Milwaukee, Wis. ? For the first time, researchers have grown eggs from embryonic stem cells, showing a potential new way to treat diseases and challenging notions of how life begins.

The research was in mice but has implications to radically change the debate about using human stem cells.

Embryonic stem cells are considered to be the primordial ooze from which virtually all organs, cells and body parts arise. With the right coaxing and conditions, they can be made to transform into heart, nerve, pancreatic and other cells.

However, it has never been demonstrated that they could create gametes — eggs and sperm — until now.

Scientists from France and the University of Pennsylvania showed that mouse stem cells could form eggs, and that those eggs could spontaneously form primitive embryos.

“I think most scientists thought this was impossible,” said Hans Scholer, a developmental biologist at the University of Pennsylvania.

The research was published online Thursday by the journal Science.