Scientists have taken an important step towards creating artificial sperm that will allow infertile men to father their own genetic children.
Primitive human sperm created from bone-marrow tissue provided by male volunteers offer new evidence that it should be possible to grow working reproductive cells for men who make none of their own.
Although the cells are not mature sperm and would not be capable of fertilizing an egg, genetic tests have indicated that they are as normal as their natural equivalents.
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Karim Nayernia, of the University of Newcastle upon Tyne in northern England, who led the research, has already used artificial mouse sperm grown from stem cells to fertilize eggs and produce pups.
He now thinks that it will be possible to produce fully functioning human sperm from stem cells within five years. He hopes to transplant these into the testes of men made sterile by cancer treatment to try to restore their fertility.
However, this goal could be affected by proposals to ban the use of artificial human gametes, set out in a government White Paper reviewing the fertility laws.
The British Government has said that it intends to outlaw artificial gametes in fertility treatment, and it is not clear whether this would block Professor Nayernia's plans for clinical trials.
His findings, published in the journal Reproduction: Gamete Biology, suggest it could be possible to use stem cells to "rescue" the testes of infertile men and start the production of sperm.
In the longer term, if cloning technology progresses, it might even be possible to use embryonic stem (ES) cells to make sperm using female genetic material, and eggs using male genes, allowing homosexual couples to have their own genetic children.
Most scientists, however, consider this unlikely because embryos generally require genetic material from both a male and a female to develop normally.
It also appears that a Y chromosome, carried only by males, is necessary for sperm formation.
Other experts welcomed the findings while urging caution about their significance.
Harry Moore, of the University of Sheffield, England, said that it was doubtful whether adult stem cells were versatile enough to make working sperm.
"Nearly all the investigations claiming that adult stem cells can change into another cell type have not been substantiated when rigorously tested," he said.