It may be possible one day to create an "unlimited" supply of human eggs to help fertility treatment, U.S. doctors.
Researchers have shown that stem cells can be found in adult women who spontaneously produce new eggs in the laboratory.
Other experiments in mice showed these eggs can be fertilized, according to a study published in the journal Nature Medicine.
A British expert said the study re-wrote the rule book with "exciting possibilities" for improving fertility.
