New work in stem cell research has challenged the long-standing belief that women are born with all the eggs they will ever need. The results were published in the journal Nature Stem Cell, although the study was received with caution.
The scientists at Shanghai Jiao Tong University claim to have detected germline stem cells in the ovaries of old and young mice that produce eggs, or oocytes, and can be fertilised to produce healthy offspring. This goes against the dogma that female mammals are born with all their eggs, and that immature eggs ripen and are ovulated or die off until the supply runs out. Sperm are continuously produced in males from stem cells, but in females the number of eggs is fixed at birth.
Kang Zou and Ji Wu lead the research that isolated female germline stem cells (FGSCs) from the ovaries of adult mice and five-day old pups. They grew these cells into a colony in a culture dish and genetically modified them to produce a green fluorescent protein, a standard technique for labelling cells. They then replaced them into the ovaries of sterilised mice.
The mice produced new eggs which, when fertilised, gave rise to healthy, fertile babies, many of which contained the green fluorescent protein, indicating that they came from eggs from the FGSCs. The scientists say that 'these findings contribute to basic research into oogenesis and stem cell self-renewal, and open up new possibilities for use of FGSCs in biotechnology and medicine'.
If these findings can be replicated, they may hold major implications for fertility treatment for women made infertile by, for example, cancer treatments, or for women who wish to have children later in life. The work is controversial, however, since many experts doubt that the Chinese team have proved that the offspring actually came from the ovarian stem cells. Jonathan Tilly, of the Massachusetts General Hospital in Boston, made a similar claim about five years ago but others were unable to replicate his work.
Professor Robin Lovell-Badge from the National Institute of Medical Research, London, says that 'extraordinary claims require extraordinary evidence', implying that the work was incomplete. He continues, however, that 'if true, and especially if applicable to humans, then this is very important. For example, it could provide a means to restore fertility to women who have few eggs or who have had to undergo cancer treatments, by isolating these cells, expanding their number in culture and keeping them frozen until needed for IVF'.