A needle is injects the human cells into the pig embryo Salk Institute / Juan Carlos Izpisua Belmonte

Cells from a human have been combined with those of a pig for the first time to create a hybrid chimera. The work could eventually lead to the development of organs that can be grown in an animal and transferred to a human.

The chimera, named after the Greek mythological creature made of various different animals, was made by inserting human stem cells into a pig embryo.


A variety of different stem cells were tested in the pig embryo but "intermediate" human pluripotent stem cells survived the best, the scientists behind the work explained. Once the stem cell was placed into the pig embryo it was implanted into a sow and left to grow for three to four weeks. Of 2,075 embryos that were implanted, only 186 developed to the 28-day stage.

"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," Juan Carlos Izpisua Belmonte, a leading researcher on the work said in a statement.

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The work form Izpisua Belmonte and colleagues at the Salk Institute was published in the Cell journal.

Before the researchers created the human/pig chimera they added rat cells into mouse embryos to create a rat/mouse chimera. Using CRISPR genome editing tools the Salk team was able to delete specific genes from fertilised mouse egg cells.


Salk Institute / Juan Carlos Izpisua Belmonte

"In a given cell, they would delete a single gene critical for the development of an organ, such as the heart, pancreas, or eye," the researchers wrote. The equivalent rat cell to the deleted mouse gene was then inserted into the hybrid embryo and left to mature. Inserted cells grew to form functional tissues in the heart, eye and pancreas. Rat cells also grew to form a gall bladder in the mouse, even though rats don't have the organ.

Jun Wu, a biologist working on the project, said: "The rat cells have a functional copy of the missing mouse gene, so they can outcompete mouse cells in occupying the emptied developmental organ niches."


The work is related to a recent study where organs were grown in one animal and then used in another. A research paper published in the journal Nature showed how a functioning pancreas grown in a rat could be inserted into a mouse and cure diabetes, which had been induced by the researchers.

“If we could use a patient’s own cells to generate a matched organ in a large animal like a sheep or pig, it is possible that we could begin to alleviate this shortage while also relieving transplant recipients from a lifetime of immunosuppressive drugs that are necessary when an unmatched organ is used,” Hiromitsu Nakauchi, the Nature paper's author, told Gizmodo.

The feat of creating organs in animals and transferring them to humans is also the goal of the researchers who created the human-pig chimera. "The ultimate goal is to grow functional and transplantable tissue or organs, but we are far away from that," Izpisua Belmonte said. "This is an important first step."