THE textbooks say we mammals are born with all the eggs we will ever have. But researchers in the US say they have isolated mouse stem cells that form new eggs throughout life. They have even managed to boost the numbers of eggs in the ovaries of mice using an experimental drug.
If the same is true of humans, and women make eggs constantly just as men make sperm, the consequences could be profound. The discovery might lead to new ways of protecting the fertility of cancer patients and extending the reproductive lives of women. It could even make it possible to delay the menopause.
Jonathan Tilly of Massachusetts General Hospital in Boston published results earlier this year suggesting mouse ovaries contained stem cells that could give rise to new egg follicles. Now his team has isolated these cells and shown they express genes characteristic of stem cells.
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They have also identified a key gene that seems to control the cells鈥� activity, Tilly told a fertility conference in Berlin, Germany, last week. When the gene, which Tilly will not name prior to publication, is 鈥渒nocked out鈥� in mice, the adult ovaries contain 40 per cent more follicles. And if normal female mice are given a drug dubbed GSA-8 that blocks the gene, their ovaries, too, develop more follicles.
鈥淚 think it is a great piece of work,鈥� says Alan Trounson, a fertility expert from Monash University in Melbourne, Australia. But to convince the many sceptics, Trounson says Tilly鈥檚 team must prove directly that these stem cells give rise to follicles.
The big question, of course, is whether women have stem cells like these. Female fruit flies, fish, birds and, it seems, mice all renew their egg supply throughout most of their lives, says Tilly, so why should primates be different? 鈥淚t just doesn鈥檛 make sense.鈥� He points to a 1956 study that suggests rhesus macaques do produce eggs throughout life.