Primitive human sperm and eggs and the germ cells that make them have been created from embryonic stem cells in an experiment that promises new treatments for infertility.

The achievement transforms scientists’ ability to study the development of the human reproductive system and has already helped to confirm the importance of several genes to egg and sperm (gamete) formation. It could eventually open new approaches to restoring or preserving fertility, such as gene therapies that stimulate gamete production in the testes or ovaries to allow natural conception.

The research, by a team at Stanford University in California, also advances the prospect of creating synthetic sperm and eggs in the laboratory to allow men and women who make none to have their own genetic children.

This, however, remains at least five years away, and would have to clear significant ethical and safety hurdles. The use of artificial gametes in reproduction was banned in Britain last year. Another potential benefit could be insights into spontaneous genetic mutations that cause disease and disability.

“Our goal is to understand how you make eggs and sperm,” said Renee Reijo Pera, who led the research. “We know almost nothing about human reproductive development, and this gives us a new way to investigate it. The hope is some day to help those who are infertile.”

The study, which is published in the journal Nature, offers the strongest evidence yet that it is possible to grow human reproductive tissue from embryonic stem cells, and even to make eggs and sperm.

Scientists at Newcastle University announced in July that they had made reasonably mature sperm from embryonic stem cells, but their claims did not convince other experts and their paper was withdrawn subsequently because of errors.

Independent researchers who were critical of the Newcastle work said yesterday that the Stanford study was much more conclusive. Allan Pacey, senior lecturer in andrology at the University of Sheffield, said that it was “very convincing”, and Robin Lovell-Badge, of the National Institute for Medical Research, in London, described it as a “much better study”.

Professor Reijo Pera cautioned that while her research had generated immature eggs and sperm, it would be unsafe and unethical to use these in reproduction.

“It’s an ultimate goal of the work, but these are highly genetically modified cells and it would not be appropriate to use them to make an embryo,” she said.

Human eggs and sperm are produced from germ cells in the ovaries or testes, which themselves start to form in early embryos. This has limited scientists’ ability to study germ cells to establish how they develop, and how genetic defects or environmental exposures cause fertility problems.

Professor Reijo Pera said: “Ten to fifteen per cent of couples are infertile. About half of these cases are due to an inability to make eggs or sperm, and yet deleting or increasing the expression of genes in the womb to understand why is both impossible and unethical. 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.”

Her research, which took five years, has identified a cocktail of proteins that can coax embryonic stem cells to form germ cells. Colonies of these germ cells can now be used to investigate how they give rise to sperm and eggs and genetic and environmental influences on this process.

The Stanford team is now trying to create germ cells using reprogrammed adult cells with embryo-like properties. If this works, it could become possible to use these to grow in-vitro derived eggs and sperm with the genetic characteristics of an infertile person.

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