Women may indeed be limited to the number of eggs their ovaries contain at birth, a new study finds, directly contradicting recent research that suggests otherwise.
Scientists have long thought that female mammals, including humans, are born with all the egg cells they will ever have. But a few papers, culminating with a study published earlier this year in Nature Medicine, have suggested that ovaries contain rare stem cells that can replenish egg supplies (SN: 4/7/12, p. 8). These egg-producing stem cells could lead to new treatments for fertility problems, ways to delay menopause and advancements in the basic understanding of human egg cells.
Not so fast, says Kui Liu, a molecular reproductive biologist at the University of Gothenburg in Sweden. Working with mice, Liu and his colleagues used a technique to identify egg cells and their precursors in ovaries. The team found no evidence of the stem cells in ovaries that reproductive biologist Jonathan Tilly of Massachusetts General Hospital and colleagues recently described in Nature Medicine.
To look for egg-making stem cells in mouse ovaries, Liu and his colleagues genetically engineered mice so that every cell glows green with a fluorescent protein, except for eggs, sperm and cells destined to become gametes. Those cells glow yellow, blue or red. The scientists found cells glowing red — purported gamete precursors — in the ovary, but those cells did not divide the way stem cells would and did not produce new eggs, leading the researchers to conclude that stem cells don’t exist in the ovary. The researchers report their work online July 9 in the Proceedings of the National Academy of Sciences.
The new finding in no way disproves the existence of egg-making stem cells in the ovary, says Evelyn Telfer, a reproductive biologist at the University of Edinburgh. For one thing, “these cells are absolutely not the same as the ones Tilly’s got,” she says. The cells Liu found are much larger and are probably already nondividing egg cells.
Liu says his point in publishing the paper is not to say that Tilly is wrong, but to urge scientists to take a hard look at the evidence for and against stem cells in the ovary.
