The mature egg is the rarest cell in the body. Credit: © SPL

It normally takes an adult female mouse to produce a mouse egg ready to be fertilized. Now Japanese researchers have taken immature eggs from a mouse fetus, completed their development in the lab, and bred new mice from them1.

The techniques could one day save the fertility of women and girls undergoing chemotherapy or radiotherapy. Removing an ovary before treatment, researchers could then produce functioning eggs without transplanting the ovary back into the patient.

Maturing eggs in the lab can provide more eggs than can an animal. This might help breeding programmes for endangered species and cloning efforts for reproduction or to produce genetically modified animals.

"The mature egg is the rarest cell in the body," comments reproductive biologist Roger Gosden of Eastern Virignia Medical School, Norfolk. Large mammals' cells will need much more tending. Mouse eggs take less than a month to mature; human eggs could need six months.

"It will take five years," before the technique can be applied to humans, says its developer Izuho Hatada, of Gunma University, Maebashi City, Japan.

Mammals make millions of eggs before birth, but only a few thousand make it to maturity. Those that do go through complex changes: they grow and switch many genes on or off in preparation for future development.

"It's been a big question whether conditions in culture would allow these changes to occur," comments cell biologist John Eppig of the Jackson Laboratory, Bar Harbor, Maine. "This is a nice technical achievement."

But the mice may have trouble in store, Eppig warns. His team produced one mouse - called Egbert - by fertilizing an egg from a newborn mouse, although they were unable to get this technique to work consistently. Egbert seemed normal, but became obese and developed other health problems later in life.

The Japanese mice are now a year old - about middle-aged, in mouse terms - and still healthy.

Nuclear family

Hatada and his colleagues removed the ovaries of a 12.5-day-old mouse fetus. They kept them in culture, while the immature eggs within continued to develop.

You can look at the whole spectrum of egg development Roger Gosden , Eastern Virignia Medical School

After 28 days the DNA in the nuclei of the immature eggs was ready for fertilization, but the rest of the cell was not. So the researchers transferred each nucleus into an adult mouse egg from which the nucleus had been removed.

They fertilized these new eggs in vitro and put them into surrogate mothers, who produced healthy, fertile offspring.

Lab-grown eggs will give us a better view of development, Gosden adds. "You can look at the whole spectrum of egg development to see what might perturb fertility or cause birth defects."