April 12 (HealthDay News) -- Researchers in China have demonstrated that female ovaries may be capable of producing new eggs in adulthood and subsequently producing offspring.
That runs counter to the long-held belief that female mammals, including humans, are born with a finite number of the eggs or oocytes needed to produce offspring.
"These stem cells are continuous. They were still around through life and actually transformed to make oocytes. Then they were transplanted into infertile females and produced offspring," said Paul Sanberg, a stem cell researcher and distinguished professor of neurosurgery and director of the University of South Florida Center for Aging and Brain Repair in Tampa. "The study is fascinating," said Sanberg, who was not involved in the research.
Could doctors someday use stem cells to help adult women produce brand-new oocytes? One reproductive medicine expert isn't sure.
The new finding is "very, very exciting and opens up a big area of discussion," said Dr. George Attia, associate professor of reproductive endocrinology and infertility at the University of Miami Miller school of Medicine. "If it would ever come to fruition in humans, I really don't know. It's far, far out there," he said.
Another expert was more cautious.
"It's a cute experiment, but I don't think it's going to have anything to do with humans," said Dr. Darwin J. Prockop, director of the Texas A&M Health Science Center College of Medicine Institute for Regenerative Medicine at Scott & White. "There are too many steps, too many things could go wrong."
But the findings, published online April 12 in Nature Cell Biology, could still have interesting implications for future stem cell and other research, Prockop added. "Any new kind of cell is interesting," he said.
For years, scientists had believed that the capability to produce oocytes was lost in most mammalian species at birth.
That line of thought was tested with the recent discovery of actively dividing germ cells (those that give rise to sexual reproduction) in the ovaries of both juvenile and adult mice. The presence of these germ cells could indicate reproductive capability.
Still, researchers disagreed as to whether female germline stem cells (FGSCs) do exist in mammalian ovaries after birth.
So, the Chinese team isolated active female FGSCs from adult and five-day-old mice. They say that they were able to generate new FGSC lines that proliferated even after being cultured multiple times.
These FGSCs restored fertility (by producing new oocytes) when transplanted into the ovaries of female mice that were previously rendered infertile by chemotherapy.
The females then gave birth to normal, young mice.
Even if the breakthrough could apply to humans, it likely would only apply to younger women experiencing infertility, Attia said. "Pregnancy is a heavy load on the human body. A 60-year-old might not be able to be pregnant," he noted.
In other stem cell news, researchers reporting Sunday in the journal Nature Biotechnology said that they were able to use bits of genetic material called microRNA to revert adult mouse cells back into embryonic cells.
These new embryonic cells are, like stem cells, capable of transforming into multiple different types of tissue.
Currently, retroviruses and genes are used to complete this transformation, but this carries the risk of cancer and other problems. Using microRNAs, which regulate gene expression, would be a potentially safer method, said researchers from the University of California, San Francisco.
SOURCES: Darwin J. Prockop, M.D., Ph.D., director, Texas A&M Health Science Center College of Medicine Institute for Regenerative Medicine at Scott & White, and Stearman Chair in Genomic Medicine and professor of molecular and cellular medicine, Texas A&M Health Science Center College of Medicine; George Attia, M.D., associate professor, reproductive endocrinology and infertility, University of Miami Miller School of Medicine; Paul Sanberg, Ph.D., D.Sc., distinguished professor, neurosurgery and director, University of South Florida Center for Aging and Brain Repair, Tampa; April 12, 2009, Nature Cell Biology, Nature Biotechnology