Let’s all take a deep breath. The lab-created “pig/human hybrid” being reported in the news this week is real, but it’s not quite the monster you might imagine. Researchers from the Salk Institute, who published their results in the journal Cell, have successfully coaxed human cells to grow inside pig embryos.

Chimeras (hybrid organisms) have always been a sticky issue both scientifically and ethically. Public opinion about this kind of science is hardly favorable, and the National Institutes of Health and other research bodies will not fund studies that involve the implantation of human stem cells into the eggs and embryos of other animals.

 

But many scientists, including the authors of the new paper, feel it’s important to keep doing it anyway. The first phase of the current research, which was funded by supporters of the Salk Institute, involved creating a cross between a rat and a mouse by implanting rat cells into mouse embryos. (Earlier this week, we reported on similar research in which scientists grew mouse organs inside rats, then transplanted them back into mice.) The researchers used gene editing to encourage those cells to develop into specific parts of the mice, including their eyes, hearts, or pancreata. They even coaxed the rat cells into becoming gallbladders—a very impressive feat, since rats don’t actually have gallbladders.

"This suggests that the reason a rat does not generate a gallbladder is not because it cannot,” co-author Jun Wu of the Salk Institute said in a statement, “but because the potential has been hidden by a rat-specific developmental program.”

Next, the team attempted to try the same technique with human cells and non-human animal hosts. They decided to use cows and pigs, since their organs are naturally similar in size to our own.

But rats and mice are much more closely related to each other than pigs and humans are, so the process proved much more complicated. Part of the difficulty involved timing: Pig embryos develop faster than humans do.

"It's as if the human cells were entering a freeway going faster than the normal freeway," said lead investigator Juan Carlos Izpisua Belmonte. "If you have different speeds, you will have accidents."

 

After four years of work by more than 40 people, the researchers achieved their goal. Human cells acclimated to pig embryos and grew inside them alongside the pigs’ own parts. The growth period was brief (3 to 4 weeks); the researchers cut the experiment short well before the embryos became piglets. They were not about to create actual ManBearpigs.

"The ultimate goal is to grow functional and transplantable tissue or organs, but we are far away from that," Izpisua Belmonte said. "This is an important first step."