Jonathan Tilly defied decades of dogma by suggesting that women can make new eggs throughout their lives. Now some of his critics are taking a second look.
Jonathan Tilly, a reproductive biologist at Massachusetts General Hospital in Boston, was explaining a procedure to retrieve stem cells from the ovaries of a sterile woman. This experiment, he hopes, will help to quell criticism of his most controversial claim: that ovaries have the potential to make eggs indefinitely. This defies the long-held dogma that female mammals are born with all the oocytes (precursors to eggs) they will ever produce, a population that dwindles with age and is exhausted at menopause.
Tilly first challenged that doctrine in 2004, in a paper suggesting that the oocytes in mouse ovaries are being replenished by stem cells. If properly understood, such cells could be harnessed to generate fresh eggs for women with fertility problems, or even achieve a goal Tilly has been pursuing for 25 years: delaying or halting menopause.
He has since published a parade of headline-grabbing papers, culminating this year in a report that he had isolated the elusive stem cells from human ovaries and coaxed them to develop into bona fide oocytes. But his work has been dogged by doubt. Some researchers question his methods and reasoning. Others have tried, and failed, to repeat his experiments.
But now the stand-off of mistrust, and sometimes open contempt, has taken a strange twist. Two of Tilly’s most vociferous critics have become his collaborators: one serving on the board of advisers at his start-up company, OvaScience in Cambridge, Massachusetts; the other working directly with the stem cells that Tilly had isolated. “These cells are doing things in vitro that can really start to address scientific problems,” says Evelyn Telfer, a reproductive biologist at the University of Edinburgh,UK, who was doubtful of Tilly’s work in the past. “If we are really interested in the science … then this is a great tool.”
The ‘no new eggs’ doctrine has a long history. In 1951, the influential anatomist Solly Zuckerman, at the University of Birmingham, UK, performed an in-depth analysis of evidence available at the time. He concluded that none of it effectively countered a proposal from the 1870s stating that female mammals stop producing oocytes after birth3.
For the first 15 years of his career, Tilly focused mainly on programmed cell death, or apoptosis, and he was struck by the fact that no one had ever quantified the loss of eggs due to ovulation and natural oocyte death over time. So beginning around 1999, Tilly commandeered a microscope and mouse ovarian tissue in order to count the follicles, the cellular compartments in which oocytes develop, in mice at different ages. He found a mathematical imbalance: the number of degenerated follicles was three times higher than expected on the basis of the starting pool. If the mice were losing oocytes at this rate, their eggs should be depleted far sooner than they actually were. Something had to be replacing them, he concluded: stem cells were the likely culprit.
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Few were willing to accept the idea. It took Tilly two years — and numerous rejections and revisions — to get the data published in Nature, in 2004. Controversy ensued over his methods as well as his conclusions. One critique said, for example, that it was “alarming” that Tilly used the rate of follicle disappearance in one mouse strain to calculate loss for another4.
Tilly dropped most of his apoptosis work and steered his entire lab towards proving the existence and functionality of these stem cells. “You are sort of standing on the precipice wondering whether or not you should make the jump,” he says. “Getting the 2004 paper published was for me the jump, because there was no turning back at that point.”
A year later, Tilly reported that he had identified the source of these putative cells: bone marrow5. When he transplanted either marrow or blood from healthy mouse donors into sterile mice, the animals could produce cells that looked like oocytes. But he could not yet fertilize the resulting eggs and create embryos — the true test of an egg stem cell.
At least six groups challenged the bone-marrow finding. In one critique, a group led by Telfer wrote that none of Tilly’s experiments had successfully been replicated, and that the results could be interpreted in other ways. Critics also asserted that Tilly was overreaching, particularly in media interviews.
There are a lot of people struggling to understand how this can possibly work.
In The Boston Globe in 2005, for example, Tilly is quoted as saying: “They’re your own cells; you don’t need anybody’s approval. They go right into your blood supply and go right to your ovaries, where they mature into eggs.” David Albertini, a reproductive biologist at the University of Kansas Medical Center in Kansas City, calls such claims outrageous: “A lot of us reproductive biologists feel that this is a frank travesty that has falsely raised the hopes of many women.”
Tilly defended his comments and challenged his peers to go back to their labs and reproduce his experiments. Several did. In 2006, stem-cell biologist Amy Wagers at Harvard University in Cambridge, Massachusetts, and her collaborators stitched together the circulatory systems of two mice. One, the donor, expressed green fluorescent protein (GFP) in its cells. The other did not. The scientists found that although green, glowing, blood-borne cells could infiltrate the ovaries of the recipient mice, these cells acted like blood cells, not oocytes.
Tilly, in response, performed a similar experiment, showing that mice sterilized by chemotherapy could give birth after a bone-marrow transplant. But the babies did not express GFP, indicating that the eggs from which they were derived came from the recipient, not the donor. Tilly argued that the bone marrow either protected existing oocytes or revived oocyte formation, but critics argued that the chemotherapy probably didn’t kill off all the recipient’s oocytes in the first place.
Shanghai surprise
With little independent replication of his work, Tilly was standing alone through much of the fray. Then, in 2009, Ji Wu at Shanghai Jiao Tong University in China and her colleagues reported that they had isolated from mice what she called “female germline stem cells” — not from bone marrow, but from ovarian tissue. When her team transplanted the cells into chemotherapy-treated female mice, they developed into mature oocytes, then fertilizable eggs and, the clincher, healthy pups.