A woman is born with all the eggs she will ever have—or maybe not.
Harvard Medical School reproductive biologist Jonathan Tilly made headlines in March when he suggested mammalian ovaries contain stem cells that give rise to new potential eggs, or oocytes, throughout adulthood. He was conducting laboratory experiments aimed at finding ways to lessen the effects of cancer therapy on ovaries when he noticed that healthy female mice seemed to be losing eggs at unsustainable rates. “We counted the dying eggs in every ovary, and it was a nearly a third of the total egg cells present. An oocyte takes roughly three days to die completely and vanish. Without replenishment, the ovaries would become exhausted within two weeks,” he says.
Many of Tilly’s colleagues were skeptical, but in June he announced direct evidence: Not only had he isolated ovary cells that have the distinctive genetic signature of stem cells, but he had also found a gene that seems to prevent the body from creating too many new eggs. When a drug blocking the gene was given to adult female mice, their ovaries started producing more eggs.
Some researchers still want conclusive proof that Tilly’s stem cells give rise to new egg cells. Others question why no one has come across these cells before. “This dogma has been so accepted that nobody has looked for them since the 1950s,” Tilly says. “If I’m right, which I am, this is going to open up a whole new avenue of infertility treatments.”