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Summarizing, the process works by using injected human pluripotent stem cells into the developing fetus. These stem cells have not been designated as heart or liver cells, e.g., instead they are assigned roles at a later date. So to the host pig, these cells are just like any other that have grown along with the rest of the body. As the fetus develops, the human stem cells will propagate, grow, and eventually make up a good portion of the pig's total cell count. In the scientists' experiment as detailed in the journal Nature, the ratio of human to pig cells was 1:100,000 which is an extremely high proportion if you factor in the sheer quantity of matter that makes up even a simple organism.
But as of right now, this does nothing. The pig would develop just like any other even if it was 0.00001% human (that we know of, and that is if scientists were allowed to observe the pig through postnatal development, since the fetuses must be destroyed per ethical requirements. We don't want human pigs running around even if they are just technically pigs.) However, it does represent a huge change in regards to organ transplantation for two reasons:
Firstly, and most notably, we learned in biology that most vertebrate animals have a unique protein which attaches to the exterior to all of their cells, that uniquely identifies that person. This is involved in the immune system, where if the body recognizes that a virus or bacteria does not have the correct protein that the rest of the body has, it will attack it to prevent infection. However, in this experiment, the human cells, which would clearly have a different protein than the pig fetus, remained unrejected by the body. One would assume that this can take place because the pig fetus is so young, that its immune system has not developed in full yet. Although this is true, scientists at the Salk Institute for Biological Studies performed this experiment but with a rat-mouse hybrid and let it grow into an adult. Sure enough, the adult had no problems related to rejection (more on that later). And on the same note, if applied in humans, the head transplant we discussed earlier seems a tad more feasible (albeit not by much, at all--it's really not feasible still). As for sci-fi hopes, Shelley's dream comes into full swing. Had Frankenstein's monster been created as Shelley described, all of the individual parts of the monster would reject each other (that is, if the monster even had an immune system that worked properly and in unison) immunologically, and would likely have to rely on a steady supply of immunosuppressants which the thing never got. If Frankenstein could have been conceived with body parts that do not recognize those identifying proteins, the chances his body parts work together seems more probable, but he would still suffer from a deadly, AIDS-like vulnerability to common illnesses because of a lack of a functioning immune system.
Much more importantly, the experiments prove a theoretical way to grow new organs, although human tests are far off. Now back to that rat-mouse. The scientists at Salk Institute, using CRISPR technology, included the genes primarily of mouse vital organs in the mouse stem cells that were injected into the rat embryo. And since there really are little ethical protections for rats and mice, they could let this organism grow to adulthood (the lab rats deserve more credit for enduring what we put them through). What they found is peculiar, that some of the organs that grew were mouse-like and the rest specific to the rat species. So in essence, we grew mouse organs in a rat. Growing new organs for those in need has been a huge subject of debate, and this experiment has only fanned the flame; it creates another opportunity for sourcing organs but feeding off animals may not be the most desired approach. Regardless, it provides another chance to make Frankenstein a future reality. Instead of assembling a lifeform out of decaying matter, dead cells, and all that, how about growing all of the organs, fresh and having a healthier-looking (I guess if you want to call it "healthy") sewn-up being? Had all of the body parts been created in this fashion, perhaps Dr. Frankenstein may have taken a different approach to his creation, instead of the horrified and disgusted one that consumed him.
Anyway, this is becoming more and more of a possibility for organ transplantation, and can save countless lives once the technology advances. Researchers are exploring possible methods to turn pigs into organ donors for humans by basically "turning off" all of the pig proteins in the organism that could harm humans, again using CRISPR-Cas9 gene editing practices. In any case, we can only hope the new technology is used justly.
