Theory that fertility can be regained is dealt blow

Just a year ago, Harvard Medical School scientist Jonathan L. Tilly set off a worldwide sensation. Defying the biological dogma that women are born with a limited lifetime supply of eggs, he reported that he could inject infertile mice with blood cells, and that some of the cells became new eggs. He had discovered, he said, evidence that simple injections might someday help legions of women regain their fertility.

But now a team of scientists, led by another Harvard researcher, has followed up Tilly's work and shown that the excitement was unfounded. In their experiments, which were published online by the journal Nature to make the results available faster, the eggs Tilly identified were not able to develop into the fully mature eggs females need to procreate. Scientists said the new report dealt a crushing blow to Tilly's idea -- and the hopes that it had raised.

``I think this argues very strongly against the Tilly hypothesis," said John J. Eppig , a Jackson Laboratory senior staff scientist who was not involved in the research.

The team is not accusing Tilly of misconduct -- his original data may all be correct -- but says that he overstated the significance of what he found. Biology has been roiled by the idea that the body can regenerate itself in entirely unexpected ways, including a wave of recent findings about adult stem cells. In some cases, these claims have survived the test of time: For example, one of the most important ideas in modern neuroscience is ``plasticity," the recognition that the brain's neurons are far more flexible than once thought. But other claims, perhaps fueled by the overheated atmosphere, have failed on closer inspection.

At the heart of this dispute between two respected Harvard scientists is a crucial, but profoundly inexact, part of the scientific process: interpretation. Many scientists have accused Tilly of making leaps of interpretation, such as claiming he had found eggs before he knew what the cells were capable of, and ignoring other explanations for the surprising observations he has made.

Reacting to the new paper, Tilly insisted that the new work does not contradict his own findings, and that he will continue his quest to prove that ovaries have a mysterious ability to generate new eggs after birth.

``I am not planning on giving up," said Tilly, an associate professor at Harvard Medical School and the director of the Vincent Center for Reproductive Biology at Massachusetts General Hospital.

Tilly's challenge to the traditional view of fertility came in the form of two papers. The first, published in 2004 in Nature, reported that eggs in mice die much too quickly to last the mouse's reproductive lifetime, implying that the body must be creating new eggs after birth. The paper also provisionally identified a cell in the ovary that might be generating the eggs.

This paper was controversial, but his second paper, published last year in the journal Cell, was even more so. In the Cell paper, he reported more evidence of new eggs, and said that the cells responsible circulate in the blood, coming to the ovary when it needs more eggs. The paper reported that these ``germ cells" could create eggs in a day, much faster than eggs are created during development, before birth.

Scientists were skeptical of this finding -- with some charging that Cell should not have accepted the paper -- because Tilly did not show that the newly generated eggs he was seeing could be fertilized and mature into a new animal. Without this, it was impossible to know the importance of the new cells, or the experiment, said Evelyn Telfer, a senior lecturer at the University of Edinburgh who is co author of a letter published in Cell that was critical of Tilly's experiment.

Researchers have been eagerly anticipating an independent test of Tilly's findings, because it could be so beneficial to patients, said Catherine Racowsky , director of the in vitro fertilization laboratory at Brigham and Women's Hospital. Increasingly, women have been putting off childbearing, leading to more fertility problems. There are also many women who have been left infertile by chemotherapy or by other conditions, she said.

To test Tilly's idea, Amy J. Wagers, a scientist at the Joslin Diabetes Center , used a powerful tool in biology known as ``parabiotic mice." One mouse is genetically engineered so that all of its cells glow green, and then its blood vessels are joined with another mouse's, so that the two share a circulatory system. This allows biologists to see whether cells from the blood of the ``green" mouse become part of organs in the unmarked mouse. The team, which included Harvard stem cell scientist Kevin Eggan, then induced both mice to ovulate many eggs. If cells circulating in the blood were forming eggs, then one would expect the unmarked mouse to release some eggs that were marked green, or vice versa.

However, in trial after trial, the eggs of the green mouse were always green, and the eggs of the unmarked mouse were always unmarked. In total, the team examined 665 eggs, and every one matched the mouse it came from, according to the Nature paper.

Tilly criticized the work because the Joslin team looked only at the eggs that the ovary had released, rather than looking at the ovaries themselves, for signs of immature eggs. He said that it is possible that cells from the blood contribute to immature eggs and that these produce hormones that support the function of the ovary, prolonging fertility. If this were demonstrated, it would still be an important advance, but it would fall short of the potential Tilly ascribed to the work when it was announced.

Wagers said that in her experiment, however, the injections of blood cells did not help the mice create any more eggs. Both Wagers and Eggan are members of the Harvard Stem Cell Institute .

The Jackson Laboratory's Eppig said that he is in the middle of an experiment that, like the Joslin work, is showing that the cells circulating in the blood do not become mature eggs. He cautioned that the experiments are not finished, and have not been peer reviewed.

The Joslin experiment does not address the possibility that there are cells in the ovaries that generate eggs -- the topic of Tilly's first paper. However, Telfer, the Edinburgh scientist, said that Tilly's original paper could be misleading if the criteria he used to count dying eggs were incorrect.

The scientists involved stressed that the disagreement is a professional one, and not a personal one. Wagers and Tilly discussed her findings at a meeting in his office that both described as friendly. Eppig even praised Tilly for challenging such a long-held idea.

``It is courageous," Eppig said. ``But my feeling is that when you do that, you need to make sure that all of your ducks are in a row."