Scientists have moved a step closer to creating human sperm and eggs from stem cells in research which could end infertility problems. Researchers at Stanford University, which harvested the stem cells from an embryo, claim the breakthrough could eventually cure infertility in men, birth defects and even extend the menopause in women. Previous studies have claimed to have reproduced egg and sperm cells in the laboratory but they were always slightly damaged or malformed. The researchers in the latest study believe their germ cells - the cells that eventually turn into sperm and eggs - are so perfect it would be possible to grow them into fully functioning reproductive cells. The scientists, who published their findings in Nature, claim to have unlocked the genetic "recipe" that leads to these unique cells being formed. They used human embryonic stem cells from excess IVF embryos and treated them with proteins to stimulate the growth of germ cells. It is estimated that one in seven couples in Britain - about 3.5 million people - have difficulty conceiving. In about a third of couples having IVF, male fertility is a contributory factor. "Figuring out the genetic 'recipe' needed to develop human germ cells in the laboratory will give us the tools we need to trace what's going wrong for these people," said Dr Renee Reijo Pera, the senior author. Eventually the researchers want to see if they can create germ cells from adult stem cells which could be harvested from any part of the body including skin. In July, the Daily Telegraph reported that a British team at the Northeast England Stem Cell Institute claimed to have become the first to create sperm in the laboratory though they admitted it was not "perfect". Researchers said that with some minor changes the sperm could theoretically fertilise an egg to create a child. Professor Karim Nayernia, a stem cell biologist who led the team, said the sperm had all the essential qualities for creating life. He said: "They have heads, they have tails and they move. The shape is not quite normal nor the movement, but they contain the proteins for egg activation.'' The team had already used the technique in mice and produced offspring, although all died shortly after they were born. Professor Nayernia added: "We hope that eventually this could help create sperm for infertile men.'' Currently the Human Fertilisation and Embryology Act 2008 bans the use of artificially created sperm and eggs in fertility treatment.