Scientists seem to have done the impossible. They have created mice with two biological fathers. The revolutionary research could open the way for new fertility treatments in the future, including reproduction in same-sex couples.

"This is the first case of making robust mammal oocytes from male cells," said Katsuhiko Hayashi, who led the research team at Kyushu University in Japan, The Guardian reported.

Their work was presented at the third International Summit on Human Genome Editing at the Francis Crick Institute in London on Wednesday. It was submitted for publication in the scientific journal Nature, reported SkyNews.

Hayashi is confident that it will be technically possible to create a viable human egg from a male skin cell within a decade. Others believe the timeline is too optimistic since we are yet to create viable lab-grown human eggs from female cells.

"Purely in terms of technology, it will be possible [in humans] even in 10 years," he said, adding he would support using the technology to clinically allow two men to have a baby. "I don't know whether they'll be available for reproduction. That is not a question just for the scientific program, but also for [society]."

If proved to be viable, the technique could also help women with Turner's syndrome – one copy of their X chromosome is partly or wholly missing. This aspect was the primary motivation for Hayashi's research.

Prof. George Daley, the dean of Harvard Medical School, believes the work is "fascinating," but adds that creating lab-grown gametes from human cells could be far more challenging than doing so in mouse cells.

"We still don't understand enough of the unique biology of human gametogenesis to reproduce Hayashi's provocative work in mice," he said.

In the study, a male skin cell consisting of an XY chromosome combination was transformed into an egg, which is the female XX combination. Male skin cells were reprogrammed into a stem cell-like stage called induced pluripotent stem (iPS) cells. Next, the Y-chromosome present in these cells was deleted and substituted with an X chromosome "borrowed" from another cell. This created iPS cells with two identical X chromosomes.

"The trick of this, the biggest trick, is the duplication of the X chromosome," said Hayashi. "We really tried to establish a system to duplicate the X chromosome."

After creating male cell-derived oocytes, they were cultivated in an ovary organoid, a system that mimics the conditions inside a mouse ovary. Once the eggs were fertilized with normal sperm, they obtained around 600 embryos, which were implanted into surrogate mice. It led to the birth of seven mouse pups.

The number of embryos that went on to produce a live birth was about 1%, which is lower than the 5% efficiency achieved with normal female-derived eggs.

The baby mice looked healthy, had a normal lifespan and even had babies of their own, researchers said.

"They look OK, they look to be growing normally, they become fathers," said Hayashi.

The research team is hoping to replicate the creation of lab-grown eggs using human cells.