By James Gallagher, Health and science correspondent, BBC News
Human eggs have been grown in the laboratory for the first time, say researchers at the University of Edinburgh.
The team say the technique could lead to new ways of preserving the fertility of children having cancer treatment.
It is also an opportunity to explore how human eggs develop, much of which remains a mystery to science.
Experts said it was an exciting breakthrough, but more work was needed before it could be used clinically.
Women are born with immature eggs in their ovaries that can develop fully only after puberty.
It has taken decades of work, but scientists can now grow eggs to maturity outside of the ovary.
It requires carefully controlling laboratory conditions including oxygen levels, hormones, proteins that simulate growth and the medium in which the eggs are cultured.
But while the scientists have shown it is possible, the approach published in the journal Molecular Human Reproduction still needs refinement.
It is very inefficient with only 10% of eggs completing their journey to maturity.
And the eggs have not been fertilised, so it is uncertain how viable they are.
Prof Evelyn Telfer, one of the researchers, told the BBC: “It’s very exciting to obtain proof of principle that it’s possible to reach this stage in human tissue.
“But that has to be tempered by the whole lot of work needed to improve the culture conditions and test the quality of the oocytes [eggs].
“But apart from any clinical applications, this is a big breakthrough in improving understanding of human egg development.”