Mouse sperm made from stem cells and used to treat infertile mice marks a step forward for regenerative medicine, according to researchers in Japan.

Stem cells are the body's master cells, used to create all cells and tissues. Scientists hope to use stem cell's ability to generate a variety of new, healthy cells to treat cancer and diabetes, among other things.

Stem cells were removed from mouse embryos and developed into a type of precursor cell, known to grow into either mouse eggs or sperm.

These new cells were transplanted into the testes of infertile male mice, which then went on to produce healthy sperm.

Lead Author Professor Mitinori Saitou, from Kyoto University's department of anatomy and cell biology, told Reuters: "The sperm were removed directly from the testes and fertilized with eggs (on laboratory dishes).

"After insemination, we made two set of embryos and these were transferred into the uterus of the foster mother and they derived healthy mice (that went on to reproduce normally)."

Previous attempts to create sperm from stem cells have resulted in unhealthy, short lived offspring but this attempt appears to have been more successful.

The research, carried out at Kyoto University in Japan and published in the journal Cell, might pave the way for scientists to develop human precursor cells that could grow into human sperm or eggs.

Professor Saitou added: "We have huge materials to work with now and ... we can accelerate our study into the cause of human infertility."

"We can possibly use this knowledge to induce human primordial germ cells (cells that grow into eggs or sperm)."

The gap between human and animal research means that more work is still needed, said Saitou. First, scientists will attempt to produce mouse eggs using stem cells.

British law bans the use of lab made mature sperm in fertility treatments. However, because the sperm is produced naturally from stem cells the new method may circumvent this technicality.

Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said: "The philosophy of the law is to stop that kind of thing happening. But in this case you're not technically creating sperm, so it might be that you can sidestep this regulation. It all depends on definition."

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