Pig-Monkey Chimeras Have Been Born in China in World First Experiment

The world's first pig-monkey chimeras have been born in China, scientists have announced. Researchers injected monkey stem cells into fertilized pig embryos then implanted them into sows and allowed pregnancy to continue to full term. The two hybrid piglets both died within a week of being born, the team said.

Chimeras are organisms made of cells from two genetically distinct individuals—normally two different species. The word comes from Greek mythology, with Chimera being a fire-breathing creature composed of a lion, goat and serpent.

In genetics, it is hoped chimeras could one day result in human organs being grown inside animals that can then be transplanted. Shortages of organs for transplants is a problem facing nations across the world—according to the United Network for Organ Sharing (UNOS), there are over 113,000 currently on the waiting list for a transplant in the U.S.

Earlier this year, Spanish newspaper El Pais announced researchers in China had created a human-monkey chimera, inserting human stem cells into monkey embryos. These embryos were not left to develop into a live birth, but biologist Estrella Núñez, from the Murcia Catholic University in Spain, told the newspaper the initial results were "very promising."

In a study published in Protein & Cell, researchers from the Chinese Academy of Sciences announced the live birth of pig-monkey chimeras. They say this research is considered to be the "most promising method" to generate organs, but "ethical issues prevent the study of human chimeras in the late embryonic stage of development." They say that by studying primate embryonic stem cells, they can get closer to understanding what might happen to human chimera embryos.

According to New Scientist, which first reported the chimeras, the team genetically modified monkey cells growing in culture. By doing this, the monkey cells produced a fluorescent protein that meant they could be tracked after being implanted in the pig embryos. The team derived stem cells from these then inserted them into 4,000 embryos, which were subsequently implanted into sows. Of all these embryos, ten piglets were born, two of which were pig-monkey chimeras.

"This is the first report of full-term pig-monkey chimeras," Tang Hai, one of the study authors, told New Scientist. Further research showed the chimeras had monkey cells in a range of organs, including the heart, liver, spleen, lung, and skin. These were at very low levels, with monkey cell ratios between 0.001–0.0001 among different tissues. Monkey cells were not found in the testis and ovaries, they added.

All 10 died within a week of being born, the reasons for which are unknown—although Hai said he thinks it is related to the IVF process. The team now hopes to create chimeras with higher proportions of monkey cells. Eventually, they want to create a pig with an organ made up of mostly monkey cells.

Concluding, they said: "Here, we have used monkey cells to explore the potential of reconstructing chimeric human organs in a large animal model… The findings could pave the way toward overcoming the obstacles in the re-engineering of heterogeneous organs and achieve the ultimate goal of human organ reconstruction in a large animal."

However, they conceded that this sort of research "still has a long way to go before clinical application is possible."

Representative image of a piglet. Two pig-monkey chimeras were born in China recently. Both died within a week. iStock