Stem cells became egg-like structures
Scientists have managed to make stem cells taken from mouse embryos develop into eggs in a test tube.
It is believed to be the first time this has been achieved outside the body.
However, there are some doubts whether these eggs are fully-functional and can be fertilised normally.
The idea of the experiment is to one day use embryos produced from these eggs as a source of stem cells for other research work.
However, there is no suggestion at the moment that scientists believe they could turn a stem cell harvested from an adult woman into an egg.
The research was carried out by researchers from the University of Pennsylvania in the US.
Stem cells are the body's "master cells" - an early embryo has cells which have the ability to differentiate into every type of tissue cell that the body needs.
This is how a five-cell embryo can become a foetus with organs, skin and bone.
Scientists are hoping to harness this ability, using stem cells harvested from embryos to produce, for example, brain cells to replace those lost through disease or stroke.
The Pennsylvania team, led by Professor Hans Scholer, took mouse embryonic stem cells, and then placed them in a petri dish and watched them divide.
After just a few weeks, cells with similar qualities to ovarian follicles developed - and these released egg-like cells.
Despite the absence of sperm, the egg-like cells developed further into embryo-like structures, although these were not left to see if they could develop beyond an early stage.
Professor Scholer said: "Most scientists have thought it impossible to grow gametes from stem cells outside the body.
"We found that not can mouse embryonic stem cells produce oocytes (eggs), but that these oocytes can then enter meiosis, recruit adjacent cells to form structures similar to the follicles that surround and nurture natural mouse eggs, and develop into embryos."
Stem Cell expert Professor Azim Surani, from the Wellcome Trust/ Cancer Research UK Institute in Cambridge, said that the finding was "interesting", but that doubts remained about what had actually been produced.
He told BBC News Online: "These structures have many of the characteristics of gametes.
"However, it will require further work to be really sure of this.
"At this stage there isn't enough scientific evidence that these structures have all the attributes of a normal gamete."
The research was published in the journal Science.