Chinese scientists have published a study in Nature Cell Biology that suggests even older females retain adult stem cells that can stimulate fertility, including the production of more eggs. The evidence is in contrast to the usual dogma that women have a finite number of immature eggs, some of which mature and are ovulated, and that the number of eggs becomes depleted with age. Tilly et al. recently reviewed the evidence for and against production of new eggs after birth. Tilly's group was one of those recently to challenge the dogma with evidence that new egg production could occur in mammals.

What the Chinese group showed is that ovaries contain stem cells that can produce more eggs, similar to the stem cells in testes that produce sperm. The group isolated these female germline stem cells (FGSCs) from newborn and adult mice and grew them in culture. When female mice that had been sterilized were injected with the stem cells, fertility was restored including birth of healthy offspring. The injected FGSCs had been labeled with a fluorescent gene so that they could be tracked and some, though not all, of the offspring contained the fluorescent gene.

The new evidence doesn't prove that these stem cells actually function in the normal ovary. They may age and that's why the ovary runs out of eggs and menopause occurs. They may stimulate egg formation by secreting factors that affect other cells to maintain eggs rather than forming eggs themselves. More studies will be needed to see what is their normal function. But if similar cells could be found in humans, it might lead to treatments that extend or rejuvenate fertility. It could also pose a concern if those who want to do cloning try to use the cells to create large numbers of eggs for experiments.