Scientists in Pennsylvania have created pig and goat sperm inside the bodies of laboratory mice, marking the first time that male reproductive cells have been produced in such distantly related species.

The researchers said they hoped to use mice as “bio-incubators” to grow sperm cells for endangered species whose survival is threatened by a lack of sexually mature males. They also want to produce sperm from valuable farm animals without waiting for them to reach puberty.

The technique, in which bits of testicular tissue from newborn pigs and goats were grafted onto the backs of mice, could also provide an unprecedented window through which scientists may watch the mysterious process by which sperm develop in various species including humans.

Indeed, several experts Wednesday predicted that it won’t be long before human sperm are grown in mice an advance that scientists and ethicists said could lead to both useful and troubling scenarios.

On the positive side, men who lost their testes before puberty to surgery or cancer treatment might use the approach to sire children: After reaching adulthood, they could thaw a preserved smidgen of their young testicular tissue, have it grafted onto a mouse and after allowing time for maturation extract from the rodents healthy human sperm.

But the ability to cultivate sperm from immature testicular tissue could lead to stranger futures than that. It could mean that a boy who didn’t survive childhood or even a male fetus that was never born could someday be a father.

Researcher Ina Dobrinski of the University of Pennsylvania School of Veterinary Medicine emphasized that the team was not pursuing such applications.

“Anything done with humans would have to undergo scrutiny first by ethics boards and the whole nine yards,” she said. “This is nothing you’d want to try at home.”

Conservationists said they were excited by the prospect of coaxing mature sperm from immature tissue taken from endangered animals.

“With endangered species, you lose some of the animals before they even reach puberty,” said Philip Damiani, staff scientist at the Audubon Center for Research of Endangered Species.

The Penn team took tiny samples of testicular tissue from newborn goats, pigs and mice and transplanted them just under the skin of “nude mice,” whose immune systems are incapable of rejecting grafts.

More than 60 percent of the approximately 800 grafts survived, and each of those produced mature sperm, the team reported in Thursday’s issue of the journal Nature.