It’s time to rewrite the textbooks. For 60 years, everyone from high-school biology teachers to top fertility specialists has been operating under the assumption that women are born with all the eggs they will ever produce, with no way to replenish that supply. But the discovery of human egg-producing stem cells, harvested from the ovaries of six women aged 22 to 33, puts that dogma in doubt.

The work, published online in Nature Medicine¹ by Jonathan Tilly and colleagues at Massachusetts General Hospital in Boston, parallels the findings of a Shanghai-based group² that isolated similar stem cells from mice in 2009. However, both this and Tilly’s earlier work in mice³ remained controversial, with many experts sceptical that such stem cells existed.

“This is unequivocal proof that not only was the mouse biology correct, but what we proposed eight years ago was also correct — that there was a human population of stem cells in young adult tissue,” says Tilly.

To address the doubts, Tilly’s team began by developing a more sensitive method for identifying and collecting mouse ovarian stem cells. Their method, based on a technique called fluorescence-activated cell sorting (FACS), attaches a fluorescently labelled antibody to a protein, Ddx4, that is present on the outer surface of the stem cells but not on the surface of the later-stage egg cells or oocytes. The FACS instrument lines up cells in single file and sorts them one by one, separating the labelled ones from the rest; it also gets rid of dead or damaged cells, such as oocytes, in which internal Ddx4 might become accessible to the antibody. This method is more selective than previous isolation methods, which did not get rid of such cells.

Once the team confirmed that it had isolated mouse ovarian stem cells by this method, it set its sights on reproductive-age human ovaries. Yasushi Takai, a former research fellow in Tilly’s lab and now a reproductive biologist at Saitama Medical University in Japan, supplied frozen whole ovaries removed from sex-reassignment patients, all young women of reproductive age. “It was 9 November when we did the first human FACS sort and I knew immediately that it had worked,” says Tilly. “I cannot even put into words the excitement — and, to some degree, the relief — I felt.”

The cells they pulled out, called oogonial stem cells (OSCs), spontaneously generated apparently normal immature oocytes when cultured in the lab. To look at the development of the putative human OSCs in a more natural environment, the team labelled the cells with green fluorescent protein to make them traceable, and injected them into fragments ofadulthumanovarian tissue, which were then transplanted under the skin of mice. After one to two weeks of growth, the OSCs had formed green-glowing cells that looked like oocytes and that also expressed two of the genetic hallmarks of this cell type.

“There’s no confirmation that we have baby-making eggs yet, but every other indication is that these cells are the real deal — bona fide oocyte precursor cells,” says Tilly. The next step, to test whether the human OSC-derived oocytes can be fertilized and form an early embryo, will require special considerations — namely, private funding to support the work in the United States (federal funding cannot by law be used for any research that will result in the destruction of a human embryo, whatever the source of the embryo) or a licence from the UK Human Fertilisation and Embryology Authority to do the work with collaborators in the United Kingdom.

“I’ve seen these cells and how they behave. They’re convincing and impressive.”

Evelyn Telfer, a reproductive biologist at the University of Edinburgh, UK, was once sceptical of the mouse work, but has become a believer. “I’ve visited [Tilly’s] lab, seen these cells and how they behave. They’re convincing and impressive,” she says. Telfer, who studies the maturation of human eggs in vitro, will work with Tilly to try to grow the OSC-derived eggs to the point at which they are ready for fertilization.

She notes that there’s still no evidence that the OSCs form new eggs naturally in the body. However, if they could be coaxed in a dish to make eggs that could successfully be used for in vitro fertilization (IVF), it would change the face of assisted reproduction.

“That’s a huge ‘if’,” admits Tilly. But, he continues, it could mean an unlimited supply of eggs for women who have ovarian tissue that still hosts OSCs. This group could include cancer patients who have undergone sterilizing chemotherapy, women who have gone through premature menopause, or even those experiencing normal ageing. Tilly says that follow-up studies have confirmed that OSCs exist in the ovaries of women well into their 40s.

In addition, growing eggs from OSCs in the lab would allow scientists to screen for hormones or drugs that might reinvigorate these cells to keep producing eggs in the body and slow down women’s biological clocks. “Even if you could gain an additional five years of ovarian function, that would cover most women affected by IVF,” notes Tilly.