Image caption A breakthrough in lab creation of mature human eggs would raise many ethical questions

US researchers say they will redouble their efforts to create human sperm from stem cells following the success of a Japanese study involving mice.

A Kyoto University team used mice stem cells to create eggs, which were fertilised to produce baby mice.

Dr Renee Pera, of Stanford University in California, aims to create human sperm to use for reproduction within two years, and eggs within five years.

Infertility affects up to 15% of reproductive-aged couples worldwide.

"I know people think it's Frankenstein medicine, but I think it's not an imagined or lessened health problem - infertility affects your whole life," Dr Pera says.

"To have sex and have a baby would be a super simple decision, but not everybody can do it."

But using embryonic stem cells for research - as Dr Pera's lab at the Institute for Stem Cell Biology and Regenerative Medicine does - is controversial because the embryos are destroyed in order to use them.

Dr Pera's lab uses embryos left over from IVF treatments.

Laboratory generation?

Stem cells have the potential to grow into any cell in the body. Creating eggs in a lab could become mainstream, much like IVF is viewed today.

One mistake in a genome can lead to devastating disease in a child Dr Renee Pera, Stanford University

Dr Pera says there are about one million or 1.5 million embryos made each year in America using IVF - and about 500,000 of those embryos are discarded. About 500 of those embryos are used for research, she said.

"And people worry about those 500 instead of the 500,000 discarded," Dr Pera says.

The Japanese study marks the first time a mammal has been created from stem cells. It is being hailed as the Holy Grail of reproductive stem cell research.

The researchers at Kyoto University say they have demonstrated how to grow eggs and sperm in a lab and combine them to produce seemingly healthy offspring.

"We are reinvigorated again. It seems that something every two years comes out that gets everyone reinvigorated," Dr Pera said of the Japanese study.

"We've been mostly working on the human system to do the same things - to make mature eggs and mature sperm in a dish."

Pregnancy age limits

By creating sperm and eggs from embryonic stem cells, scientists hope to better understand the reproductive process and embryos.

Clinically, this could eventually give new options to infertile couples who want to have biological children.

Dr Pera's lab has successfully made "primitive" sperm and eggs in the past, but have not yet mastered creating cells good enough to actually use in human reproduction.

"The cells have some errors. When you think about stem cell biology and regenerative medicine, most applications are about making large batches or large sets of cells for, say, cardiac repair," she says.

"Here, we are trying to make one cell that's perfect.

"One mistake in a genome can lead to devastating disease in a child. And so I would guess if we were not so careful we could inject the cells we make into an egg and see what happens.

"But you can do that in mice. You can't do that in women."

If successful, the technology could significantly wind back the time on a woman's biological clock.

That is great news for many women who have put off having babies to pursue careers and for women who cannot get pregnant due to cancer treatments.

But it raises a slew of new ethical issues: who would decide when a woman is too old to become pregnant?

"That is one of the major worries of technologies like this - what age should women be allowed to reproduce," Dr Pera says.

"In most countries there is an age limit - mainly based on health of being able to carry a pregnancy. It can be dangerous to the heart. It's hard to carry a baby."