Scientists have successfully created a human-pig hybrid that could one day lead to the growth of human organs inside animals for transplant use.
This is the first time that embryos combining two large, distantly-related species have been produced. This so-called chimera — named after the cross-species animal from Greek mythology — is an important first step in eventually growing human hearts, livers and kidneys from scratch.
“The ultimate goal is to grow functional and transplantable tissue or organs, but we are far away from that,” Juan Carlos Izpisua Belmonte, who led the work on the part-pig, part-human embryos at the Salk Institute for Biological Studies in La Jolla, California, said.
Researchers injected human stem cells into early-stage pig embryos, resulting in more than 2,000 hybrids that were transferred to surrogate sows. Then, researchers watched as more than 150 of the embryos developed into chimeras that were mostly pig, with a small human contribution of about one in 10,000 cells.
The human-pig embryos developed for 28 days — the first trimester of a pig pregnancy — and were removed.
“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,” said Izpisua Belmonte.
Unlike nine-month pregnancies in humans, pig pregnancies typically last about 112 days, meaning that the embryonic cells develop at different rates. Due to this difference, the team of researchers found that human stem cells needed to be injected at exactly the right moment in their own development in order to survive and become part of the animal.
“It’s like if you’re going onto a highway where the cars are traveling three times faster than you are, you need to choose the right timing, otherwise you cause an accident,” Jun Wu, the paper’s lead author and a scientist at Salk, said.
The study, published in the journal Cell, revived a debate on the ethics of this kind of scientific experiment. Some opponents fear this may one day lead to intelligent animals with humanized brains somehow being released into the wild. Last year, the U.S. National Institutes of Health (NIH) placed a moratorium on funding for these kinds of experiments while they reviewed the possible risks.
“This is a significant advance that raises opportunities and ethical questions as well,” Professor Daniel Garry, a cardiologist and head of a different chimera project at the University of Minnesota, said.
Izpisua Belmonte worries that these concerns, which threaten to overshadow this important work, stem from mythology rather than the realities of scrupulously controlled experiments.
“The idea of having an animal being born composing of human cells creates some feelings that need to be addressed,” Izpisua Belmonte said.
The scientists hope this will lead the way for incubating human organs, which would be genetically matched to patients for transplants or for testing new medicines safely.