Human eggs have been fully grown in a laboratory for the first time in what scientists hope could be a breakthrough in improving fertility treatment.
Scientists removed egg cells from ovary tissue at their earliest stage of development and grew them to the point at which they were ready for fertilisation, according to the study published in Molecular Human Reproduction.
The process could offer hope to women undergoing potentially harmful treatments such as chemotherapy - allowing immature eggs to be recovered from patients, matured in a lab and stored for future fertilisation.
Scientists have previously developed mouse eggs to produce live offspring and matured human eggs from a late stage of development.
But this study is the first time a human egg has been developed in the laboratory from its earliest stage to full maturity, researchers say.
Scientists will now focus on examining how healthy the eggs are and whether they can be fertilised.
Lead researcher Professor Evelyn Telfer, of the University of Edinburgh's school of biological sciences, said: "Being able to fully develop human eggs in the lab could widen the scope of available fertility treatments.
"We are now working on optimising the conditions that support egg development in this way and studying how healthy they are.
"We also hope to find out, subject to regulatory approval, whether they can be fertilised."
Professor Daniel Brison, of the department of reproduction at the University of Manchester, said: "This is an exciting breakthrough which shows for the first time that complete development of human eggs in the laboratory is possible, more than 20 years after this was achieved in mice.
"As the authors acknowledge, there is much more important research still to do, but this could pave the way for fertility preservation in women and girls with a wider variety of cancers than is possible using existing methods."
But Professor Simon Fishel, founder and president of leading IVF treatment providers CARE Fertility, said further research was needed to establish whether eggs developed using the method could be healthy.
He said: "This study demonstrates that there is much laboratory research to be undertaken before we can be encouraged to believe that we will achieve healthy normal eggs for clinical purposes in vitro developed follicles derived from human ovarian cortical tissue."
The study, carried out by the Royal Infirmary Edinburgh, the Centre for Human Reproduction in New York and the Royal Hospital for Sick Children in Edinburgh, was supported by the Medical Research Council.