By Michelle Roberts
BBC News, Health reporter in Lyon
Israeli scientists say that they have extracted and matured eggs from girls as young as five to freeze for possible fertility treatment in the future.
The eggs were extracted, artificially matured and then frozen
The team said that the technique could give child cancer sufferers left infertile by chemotherapy treatment a shot at parenthood later in life.
The team took eggs from a group of girls between the ages of five and 10 who had cancer.
They artificially matured the eggs to make them viable and froze them.
Experts had previously thought the eggs of pre-pubescent girls could not be used in this way.
Dr Ariel Revel, from Hadassah University Hospital in Jerusalem, is to present the team's findings at a fertility conference in Lyon, France, this week.
"No eggs have yet been thawed, so we do not know whether pregnancies will result," he said in a statement.
"But we are encouraged by our results so far, particularly the young ages of the patients from whom we have been able to collect eggs."
Childhood cancer have a good cure rate - between 70% and 90% - but often require aggressive chemotherapy which can mean the child will be sterile in later life.
Doctors have been experimenting with freezing the ovarian tissue of young girls which could be transplanted back into their bodies when they are older.
However, the ovarian tissue could also contain cancerous cells. Frozen eggs could eliminate this risk, say experts.
The new technique could give girls an option they would otherwise not have, the team said.
Professor Gedis Grudzinskas, medical director of the Bridge Fertility Centre, London, said: "This is very important because this is the first time that human eggs have undergone changes in the test-tube which normally take place during puberty."
Dr Hamish Wallace, a consultant paediatric oncologist at the Royal Hospital for Sick Children in Edinburgh, said: "Maturing in vitro a primitive egg from a five-year-old girl would be a huge advance.
"Doctors wouldn't need to worry about the ovarian tissue being contaminated with some of the original cancer cells and reintroducing the cancer into the body."
Geoff Thaxter, of the children's cancer charity CLIC Sargent, said: "This report represents interesting initial research into potential fertility treatments for children being treated for cancer, and could help to make sure that childhood cancer does not have a lifelong impact."
But Josephine Quintavalle, of Comment on Reproductive Ethics expressed concern that if the eggs were donated to a woman of childbearing age, a resulting child could have a biological mother who was only a few years older.
She said: "Are we going to end up with a child who has a mother who is just six years older? What happens if the child dies? Could the eggs be donated to someone else?
"I don't think this is the first priority for five year olds. Any intervention for a child going through cancer treatment is an added burden. I feel uncomfortable about this development."