Karim Nayernia and his colleagues at the University of Newcastle recently treated male embryonic stem cells (ESCs) with a range of substances, which converted them first into germline stem cells, and finally into spermatogonial stem cells.

The spermatogonial stem cells thus created then divided to produce "haploid" spermatocytes with just 23 chromosomes, which went on to mature into sperm.

Other scientists are now calling for more evidence.

"Although they find that some of the sperm cells have tails and can swim, this is not evidence of normality," New Scientist magazine quoted Robin Lovell-Badge, who studies sperm formation at the UK National Institute for Medical Research in London, as saying.

The seven mouse pups Nayernia's team produced in 2006, after fusing normal eggs with mouse sperm created in the lab, died within five months because chemical caps called methyl groups had blocked vital genes in the sperm.

He is currently carrying out further tests to determine whether the same thing happens with the human sperm.

The researcher has solved the problem in mice by putting spermatogonial cells into mouse testes before they mature.

"The sperm then have a normal shape and normal methylation patterns," he says.

He also claims to have developed "artificial testes" to do the same job for humans.

A more distant possibility is the creation of sperm from a woman's cells, allowing a lesbian couple to have a child.

Nayernia produced spermatogonial stem cells from female ESCs, but they lacked genes needed to mature.

An article shedding light on this field has been published in the journal Stem Cells and Development.