Mammals may continue to produce new eggs throughout their lives, a study carried out on mice suggests. The findings challenge the long-held belief that female humans, mice and other mammals are born with a finite supply of eggs, which lasts until the menopause. A team of researchers at the Massachusetts General Hospital in Boston, US, say that if their results are confirmed in humans, all current theories about the aging of the female reproductive system will have to be revisited. And the potential existence of 'ovary stem cells' could pave the way for new treatments into treating infertility, say the scientists, who published their results in the journal Nature.
For decades, scientists have believed that all of a woman's immature egg cells are all produced before she is born. Between puberty and the menopause, a few hundred of these are released as mature eggs, while the rest gradually die off. But the new study shows that in mice, there is evidence of ovary 'stem cells', which are capable of producing fresh eggs throughout the animal's reproductive life. At first, the researchers were shocked by their results: 'We had a six-month period of disbelief, when we had trouble digesting the whole thing,' said team leader Jonathon Tilly.
Tilly's team had been studying the process by which ovarian follicles (containing immature egg cells) die off in adult mice. To their surprise, they found although there was a high level of dying follicles in adult female mice, the animals still had a plentiful supply of healthy follicles. 'That's when all the whistles and bells went off,' said Tilly. The only likely explanation was that new eggs were being continuously made to replace the lost ones. To test their theory, the researchers then looked for evidence of germ cells, the stem cells that give rise to oocytes in the developing fetus. They found potential 'ovary stem cells', which appear to share many of the features of male germ cells, on the outer surface of the ovary.
Reproductive biologist Roger Gosden, of the East Virginia Medical School in Norfolk, US, said that the ability to make more eggs would be a revolution in womens' health. 'In theory, it would allow you to have better control over the timing of the menopause, to grow more eggs for one's own fertility treatment, to prevent premature menopause, to recover fertility after chemotherapy, and on and on,' he said. However, he also cautioned that it is not yet clear whether stem cells exist in human ovaries, or how prolific they might be.