Boys left infertile by childhood cancer treatment may one day be able to produce healthy sperm by using stored stem cells, monkey research suggests.
Chemotherapy and radiotherapy can kill tumours and the cells which make sperm.
A study, published in the journal Cell Stem Cell, extracted sperm-producing stem cells before cancer treatment and later placed them back into the monkey.
Sperm which could fertilise an egg were produced, which experts labelled a "milestone" in research.
Most men who have cancer treatment which could affect their fertility can choose to freeze sperm before their treatment starts. This is not an option for patients who have not yet gone through puberty.
However, they do have the spermatogonial stem cells which would start to produce sperm in their teenage years.
The researchers at the University of Pittsburgh and the Magee-Womens Research Institute took samples of the stem cells from macaques and stored them in a freezer.
The monkeys were then given a chemotherapy drug.
Their own stem cells were implanted back where they came from after the course of chemotherapy had ended. Nine out of 12 adult monkeys and three out of five prepubescent monkeys were later able to produce sperm again.
Separate experiments showed eggs could be fertilised with sperm produced after the procedure.
Dr Kyle Orwig, from the department of obstetrics, gynaecology and reproductive medicine at Pitt School of Medicine, said: "This study demonstrates that spermatogonial stem cells from higher primates can be frozen and thawed without losing their activity, and that they can be transplanted to produce functional sperm that are able to fertilise eggs and give rise to early embryos."
He said there were still many challenges before this could be used in people: "Should we re-introduce the spermatogonial cells as soon as treatment is over, or wait until the patient is considered cured of his disease, or when he is ready to start a family? How do we eliminate the risk of cancer recurrence if we give back untreated cells that might include cancer cells?
"These are issues we still must work through, but this study does show us the concept is feasible."
Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, told the BBC that the idea had been discussed repeatedly but "experiments have never come close to suggesting it might work in humans".
However, he said: "This report is a very useful step forward and clearly shows that the science of spermatogonial stem cells transplantation might one day work for humans. And, although the authors report relatively low efficiency so far, in the context of someone who does not have any banked sperm to fall back on, these odds are probably very encouraging to make this kind of approach worthwhile."
He also highlighted safety concern saying: "It would be a disaster to give the cancer back to someone because cancer cells are lurking in the transplant.
"But also we need to make sure that the genetics of sperm produced from transplanted spermatogonial stem cells is correct and leads to the birth of healthy offspring which themselves give rise to healthy grandchildren and great grandchildren."
Pierre Fouchet, a researcher at the CEA Institute of Cellular and Molecular Radiobiology in France, said the results: "Constitute a milestone in the field of reproduction and generate hope for restoring fertility in survivors of childhood cancer."
However, he said advances in fertility research needed "intense debate" about the social consequences.