Scientists at the University of Sheffield have proved for the first time that human ES cells can form the precursors of sperm and eggs in the laboratory, a critical step towards making synthetic versions for infertile men and women.

The findings suggest that it will be feasible either to use stem cell grafts to “rescue” testes and ovaries damaged by cancer treatment, or even to grow fully fledged reproductive cells for use in fertility treatment. Therapeutic cloning would ensure that the stem cells carry the patient’s genes.

Harry Moore, who led the research, said: “This is probably ten years away from the clinic. We have a lot more work to do, and we have to prove it is safe.”

The technology raises fresh ethical questions to add to those posed by the use of ES cells and therapeutic cloning. It also raises the prospect of allowing homosexual couples to have children that bear the genes of both parents — though there are still difficult technical barriers to making viable sperm from female stem cells and eggs from male ones.

If these are addressed, another potential application is to enable a single man or woman to provide both the sperm and eggs needed to create an embryo — essentially allowing a person to mate with himself — or herself. Ethicists said that while this is never likely to be popular or common, its implications need to be considered.

Anna Smajdor, a medical ethicist at Imperial College London, said: “In the future this technology would offer an obvious solution to infertile couples, obviating the need for donor gametes [sperm and eggs], But it opens new and challenging possibilities. Single men could even produce a child using their own sperm, opening the way to a new form of cloning. Women’s fertility would no longer need to be curtailed at the menopause.”

In the Sheffield study, details of which will be presented today at the European Society of Human Reproduction and Embryology conference in Copenhagen, in Denmark, Behrouz Aflatoonian and Professor Moore investigated whether human ES cells could form primordial germ cells (PGCs) — the precursors of sperm and eggs. Previous research in the United States and Japan has established that this is possible in mice, and one team has even produced mouse embryos using artificial sperm generated in this way.

Human ES cells were clumped together into balls known as embryoid bodies, which were then tested to determine how genes were switched on. Some of the cells showed a pattern of gene activity normally seen only in PGCs, and some of these also contained proteins found only in maturing sperm.

The scientists have not yet learnt how to direct this development or to create fully functional gametes. They think the most promising method is to transplant PGCs directly into a man’s testis or woman’s ovary, where the environmental and hormonal conditions are right to turn them into sperm or eggs.

If it becomes possible to grow mature eggs from stem cells, it would also solve one of the biggest barriers to widespread use of therapeutic cloning in medicine — the availability of the donated eggs needed for the procedure.

Allan Pacey, secretary of the British Fertility Society, said: “This is a really exciting step forward. We still don’t really understand why some men and women can’t produce sperm and eggs of their own, and, sadly for them, that leads to infertility. But if we could better understand the basic biology then we might be in a better position to help them one day.”

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