At any given time, there are more people needing organ donations than there are available organs. Those hoping for a kidney transplant, for example, might have to wait for three to five years. But in the future, those life-saving organs may come from an unexpected source—not a human donor, but a pig.

According to Smithsonian, researchers in China have achieved what’s believed to be the first partially human organ grown inside of another species. The results, published in the journal Cell Stem Cell, came after scientists turned off genes in pig embryos responsible for their own kidney development. Modified human stem cells were then introduced into roughly 1800 embryos, which were transferred to 13 surrogates. After 28 days, five embryos had grown kidneys that were up to 65 percent human cells.

“It took us five years,” senior study author Miguel Esteban told CNN. “We modified the pig genetically to create a space for the human cells to grow with less competition from pig cells, and we also modified the human cells to make them survive in an environment that was not their natural one.”

While the findings are promising, a number of obstacles remain. A kidney intended for a human cannot contain any pig cells; a human-pig hybrid kidney would be rejected by the human body and not be viable. There are also a variety of kidney cell types; the study worked with only two of them.

The advancement of such practices also present ethical quandaries. Some human cells were found in the brain tissue of the pig embryos, which introduces potential to disrupt the animal’s behavior. There’s also the possibility that even sperm or eggs could develop with a combination of human and animal genes.

Still, the process of utilizing non-human organs or vessels remains a focus of research. Earlier this year, surgeons were able to transplant genetically modified pig kidneys into two brain-dead human patients and observed the kidneys functioning.

Someday, scientists hope to build “custom” transplantable organs from patients’ own cells, reducing the need for immuno-suppressing drugs (which come with their own health issues). Such practices are years or decades away, but they’re closer to becoming a reality.

[h/t Smithsonian]

This article was originally published on www.mentalfloss.com as Scientific Progress Goes 'Oink': Part-Human Kidneys Have Been Grown in Pig Embryos.