Researchers in the United States have successfully transplanted immature cells with the potential of creating sperm from the testes of rats into mice. The result was that the mice went on to produce fully formed rat sperm.
The achievement has enormous implications. Rats and mice are distinctly different species, having diverged on the evolutionary path 11 million years ago.
What the scientists have demonstrated is that it is possible for one species to develop the sperm of another. If it were possible to extend the principle to humans, it could provide an extraordinary alternative to sperm banking - animals "fathering" human sperm.
The technique also opens the door to a completely new method of immortalising individuals, allowing a parent's personal characteristics to be conserved for many years, perhaps indefinitely.
The breakthrough was announced in the science journal Nature last night and will be featured tomorrow night on BBC1's Tomorrow's World programme.
Last night a spokeswoman for the Human Fertilization and Embryology Authority, the body which regulates fertility research, admitted that there was nothing in British law expressly forbidding the production of human sperm in animals if this should ever be feasible.
The test-tube baby pioneer Dr Ralph Brinster led the research at Pennsylvania University, Philadelphia, in the United States. He and his team took sperm stem cells - immature cells which are the precursors of sperm - from the testes of rats and transplanted them into the testes of 10 mice.
The process of rat sperm production was observed taking place in all the mice. A closer look at eight of the mice showed that those which had harboured the transplants for more than 110 days contained sperm with a shape characteristic of rats.
Speaking on Tomorrow's World, Dr Brinster says: "The rat-to-mouse suggests that you can go across species barriers - which species it's difficult to say. In terms of going from human to mouse it may be much more difficult than going from human to pig, and only doing the experiment will tell you. It's very unlikely I think that one could not freeze the sperm stem cells of all mammalian species. I would be surprised if the human stem cell could not be frozen and perhaps kept for many years, perhaps indefinitely."
In theory, the use of stem cells could mark a revolution in fertility treatment. The technique also raises other possibilities such as infertile fathers using their sons' stem cells to father their own grandchildren, or babies being born hundreds of years after their fathers' death.
Dr Brinster explained that freezing stem cells is totally different from freezing sperm. "When you freeze a stem cell and you can show it will grow in another animal, you have saved that individual, you have immortalised that individual. It's a totally new concept in biology," he said.
There are also implications for genetic engineering. If it became possible to produce laboratory cultures of stem cells, modification of their genetic structure should be feasible.