Cute and heartwarming videos of dogs fill the internet. My favourite is the bacon dog tease, but others catch my attention because they reveal extraordinary animal behaviours. For example, there are many of dogs helping other animals, like opening the back door to let in a friend.
Keeping pigs as pets is increasing in popularity, especially as sizes decrease in new breeds: pigs the size of dogs now include genetically-engineered micropigs. Gene-editing alterations to a hormone that controls growth locks pig development at a size that means these pets can cuddle with their owners on the couch without breaking bones.
Physiologically, pigs and humans are surprisingly similar. Enough so that pig organ donation might be viable in the future. The gap in numbers between those dying of organ failure and donors is so vast that France has just implemented a progressive ‘opt-out’ policy.
Recent work aimed at taking advantage of the pig as a model system for humans is being led by Craig Venter who hopes to alter the pig genome sufficiently to make pig lungs human-enough for transplantation without the normal risk of rejection by the human immune system. Another genomic pioneer, George Church of Harvard, broke the record for editing the most pieces of DNA using CRISPR gene-editing technology, snipping out 60+ parts of the pig genome to humanise it for transplantation.
Could scientists take the next giant step and grow human organs inside pigs? The entire complement of genes would then be human. A research group at the Salk Institute have taken a step in this direction and published early results in the journal Cell. Named after the fire-breathing chimera of Greek mythology, a lion with the head of a goat and a tail capped with the head of a snake, they’ve created the first human-pig cell hybrids.
Creating chimeras exceeds the boundary of research eligible for public funding in the United States: this work was funded by private donations. What is contentious about chimeras is the need to build them using stem cells, generalist cells that can turn into any type of adult cell. It is this ‘pluripotent quality’ that is needed to make them ‘take seed’ in the embryo of another species.
Like kittens and puppies who grow up together don’t fight each other as grown cats and dogs do, human cells injected early enough in the development of a different species play nice together with their foreign host cells.
The trick of crafting a viable chimera lies in fusing cells together at exactly the time: the human stems cells and pig cells both need to be in a compatible stage of development. Get it correct, as these authors did, and the result is a pig-human.
Well, sort of.
Only a tiny fraction of the pig is human. Still, these experiments offer critical proof-of-principle that it is possible to force a co-existence. And human cells aren’t building any one targeted organ – yet. But this could be achieved by knocking out the right pig genes with gene-editing, as was done in mice to ensure the growth of organs from injected rat cells. Finally, the ability of these chimeras to produce mature organs remains to be proven. They were destroyed in accordance with ethical regulations after only a few weeks. So, the starting gun has been fired but there is a long way to go to cross the finish line.
Human cells can grow in petri dishes, and now it seems, they can grow in other species, perhaps even to the point of forming transplantable organs. Might you one day owe your life to a pig?
Featured image credit: Pig in a bucket by Ben Salter. CC BY 2.0 via Wikimedia Commons.