Nikolaos Sofikitis explains his research

The team at the Tottori University in Japan, say they implanted the human cells responsible for producing sperm - spermatogonia - into rat and mice testes in August 1998.

Five months later human sperm was detected in the animals.

Even more remarkable, if confirmed, is the disclosure that the spermatogonia used in the experiment were taken from infertile men.

A letter has now been sent to the Japan Society of Obstetrics and Gynecology asking permission to see if the sperm can fertilize human eggs.

Eye cells

Assistant professor Nikolaos Sofikitis said the key to success was the use of the animals' own eye cells in the operation.

"Human sperm cells were protected by the eye cells," he said.

BBC's Matt McGrath reports

There are a number of teams working in this area around the world. Previous attempts to grow human spermatogonia in rats and mice had failed due to the animals' white blood cells attacking the human transplant within the testes.

But when some of the animals' own eye cells were implanted in the testes they killed off the white blood cells and allowed the human spermatogonia to grow and eventually produce sperm.

The spermatogonia were taken from 18 infertile human patients and used in 10 rats and eight mice, Sofikitis said.

There was much work ahead before it could be applied to humans, said the researcher, describing the experiment as a breakthrough.

"I have to conduct many experiments before determining that these sperm can function properly," he said.

Human eggs

Another researcher at the university, gynaecologist Yasuyuki Mio, said he had applied to the Japan Society of Obstetrics and Gynecology for permission to use the sperm produced by the rodents on human unfertilised eggs.

"I would like to conduct many experiments by using sperm [grown in animals] to see how they grow and function in eggs," he said.

Kazunori Ochiai, an assistant professor at Tokyo's Jikei University Hospital who serves as spokesman for the Japan Society of Obstetrics and Gynecology, declined to comment.

"We have yet to receive a request letter, so I can't comment," he said.