STEM cells placed in the ovaries of sterilised adult mice have apparently given rise to fresh eggs and healthy offspring, reigniting the debate over whether women are born with all the eggs they will ever have.
If the finding can be replicated in women, it would revolutionise reproduction, opening the door for later child-bearing and providing a source of eggs for infertile women. But despite early signs that similar egg-producing cells may exist in women, there are doubts over whether the mouse and human cells are what they seem.
To isolate ovarian stem cells in mice 鈥� dubbed female germline stem cells (FGSCs) 鈥� Ji Wu and his colleagues at in China looked in the ovaries of adult and 5-day-old mice for cells producing a protein called MVH, which should be expressed both by egg cells and egg-producing stem cells. They then extracted those MVH cells that were also rapidly dividing, a key characteristic of stem cells, and grew them in culture.
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These cells were labelled with the gene for a fluorescent protein before being injected into the ovaries of mice that had been sterilised by chemotherapy. The team found that this led to the ovaries being repopulated with eggs. The mice subsequently gave birth to healthy pups, some of which carried the fluorescent protein, showing they had come from the injected cells (Nature Cell Biology, ).
鈥淏y producing live young, these cells have passed the ultimate test to prove their germline credentials,鈥� says of the University of Edinburgh, UK. 鈥淚t鈥檚 very, very exciting.鈥�
鈥淏y producing live young, these cells have passed the ultimate test to prove their germline credentials鈥�
If Wu has identified FGSCs in adult mice, do other mammals also carry similar cells in their ovaries? There is early evidence that women might.
Irma Virant-Klun at the in Slovenia and her team took cells that seem to express genes associated with embryonic stem cells from the surface of the ovaries in post-menopausal women (Differentiation, ).
While growing these cells in culture led to the production of egg-like cells, it is not yet clear whether they are FGSCs, nor whether the 鈥渆ggs鈥� they produce are capable of yielding offspring. 鈥淎lthough they may look like eggs, that doesn鈥檛 necessarily mean that鈥檚 what they are,鈥� warns of Weill Cornell Medical College in New York.
If they are shown to be FGSCs, it would open the door to new fertility treatments, including growing eggs to implant into infertile women, or stimulating the generation of new eggs inside adult ovaries. It could also lead to a new way to create embryos for embryonic stem cell research.
Other research groups must first reproduce both sets of results. 鈥淪tem cell biology has been mired in the problem of replication,鈥� says Gosden. 鈥淎 very high standard of proof is needed.鈥�
Some scientists have already pointed to several puzzling aspects of Wu鈥檚 work. For example, if the dividing cells Wu extracted from the mice really are FGSCs, why are mammalian egg supplies not kept topped up indefinitely? Also, Wu鈥檚 team used antibody-coated beads to extract cells that express MVH, a technique that relies on a protein being expressed on the cell鈥檚 surface, but MVH is usually expressed inside cells, prompting some to question whether the cells extracted by Wu really are FGSCs.
鈥淚f there are cells that can give rise to eggs it would be fantastic, but it鈥檚 an extraordinary claim and the evidence needs to be cast iron,鈥� says of the National Institute for Medical Research in London.