Human-Pig Chimera Alert — Human Muscle Tissue Growing in Adult Pigs

“The generated intraspecies chimaeras were viable and displayed normal histology, morphology and function. Human:pig chimaeras generated with TP53-null human induced pluripotent stem cells led to higher chimaerism efficiency, with embryos collected at embryonic days 20 and 27 containing humanized muscle, as confirmed by immunohistochemical and molecular analyses. Human:pig chimaeras may facilitate the production of exogenic organs for research and xenotransplantation.”

Science-speak is often so confusing that it makes seemingly horrific things less so, but one phrase in the opening abstract of a new study jumps out in a way that anyone can understand – “Human:pig chimaeras.” The chimera (or chimaera) of Greek mythology was a monstrous fire-breathing hybrid creature made from parts of a lion, goat and serpent. Modern-day chimeras are creatures, organisms or tissue that are one body with two sets of cells and two different sets of DNA. A whole-body chimera is created when two embryos merge at a very early stage and the two embryos develop as one being. This can occur in humans when a fetus absorbs a dead fraternal twin. For comparison, a hybrid is the offspring of two different species with each of their cells containing DNA from both species — mules (horse-donkey) and ligers (lion-tiger) are examples.

Classic chimera

Human-pig chimeras in the U.S. were first created in 2017 by placing human cells in pig embryos. In a few, the cells grew into muscle and organ cells before the embryos were destroyed. The goal was to eventually grow human organs in pigs for transplanting, and more experiments involving human-rodent and human-chimpanzee chimeras allegedly occurred in China and Japan. On the other hand, these experiments in the U.S. have been tightly regulated and are quite rare. Recently, researchers at the University of Minnesota focused on some less complex but just as needed tissue – muscles to replace those lost in accidents, combat or surgical procedures. That focus resulted in the creation of more human-pig chimeras with one new twist – the embryos were allowed to grow to full term!

“It is important to note that we demonstrate that the human donor cells are located only where the pig skeletal muscle [now genetically deleted] once was. The human donor cells do not migrate to the brain or to the reproductive cells of the pig.”

Mary Garry, associate professor of medicine at the University of Minnesota’s Cardiovascular Division and co-author of the study published in the journal Nature Biomedical Engineering, answers in Inverse the chimera-elephant-in-the-room question – the human stem cells were implanted in pig embryos that had the genes required to develop pig skeletal muscle tissue removed via CRISPR gene editing. Because of that, they never strayed to the pigs’ brains or reproductive organs. Also, these were adult-derived stem cells, not embryonic stem cells.

You know what this means?

That still left a lab full of human-pig chimeras running around. Transplanting their human muscles is not yet technologically feasible nor ethically approved — Garry estimates that both will happen in the next 3 to 5 years in the U.S. What about outside the U.S.? We’ve already seen that regulations don’t matter in some countries and money talks in just about all of them. Could human-pig chimeras be supplying organs already somewhere else? That needle is moving quickly on the probability meter. Organ transplants of all kinds are in short supply, even in countries with illegal or look-the-other-way harvesting. Do the benefits outweigh the risk of unethical development or an accidental escape into the normal population? Is there a twisted scientist with the goal of creating a real Orwellian “Animal Farm” or Seinfeldian Pig Man?

Stranger things have happened … and they’re not going to stop.

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