They then applied the same method to human ovaries donated by women at the Saitama Medical Center in Japan who were undergoing sex reassignment because of a gender identity disorder. As with the mice, the team was able to retrieve reproductive cells that produced immature egg cells when grown in the laboratory. The egg cells, when injected into mice, generated follicles, the ovarian structure in which eggs are formed, as well as mature eggs, some of which had a single set of chromosomes, a signature of eggs and sperm. The results were published online Sunday by the journal Nature Medicine.

Image Jonathan L. Tilly led the study, which, if confirmed, could pave the way for new infertility treatments. Credit... Brian Snyder for The New York Times

Dr. Tilly and colleagues wrote that their work opens up “a new field in human reproductive biology that was inconceivable less than 10 years ago,” and that access to the new cells will make possible novel forms of fertility preservation.

David Albertini, an expert on female reproduction at the Kansas University Medical Center, called the report “a real technological tour de force,” but added that it was not yet clear whether the procedure yielded real egg cells that could be used in human fertility. “None of the criteria that we in the field use to establish that a cell is a high-quality oocyte are satisfied here,” he said, using the scientific term for an unfertilized egg.

Dr. Tilly has long disputed the accepted belief that a woman makes no new egg cells after she is born. In 2005, he reported that women possess a hidden reserve of cells in the bone marrow that constantly replenish the ovaries with new eggs. But other researchers have been unable to confirm these results, Dr. Albertini said.

The new study is “along a completely different line” but still should be interpreted with caution, Dr. Albertini said, until other researchers have been able to repeat it, a routine verification procedure in research.