Human/Pig Chimera Embryos Created Using Stem Cells

Experiments with cow embryos were more difficult and costly than pigs, so the team zoomed in on pigs. The effort required to complete studies of 1,500 pig embryos involved the contributions of over 40 people, including pig farmers, over a four-year period. “We underestimated the effort involved,” says Izpisua Belmonte. “This required a tour de force.”

Not only are pigs and humans about five times more distant evolutionarily than mice and rats, but pigs also have a gestation period that is about one-third as long as humans, so the researchers needed to introduce human cells with perfect timing to match the developmental stage of the pig. “It’s as if the human cells were entering a freeway going faster than the normal freeway,” says Izpisua Belmonte. “If you have different speeds, you will have accidents.”

The researchers injected several different forms of human stem cells into pig embryos to see which would survive best. The cells that survived longest and showed the most potential to continue to develop were “intermediate” human pluripotent stem cells. So-called “naïve” cells resemble cells from an earlier developmental origin with unrestricted developmental potential; “primed” cells have developed further, but still remain pluripotent. “Intermediate cells are somewhere in between,” says Wu.

The human cells survived and formed a human/pig chimera embryo. Embryos were implanted in sows and allowed to develop for between three and 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,” says Izpisua Belmonte.

Even using the most well-performing human stem cells, the level of contribution to the chimerized embryo was not high. “It’s low,” says Wu.

Izpisua Belmonte considers this good news. One concern with the creation of human/animal chimeras is that the chimera will be too human. For instance, researchers don’t want human cells to contribute to the formation of the brain.

In this study, the human cells did not become precursors of brain cells that can grow into the central nervous system. Rather, they were developing into muscle cells and precursors of other organs. “At this point, we wanted to know whether human cells can contribute at all to address the ‘yes or no’ question,” he says. “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.”

To do this, the researchers are using CRISPR to perform genome editing on the pig genome, as they did with mice, to open gaps that human cells can fill in. The work is in progress.

This research study was supported by The Fundación Séneca in Murcia, Spain, the Universidad Católica San Antonio de Murcia (UCAM), the Fundacion Dr. Pedro Guillen, the G. Harold and Leila Y. Mathers Charitable Foundation, and The Moxie Foundation.