Scientists have generated the world’s first monkey embryos incorporating human cells to study how the two types of cells develop in tandem. The embryos were derived from a macaque and then injected with human stem cells in the lab before being destroyed after 20 days.

Scientists have generated embryos in a lab for the first time that contain cells from both humans and monkeys.

Scientists anticipate that by producing chimeric embryos (embryos having cells from two distinct species), they may be able to manufacture organs for those in need of transplantation.

Over 100,000 people in the United States alone are currently on a waiting list for life-saving organ transplants but the supply of donor organs has decreased considerably since the pandemic began.

In recent years, researchers have sought to inject human stem cells into pig and sheep embryos in the hopes of producing organs for transplants but this has not given positive results. Scientists are expecting better results by turning to macaque monkeys who have a higher genetic similarity to humans.

In a study published Thursday in the journal Cell, researchers from the United States and China implanted 25 pluripotent stem cells from humans into macaque monkey embryos.

After one day, the researchers discovered human cells growing in 132 of the embryos. The embryos lived for a total of 19 days.

However, bioethicists have expressed concern about the likelihood of violating medical standards that currently regulate the care of animal and human subjects, as well as the risk that rogue scientists would inject living organisms with human cells.

“My initial thought is, why?” Kirstin Matthews, a science and technology fellow at Rice University’s Baker Institute, said NPR. “I believe the public will be concerned as I am, that we are simply moving on with science without having a genuine conversation about what we should or should not do.”

Researchers say that the study is solely humanitarian in nature with the potential to save countless lives in the future.

“This work is a significant step that provides very convincing evidence that someday when we completely understand the process we could be able to make them develop into a heart, kidney, or lungs,” explained University of Michigan professor Jeffrey Platt, who was not involved in the study.