Scientists grow the world’s first human pig chimera

Sometimes it’s hard to help but wonder what scientists would do with their time if they didn’t have stem cells to experiment with, or weren't busy creating new, completely alien species. Now a team of said scientists at the Salk Institute have used stem cells to create their own genetic chimera’s, and we’re not talking about the multi-headed, fire-breathing monster from Greek mythology. And in this case their latest chimera is a common a garden pig that grows human organs.

However, the teams research is still in the early stages and the pig embryos that the team had injected with human stem cells weren’t allowed to grow beyond the foetal stage. The success of the experiment though suggests that one day these chimeras, and other creatures like them – that are part animal, part human - could be used to grow fully human organs that could be used in transplants. And for over 120,000 American’s on donor waiting lists that could literally mean the difference between life and death.

“Our ultimate goal is to grow functional and transplantable tissue or organs, but we are far away from that,” said Juan Carlos Izpisua Belmonte who led the team, and the author of the study published in the journal Cell, "this is an important first step.”

As expected, developing fully grown 3D tissues and organs from stem cells in a petri dish wasn’t easy.

 

“It’s like when you try to duplicate a key. The duplicate looks almost identical, but when you get home, it doesn’t open the door. There is something we are not doing right,” said Belmonte, “we thought growing human cells in an animal would be much more fruitful but we still have many things to learn about the early development of cells.”

Belmonte and Salk scientist Jun Wu’s experiment initially began with the production of a mouse-rat chimera, then, over time and 1,500 pig and cow embryos later they slowly introduced human stem cells into the mix.

The team tried several different types of human stem cells, injecting them into pig embryos to determine which could best survive. The ones that showed the most promise were the “intermediate” pluripotent stem cells that are cells that are somewhere in between the two cell types, “Naive” stem cells, which are in an early stage of development, and “Primed” stem cells that are in a later stage of development.

The human cells that survived went on to form a human-pig chimera embryo which were implanted into sows and were allowed to develop for three to four weeks.

“This is long enough for us to try to understand how the human and pig cells mix together early on without raising ethical concerns about mature chimeric animals,” Izpisua Belmonte said.

However, even with the best-performing human stem cells, Wu stated that the level of contribution to the chimeric embryo was low. But this isn’t necessarily a bad thing. One concern about human-animal chimera embryos is that they would develop to be too human.

“At this point, we wanted to know whether human cells can contribute at all to address the ‘yes or no’ question,” said Izpisua Belmonte, “now that we know the answer is yes, our next challenge is to improve efficiency and guide the human cells into forming a particular organ in pigs.”

The researchers are now relying on CRISPR, a revolutionary gene editing technique, to do this, editing pig genomes to open gaps that the human cells can fill in.

The goal of growing fully functional and transplantable human tissues and organs is still a way off, but with governments flagging the arrival of self-driving cars, which will dramatically reduce the number of road related deaths, and as a consequence the number of available donor organs, and with 3D printed organs still some way off, chimeras might be a way to fill the gap.

For now the team have no intention of creating fully grown chimera organisms - they simply want to prove the model, so for now at least you can sleep, safe in the knowledge that there won't be any eagle, human, lion, rat hybrids trying to hunt you down.