For a long time scientists have been puzzled by the difference between male and female reproductive biology; why do males make fresh sperm daily, yet females are born with all the eggs they will ever have? The dogma was that female mammals lack renewing egg stem cells. But now an astonishing discovery by Jonathan Tilly and his team at Massachusetts General Hospital, USA, shows that germline stem cells do exist.

Tilly and colleagues studied the number of dying follicles – the tiny sacs in which eggs grow – in female mice, which are fertile for more than a year. Surprisingly, follicles died so rapidly that mice should have become infertile after only weeks. This provided indirect evidence that new egg cells were being produced.

When fragments of ovaries from normal mice were transplanted into transgenic mice that expressed fluorescent protein in all cells, glowing follicles were observed in the transplanted tissue after a few weeks, which confirmed that new eggs can develop from existing stem cells.

“We had a six-month period of disbelief, when we had trouble digesting the whole thing,” claims Tilly. “The shock people may feel on seeing this paper, trust me, we went through it as well.” (The New York Times, 11 March 2004).

The researchers are confident that they will find germline stem cells in human ovaries, which would open the way to treating infertility problems. “Women could essentially grow back their ovaries after [chemo]therapy.” says Tilly. “The possibilities are almost too numerous to mention.” (NewScientist.com, 10 March 2004).