By Michelle Roberts
BBC News health reporter in Copenhagen
British scientists say they have taken a step towards showing human eggs and sperm can be created from stem cells.
Lab-grown sperm may be a possibility in the future
The finding may help IVF treatment, hit by a shortage of egg and sperm donors.
Sheffield researchers told a European fertility conference they had shown embryonic stem cells could develop into the earliest stages of eggs and sperm.
In principle, this means it may be possible to clone stem cells from an infertile patient and turn these into the required sperm or eggs for IVF.
In their study of the human embryos, the Sheffield University team were following the same principle that had previously been observed in studies of stem cells taken from mice.
They found that some of the human cells developed the genetic signature of primordial germ cells, the ancestors of eggs and sperm.
The work, presented at the European Society of Human Reproduction and Embryology conference in Copenhagen, may one day compensate for a shortage of egg and sperm donors for IVF treatment.
This lack has led to the growing trend of fertility tourism - couples seeking help abroad.
The Sheffield researchers say many more tests are needed to check the technique is safe.
Typically, sperm banks have relied on donations from younger men in their 20s, targeted through football programmes, magazines and student unions.
Egg donation is more intrusive and riskier, making it more difficult to recruit donors.
But numbers of both donors have been dwindling.
Some say that laws lifting anonymity on donations from 1 April this year might put off even more potential donors.
The team at the Centre for Stem Cell Biology in Sheffield used stem cells taken from embryos donated for research by couples undergoing IVF.
They found some formed into a collection of cells called embryoid bodies.
When they looked at these cells in detail they found that within two weeks a small number of cells expressing some of the genes found in human primordial germ cells were present.
Some cells also expressed proteins only found in maturing sperm.
Mr Behrouz Alfatoonian, one of the team which carried out the study, said: "This suggests that human stem cells may have the ability to develop into primordial germ cells and early gametes as has been shown previously for mouse embryonic stem cells."
He said the challenge now was to choose the cells that were going to develop into primordial germ cells and then work out how to encourage them to grow into mature sperm and egg.
"Producing functional gametes is much more difficult because we have to recreate for the cultured cells the environment of the developing follicle for the egg or the tissue of the testis for the sperm," he said.
Professor Harry Moore, who runs the centre, said the research meant it might ultimately be possible to produce sperm and eggs to use in assisted conception treatments.
British Fertility Society secretary Dr Allan Pacey called the research an "exciting step forward that has huge implications for the way we could undertake research studies to investigate the processes of egg and sperm development.
"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."
Laura Witjens of the National Gamete Donation Trust said: "Donation has gone down over the last eight or nine years.
"Any research that could ultimately help patients requiring egg or sperm is fantastic news."
Clare Brown, of Infertility Network UK, said: "Although this is good news for couples who cannot produce their own sperm or eggs there is a lot of work still to be done before it can be used in fertility treatment."
However, the charity Life said the work was less about helping infertile couples to have children than about helping to solve the acute shortage of eggs and sperm needed for human cloning and embryonic stem cell research.
Spokesman Matthew O'Gorman said: "Just because something may be possible does not mean that it is ethical.
"Destructive research on human embryos erodes respect for the value of human life and must be stopped."