Is there any way to read this article without having to subscribe?
I have a WaPo digital subscription, Sunny, so I copy & pasted a bit for you. (It's too long for SF.) Note that it's 3-1/2 years old.
For the first time, scientists have grown an embryo that is part-pig, part-human.
The experiment,
described Thursday in the journal Cell, involves injecting human stem cells into the embryo of a pig, then implanting the embryo in the uterus of a sow and allowing it to grow. After four weeks, the stem cells had developed into the precursors of various tissue types, including heart, liver and neurons, and a small fraction of the developing pig was made up of human cells.
The human-pig hybrid ā dubbed a āchimeraā for the mythical creature with a lion's head, a goat's body and a serpent's tail ā was āhighly inefficient,ā the researchers cautioned. But it's the most successful human-animal chimera and a significant step toward the development of animal embryos with functioning human organs.
In a study published a day earlier, an international team of researchers demonstrated that organs for transplant can be grown in chimera embryos that are part-mouse, part-rat.
Writing in Nature, the researchers reported Wednesday that they were able to grow a mouse pancreas inside a rat embryo, then transfer insulin-secreting tissue from that organ into diabetic mice, alleviating their illness without triggering an immune response.
It was the first demonstration that such an interspecies organ transplant is possible. Researchers hope that one day doctors may be able to grow human tissue using chimera embryos in farm animals, making organs available for sick humans who might otherwise wait years for a transplant.
The technique is already the subject of a vigorous debate about the ethics of introducing human material into animals; since 2015, the National Institutes of Health has had a moratorium on funding for certain human-animal chimera research. (The new study was performed in California at the Salk Institute without federal funds.) Some argue that, since stem cells can become any kind of tissue, including parts of the nervous system, chimeras raise the specter of an animal with a human brain or reproductive organs. Others think there's a symbolic or sacred line between human and animal genetic material that should not be crossed.
The model for using chimeras for organ transplant would probably look something like the technique reported in Nature. In that experiment, researchers took
induced pluripotent stem cells (ordinary cells that have been reverted to an early embryonic state, so that they have the potential to develop into any tissue type) from mice. These cells were then injected into rat embryos that had been genetically modified so that they were unable to grow their own pancreas ā āemptying a nicheā for the mouse stem cells to fill.
The embryonic rats developed normally and were born healthy. Each had a rat-sized pancreas made of mouse cells. The whole pancreases were too big to transplant into tiny mice, so the researchers extracted just the islets ā the region of the pancreas that produces hormones like insulin ā and planted them in mice that had been induced to have diabetes.
Because the transplanted cells were grown from stem cells taken from mice, the animals required just five days of immunosuppressive drugs to keep their bodies from rejecting the new tissue. After that, they were able to live normally with healthy blood glucose levels for over a year ā half a lifetime in human terms.
The study showed that interspecies organ transplants are not only possible, but they can be done effectively and safely, said
Hiromitsu Nakauchi, a stem cell researcher at Stanford University and the University of Tokyo who is the senior author of the study.
Though researchers have had great success producing rat-mouse chimeras (top), it has been more difficult to achieve chimerism with human and pig cells (bottom). (Wu et al./Cell 2017)
The Cell study was the result of four years of work involving some 1,500 pig embryos. These embryos were not genetically modified, like Nakauchi's rat embryos, but the Salk scientists used a similar technique to inject human stem cells.
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Pigs are an ideal animal for chimera research, said co-author
Pablo Ross, an associate professor in the department of animal science at the University of California, Davis. Their organs are roughly the same size as those of humans (recall that the pancreases grown in Nakauchi's rats were rat-sized, even though they were grown with mouse cells), but they reach their full size far more quickly than humans and other primates.
āYou go from one cell [at] fertilization to 200 pounds, the average size of an adult [pig], in nine months,ā Ross said. āI think that's very reasonable, when you think about the fact that the average wait for a kidney transplant is about three years.ā
Scientists turned mouse skin cells into egg cells ā and made babies
Still, pigs' rapid gestation means that their organs develop much more rapidly than those of humans. If researchers want to create a successful chimera, they have to consider timing.
So Ross and his colleagues used three different types of stem cells for their experiment: ānaiveā cells that were at the very earliest stages of development, āprimedā cells that have developed further (but are still pluripotent), and āintermediateā cells that are somewhere in between.
Dozens of cells of each type were injected into pig embryos, which were then implanted in sows and allowed to develop for three to four weeks (about a quarter of a pig's gestation period). The primed cells never really took hold in the host embryo. The naive cells were initially incorporated into the growing animal, but were indistinguishable in the developing pig four weeks later.
The intermediate cells were most successful; by the time the embryos were removed from the sow and analyzed, about one in every 100,000 cells was human rather than pig, lead author
Jun Wu estimated. The human cells were distributed randomly across the chimera: Many wound up in what would become the heart (where they made up about 10 percent of tissue), some in the kidneys and liver (1 percent or less). A few developed into the precursors of neurons, a fear of bioethicists who worry about creating an animal with human or even humanlike consciousness.
But Izpisua Belmonte said that prospect is still a long way off. The contribution of human cells to the chimera was tiny, and research protocols were in place to prevent the development of any human-animal chimera to maturity.
āWe were just trying to answer the yes or no question of, can human cells contribute at all?ā he said. āAnd the answer to that question is yes.ā
, and you pass out, you get up, pass out again, until you just die. Similar to what would happen to you and weād both lose weight 
.
