Eggs from stem cells

Eggs capable of being fertilised and giving rise to healthy baby mice have been produced in the culture dish using stem cells for the first time, Japanese scientists showed this week.

Eggs, or 'oocytes', found in the ovary are formed early on in an individual's embryonic development from a population of stem cells. These stem cells migrate into the forming ovary and, under the influence of signals from the surrounding ovarian tissue, turn into egg cells. But the nature of these signals, and their origins, have remained shrouded in mystery.

Unlocking these mysteries would hold the key to turning any cell into an egg cell capable of being fertilised and giving rise to a new individual. This means that it could be used to combat fertility problems, rescue threatened or extinct species and even confer on individuals - such as men - the ability to have children of their own where previously they may have been unable to do so.

Scientists had shown already that stem cells implanted into the ovary of a mouse could be persuaded to develop into egg cells, but no one had succeeded in making the process happen exclusively in a dish.

Now, Kyushu University's Katsuhiko Hayashi has done just that. Writing in Nature, he and his colleagues present a culture system that they have developed which can turn a skin cell from an adult mouse into a viable egg cell. Fertilised in the dish, these egg cells can produce viable mouse pups that grow up normally.

To achieve what many scientists are dubbing 'a massive breakthrough', Hayashi and his colleagues first turn their adult skin cell into what is known as an "induced pluripotent stem cell" or iPS cell.

This is achieved by reprogramming the cell to cause it to 'unspecialise'. iPS cells like this were them added to cells cultured from the ovary of a mouse. There they formed structures similar to those seen in a real developing ovary.

Under the influence of the surrounding ovarian tissue, the iPS cells became viable oocytes that can be fertilised to produce offspring.

It should be emphasised that the technique is not efficient. Just 11 of 316 embryos produced this way actually took, making the success rate of a term pregnancy just 3.5%.

To explore this further, the researchers studied the pattern of gene expression taking place as the eggs grew and compared it with normal developing ovarian tissue. This highlighted some differences, showing that that the profile of genes being turned on and off during the process is not yet quite normal. Understanding why is the next challenge.