Seven mouse pups, six of which survived to adulthood, were born in a laboratory in Germany after scientists fertilised eggs with sperm that had been grown from embryonic stem (ES) cells.

The births provide the strongest evidence yet that it will eventually be possible to use ES cells to treat infertile men who make no sperm of their own.

Stem cell grafts could repair malfunctioning testes, or artificial sperm could be grown outside the body for IVF, while therapeutic cloning would ensure that the ES cells used carried the patient’s own genes.

Other experiments have suggested that artificial eggs can be made in the same way, though no offspring have yet been born.

In the longer term, it may even prove possible to produce sperm from female stem cells, and eggs from male ones, allowing homosexual couples to have children that bear the genes of both parents.

This would also enable a single man or woman to provide both the sperm and eggs needed to create an embryo, so that a person could essentially mate with himself or herself.

The creation of “male eggs” and “female sperm”, however, still faces difficult technical barriers, as embryos need genetic material from both a mother and a father to develop normally.

The immediate benefit of the new research will be to deepen understanding of how mature sperm are formed, potentially improving treatments for male infertility.

One promising idea is to remove tissue from the testes, culture immature sperm in the laboratory and then transfer them back to the patient to restore his fertility.

The study, which was published today in the journal Developmental Cell, was led by Professor Karim Nayernia, who has recently joined the Newcastle-Durham Institute for Stem Cell Biology and Regenerative Medicine from Georg-August University in Göttingen, Germany.

His team first created male mouse embryos, which were allowed to develop for a few days until they became small balls of cells known as blastocysts.

The cells were then stained with two marker dyes, one to identify spermatogonial stem cells (SSCs), which will go on to form sperm, and the other that would show up more mature sperm as they developed.

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