BEIJING, June 20 -- Scientists at the University of Sheffield have proved for the first time that human embryonic stem (ES) cells can form the precursors of sperm and eggs in the laboratory, a critical step towards making synthetic versions for infertile as well as homosexual couples.

    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.

    It also raises the prospect of allowing homosexual couples to have children that bear the genes of both parents ¡ª though there are still difficult technical barriers to making viable sperm from female stem cells and eggs from male ones.

    Some researchers believe that the breakthrough means an egg could, in theory, be generated from a man's own stem cells and even that a child could be created from sperm and an engineered egg from the same man.

    Harry Moore, who led the research, said : "One of the reasons for doing this research is that it may allow us to investigate the very earliest processes of how a human gamete and gonad (ovary and testis) develops.''

    Prof Moore added: ¡°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 ¡ª though there are still difficult technical barriers to making viable sperm from female stem cells and eggs from male ones.

    However, Anna Smajdor, researcher in medical ethics at Imperial College London, said: "Because the technique can be used to generate eggs from a man's somatic cells, gay couples could have children genetically related to both. Single men could even produce a child using their own sperm and an engineered egg.

    "Women's fertility would no longer need to be curtailed at the menopause. These possibilities raise new questions about how we define parenthood."

    (Agencies)