Stem Cells Produce Mammalian Ova Throughout Life

Nature Cell Biology, April 12, 2009

Researchers in China have demonstrated in female mice that ovaries produce new eggs throughout adulthood, which can then develop into offspring. The discovery overturns a previous theory which was applied not just to humans but to all mammals in general and which held that female mammals are born with a finite number of oocytes (the female gametocyte that develops into ova, or eggs), a number which cannot increase throughout life. Now, however, there is evidence to indicate that the number of oocytes can, and does, increase throughout the lifetime of the female. Apparently, it would seem as though the previous theory did not take into account the concept of a stem cell.

In a study led by Dr. Ji Wu, a professor at Shanghai Jiao Tong University, the researchers isolated female germline stem cells (FGSCs) from adult mice and also from five-day-old infant mice. Even after being cultured several times, regardless of age, all the FGSCs were still found to proliferate. Most interestingly, the adult FGSCs continued to produce new oocytes and thereby restored fertility when transplanted into the ovaries of female mice who were previously infertile, and who subsequently gave birth to normal mice.

According to Dr. Paul Sandberg, professor of neurosurgery and director of the University of South Florida Center for Aging and Brain Repair in Tampa, "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."

Scientists now hope that applications of this discovery could be extended to the development of a new stem cell therapy for infertility in human females. Some researchers are cautious of such high hopes, however, such as Dr. George Attia, associate professor of reproductive endocrinology and infertility at the University of Miami Miller School of Medicine, who states, "If it would ever come to fruition in humans, I really don't know. It's far, far out there." As Dr. Darwin J. Prockop, director of the Texas A&M Health Science Center College of Medicine Institute for Regenerative Medicine, adds, "There are too many steps, too many things could go wrong."

Nevertheless, the idea that female mammalian babies are born with all the ova that they will ever have in their entire lives - a theory which never made much sense to a lot of people - has now been disproven. As with spermatocytes, the male gametocytes from which spermatozoa develop, there is now evidence to indicate that oocytes in females are also replenished throughout life.