Frozen testicle 'live birth first'
A sample of frozen testicle has been used to produce live offspring in experiments on mice, Japanese researchers report.
The breakthrough could have important implications for boys with cancer who become infertile due to chemotherapy treatments.
Fertility experts said the data was "very encouraging" and they hoped human trials would not be too far away.
The findings have been published in the journal Nature Communications.
Sperm samples can be frozen before cancer treatment starts in order to allow men to have children at a later date should the drugs damage the testes.
But this is not an option for boys who have yet to go through puberty.
"With the increasing cure rate of paediatric cancers, infertility has become an important concern for patients and their families," the report's authors said.
In the study, scientists froze a sample of testicle from mice five days after birth.
A range of assisted reproduction techniques - including artificial insemination and injecting partly developed sperm into an egg - were used.
Baby mice were produced, which were able to mate and produce a further generation of healthy mice.
Prof Takehiko Ogawa, from Yokohama City University, told the BBC: "This is the first time in animals.
"I predict it will take at least a couple of years before it is done in humans, it's not so easy.
"We are now working on human samples, which are very different from mice tissue, I have to find some trick to make it work, so it's very difficult to predict how long that will take."
He said succeeding would be "really encouraging" for a child being treated and their family.
He said he had seen the huge difference it made to the lives of older men who were able to freeze their sperm.
Dr Allan Pacey, a fertility researcher at the University of Sheffield, told the BBC: "Growing sperm in the laboratory for men who were sterilised as boys during cancer treatment is arguably preferable to trying to work out a way to transplant their tissue back into their testicle when they are ready to become dads.
"This is because we want to avoid inadvertently giving them the cancer back if there are cancer cells lurking in the stored testicular tissue.
"However, we would need to check that lab-made sperm were genetically normal and that any babies born are going to be healthy and fertile themselves.
"But based on this research in mice, the data looks encouraging and I hope that proper trials in humans will soon begin."
Another challenge that will need to be overcome is getting the frozen testicular tissue to produce sperm.
Mice begin producing sperm very early on, while the delay is more than a decade in human males.
A way of cajoling the underdeveloped tissue to produce sperm will need to be developed, although it is thought exposure to testosterone should work.