The possibility of men - and women - growing their own sperm in the laboratory has moved closer, with scientists creating human sperm from embryonic stem cells for the first time.
The controversial research is aimed at understanding and overcoming male infertility, but may eventually allow two women to have their own biological daughter.
British researchers used embryonic stem cells extracted from IVF embryos and converted them into sperm cells by growing them in a culture containing vitamin A.
The team, led by Karim Nayernia of Newcastle University, said their advance would allow researchers to study how sperm develop.
"It will also allow scientists to study how cells involved in reproduction are affected by toxins, for example, why young boys with leukaemia who undergo chemotherapy can become infertile for life," Professor Nayernia said.
In future, skin cells from patients could also be used to create sperm to find out the cause of their infertility.
Producing a baby with the new sperm is illegal in Britain and at least five more years of research was needed to see whether the technique would be safe or suitable for fertility treatment.
While the team had succeeded only in producing sperm from male embryonic stem cells, it may eventually be possible to produce them from female cells as well, he said.
Some scientists, however, questioned whether fully formed sperm had been created by the researchers, whose study is published in the journal Stem Cells and Development.
"As a sperm biologist of 20 years' experience, I am unconvinced from the data presented in this paper that the cells can be accurately called spermatozoa," Dr Allan Pacey, of the University of Sheffield, told The Guardian.
Stephan Millett, the director of Curtin University's Centre for Applied Ethics and and Philosophy, said the research also raised many scientific, ethical and legal issues, and required considered debate.
Creating sperm for study could benefit infertile couples, but also released the "genie from the bottle" of producing children in new ways.
"I think it is highly dangerous territory," Professor Millett said.
"We've seen enough science fiction become science fact to ask the question, should we do this?"
Loane Skene, of the University of Melbourne, said creating sperm to help infertile couples have children was a laudable aim.
IVF had once been seen as unnatural, yet was now routine, Professor Skene said.
"I have no difficulty with the [sperm] research continuing."
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