Growing eggs outside the body has long been an aim of fertility doctors. For starters, it might spare women undergoing IVF the risky and painful process of egg collection, which yields only a few eggs per cycle. It might also enable women to guarantee their future fertility by freezing ovarian tissue when they are young.

But no one has been able to grow eggs in the lab, even from slices of ovarian tissue containing immature egg cells. Keeping the tissue alive for the six months it takes egg cells to mature has proved impossible.

Now a team led by Antonin Bukovsky at the University of Tennessee Graduate School of Medicine in Knoxville claims to have produced "ready to be fertilised" egg cells in just five days. All the team did was to grow surface cells from the ovaries of five women aged between 39 and 52 in a medium containing the hormone oestrogen.

"It is very simple," Bukovsky told New Scientist. "I hope there will be people who try it and confirm it." He thinks the surface cells are progenitor cells that can give rise to limitless numbers of both egg cells and the cells that nourish them and form follicles. Such progenitors are comparable to those in the testes that give rise to sperm throughout life.

But Jonathan Tilly of Massachusetts General Hospital in Boston is far from convinced, even though his team reported last year that progenitor cells exist in mouse ovaries, challenging the textbook idea that mammals have only a limited supply of eggs. "It will raise lots of false hopes in patients, and I have a problem with that," Tilly says.

He thinks that the scraping procedure might have accidentally scooped up immature eggs, which are stored just below the surface layer. But Bukovsky says that if this had happened, the eggs would have been obvious after just a couple of days.

Tilly is also astonished that exposure to oestrogen seems to trigger the transformation. Researchers studying ovarian cancer routinely grow surface cells in the lab, as these are the ones that usually turn cancerous, and they often add oestrogen to the medium. "I'm surprised no one has seen [eggs] formed spontaneously in culture," Tilly says.

There is also the question of why such a major finding was published in Reproductive Biology and Endocrinology (DOI: 10.1186/1477-7827-3-17), a little-known journal edited by Bukovsky. He says the paper went through two rounds of peer review and was handled by the deputy editor.