Japanese scientists have created eggs from the cells of male mice and produced healthy mice pups.
The scientific journal Nature published details of the study, led by Professor Katsuhiko Hayashi of Kyushu University and Osaka University, on Wednesday.
In a commentary published alongside the study, Diana Laird, a stem cell and reproductive expert at the University of California, and her colleague Jonathan Bayerl said the work "opens up new avenues in reproductive biology and fertility research."
In the future, it might be possible to reproduce endangered mammals from a single male.
"It might even provide a template for enabling more people," such as male same-sex couples, "to have biological children, while circumventing the ethical and legal issues of donor eggs," they wrote.
Hayashi himself, however, warned the research was at a very early stage.
"There are big differences between mice and humans," he told a human gene-editing summit at the Crick Institute in London last week.
Stem cells from male mice tails
A Chinese study in 2018 reported that mice with two mothers were born, but when they tried it with male mice, their pups survived only a few days.
The Japanese scientists used a different approach, and the pups in their study appeared to grow normally and were able to become parents themselves in the usual way.
The technique involves first taking a skin cell from male mice's tails and transforming them into a stem cell.
Then, through a process that involved growing them and treating them with a drug, they converted male mouse stem cells into female cells and produced functional egg cells.
Finally, they fertilized those eggs and implanted the embryos into female mice.
Laird described it as "an important step in both stem cell and reproductive biology."
Human trials far off
The research is still in its early stages and the method is still extremely inefficient.
Only seven of 630 embryos transferred to surrogate mothers produced living pups.
Researchers have not determined why only a tiny fraction of the embryos placed into surrogate mice survived.
They also stressed that it's still too early to know if the protocol would work in human stem cells.
In her commentary, Laird also said scientists need to be mindful of the mutations and errors that may be introduced in a culture dish before using stem cells to make eggs.
