilised a group of female mice using busulphan, a chemotherapy drug. They then injected 10,000 stem cells into both ovaries of each mouse. The cells had been frozen and thawed out before being used, to simulate the conditions they would be kept in if stored for human patients.

A few months after leaving the mice to mate naturally, 18 out of 22 that received stem cell transplants from newborn mice gave birth to pups. A further 12 out of 15 females injected with stem cells from adult mice also gave birth. All of the offspring appeared to be healthy.