A drug used to treat cancer patients could also hold the key to rejuvenated fertility after it appeared to "revive" ovaries.
Scientists found the drug turned back the developmental clock, making the ovaries look as if they belonged to pre-pubescent girls.
The "astonishing" result overturns the widely held view that women are born with a finite supply of eggs to last their lifetime.
The Scottish team who carried out the research are still investigating whether the newly generated eggs have the potential to mature and gain the capacity to create babies.
But it raises hopes for fertility treatments and premature menopause.
Eight women, who had an age range of 16 to 29, had been treated for Hodgkin's lymphoma with a drug combination known as ABVD.
They were among 14 women having chemotherapy who donated ovarian tissue for research.
When the researchers examined the tissue, expecting to see evidence of damage by the drugs, they got a shock.
It yielded surprising numbers of non-growing follicles containing immature eggs that seemed to be newly formed.
There were far more of them than could be seen in ovarian tissue from women who received other forms of chemotherapy, or even healthy women of the same age.
Lead scientist Professor Evelyn Telfer, from the University of Edinburgh, said: "This was an astonishing result that we didn't expect. It's not what we set out even to look at.
"The tissue looked more like that of pre-pubescent than adult ovaries. It wasn't just the fact that there were more immature eggs, it was the way they were organised and clustered. There were features that we only see in the young ovary.
"We don't know what the mechanism is. Our working hypothesis is that the drugs destroyed eggs in the ovaries and at the same time induced the activation of cell populations in a way that's compatible with making new eggs."
Although the study only involved a few patients, the findings published in the journal Human Reproduction could be "far reaching", she added.
At birth, a girl's ovaries hold one to two million follicles, the "sacs" within the ovaries that contain immature eggs.
But by the time she reaches puberty, only about 400,000 remain.
With each menstrual cycle, about 1,000 follicles are lost and only one will develop a mature egg ready to be fertilised.
Very few follicles remain by the time a woman reaches the menopause, usually between 48 and 55.