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 The pigs, all 14 of them, are doing fine. Considering they’d been retrofitted with bone grown in a laboratory, that came as a pleasant surprise. 
 
 “The pigs woke up, and a half-hour later they were eating,” said Gordana Vunjak-Novakovic, a professor of biomedical engineering at Columbia University. “We thought they would be in pain. But no, they’re doing great.” 
 
 Dr. Vunjak-Novakovic and her colleagues have managed to create living bone from stem cells. First, they made a CT scan to create a 3-D image of each pig’s jaw. From cow bone, they sculpted a “scaffold” — a three-dimensional copy of the pig bone. 
 
 They put the scaffold in a nutrient solution along with stem cells extracted from the pigs. The cells attached to the scaffold, forming a new bone identical to the original. Then the researchers implanted the new bone in each pig. They reported their results in Science Translational Medicine. 
 
 There was no problem with rejection, and the new bone integrated with the pig’s own tissue. Clinical trials in humans, however, are at least three years away. ||||| Break your arm, and it will probably heal—but lose a big chunk of your chin, and that’s a different story. Now, researchers have made a significant advance toward replacing large pieces of bone in humans by growing a pig’s own cells into bone in the lab and then using it to restore a piece of missing jaw. 
 
 “Being able to translate into a large, functional animal model is an important step for this technology,” says Scott Hollister, a biomedical engineer at the University of Michigan, Ann Arbor, who was not involved in the study. But he says the new technique, now inching toward testing in humans, will still have to prove its advantages over other experimental bone regeneration approaches. 
 
 The face is an especially tricky testing ground for bone reconstruction. It takes precision to match our subtle features and symmetry, and bones in the jaw must be strong enough to withstand the force of chewing. Today, surgeons often repair facial damage by grafting a piece of bone from somewhere else, such as the patient’s pelvis or rib. But that procedure, called an autograft, creates a whole new bone injury that can be painful and slow to heal. The extracted bone is also tough to carve into the desired shape, and surgeons sometimes struggle to find a piece big enough to replace what’s missing. 
 
 Instead, researchers have been experimenting with ways to grow new facial bone, many of them based on synthetic scaffolds—porous structures shaped like the missing region that can be grafted onto existing bone to encourage the body to rebuild. Some experiments have implanted these empty scaffolds on their own and let new bone grow into them. Others have loaded scaffolds up with growth factors or cells that can transform into bone, or both. Several such techniques have made it to large animal testing—a key step in proving they’re worth trying in humans. But these approaches have involved adding precursor cells or growth factors to the scaffold right before it’s implanted and allowing the bone to develop inside the body. 
 
 Growing new bone in the scaffold for weeks before implanting it could have advantages, says Gordana Vunjak-Novakovic, a biomedical engineer at Columbia University and senior author on the new paper. It would give growth in the scaffold a kind of head start, so that the synthesis of bone matrix is more likely to keep up with the body’s natural process of degrading bone. And if cells are already part of a growing tissue, rather than a smattering of precursor cells recently injected into the scaffold, she says, they may be less likely to disperse out of the scaffold (and go to waste) after they’re grafted. 
 
 In the new study, Vunjak-Novakovic and her colleagues tested that idea in 14 pigs with facial bone damage. They cut a hammer-shaped chunk roughly 6 centimeters long out of the back of each pig’s jawbone—the vertical piece that extends up toward the ear and bears the most weight when an animal chomps down. To replace those chunks, the team first crafted scaffolds out of cow leg bone by removing all the living cells to leave behind a porous matrix of minerals and proteins. These were carved with a tiny computer-guided pin that shaved away material to match 3D images of each pig’s jaw, created from a computed tomography scan. 
 
 Next, the researchers sucked some fat from the pigs’ backs and isolated precursor cells that can become bone, known as mesenchymal stem cells. Forcing these cells—and culture medium to nourish them—into the tiny spaces of the scaffold is no easy task. By simply soaking the scaffold in the medium, “you’d get something that looks like M&M candy,” Vunjak-Novakovic says: Healthy bone cells might coat the outside, but they wouldn't penetrate to the center. So the team designed a silicon chamber cut to fit snugly around each scaffold so that the injected medium would be forced inside it instead of diverted around it. The cells were permeated with culture medium as they grew in this bioreactor for 3 weeks. 
 
 Six pigs received a personalized graft seeded with their own cells. To test the specific benefit of these cells, six other pigs received a custom-carved scaffold with no cells growing inside. The remaining two were left to heal on their own. Over 6 months, pigs in all three groups regrew a portion of the missing jaw, but regrowth was more complete in the seeded grafts, the researchers report online today in Science Translational Medicine . In tests of mechanical strength, only these grafts could withstand the same stress as the original bone, and only they reached and maintained the height of the original bone. The unseeded scaffolds, meanwhile, actually shrank slightly over time, which suggests the body was degrading bone matrix faster than it could be formed, Vunjak-Novakovic says. “This is convincing evidence that you do need provisional bone to get new bone.” 
 
 The use of a bioreactor to grow bone before grafting holds definite advantages, says Dietmar Hutmacher, a biomedical engineer at Queensland University of Technology in Brisbane, Australia, who was not involved in the study. But there is still debate over the precise recipe for a bone-stimulating scaffold. He suspects, for example, that adding growth factors known as bone morphogenetic proteins—a step this group avoided out of concerns for uncontrolled bone growth—would make for a stronger and more complete bone. 
 
 The group will also need to show that the technique is an improvement over other approaches, including the currently used autografts, Hollister notes. “There will be lots of questions whether, for example, growing [bone] in a bioreactor gives you an advantage.” Hollister and his collaborators have managed to regrow pig facial bone by adding precursor cells to a scaffold right when it’s implanted, with no cultivation ahead of time. 
 
 Vunjak-Novakovic, who founded a company called EpiBone in Brooklyn, New York, to commercialize the technology, is now planning to gather more evidence that the procedure is safe to try in humans. She hopes it will be in human trials in roughly 3 years. ||||| Injection of human iPS cells into a pig blastocyst. A laser beam (green circle with a red cross inside) was used to perforate an opening to the outer membrane (zona pellucida) of the pig blastocyst to allow easy access of an injection needle delivering human iPS cells. (Salk Institute) 
 
 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 — dubbed a “chimera” for the mythical creature with a lion's head, a goat's body and a serpent's tail — was “highly inefficient,” 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. 
 
 [Human embryo experiment shows progress toward 'three-parent' babies] 
 
 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. 
 
 But Vardit Ravitsky, a bioethicist at the University of Montreal's School of Public Health, said that the two studies published this week could help make a case for further human-animal chimera research by demonstrating the field's potential benefits. 
 
 “I think the point of these papers is sort of a proof of principle, showing that what researchers intend to achieve with human-non-human chimeras might be possible,” she said. “The more you can show that it stands to produce something that will actually save lives … the more we can demonstrate that the benefit is real, tangible and probable — overall it shifts the scale of risk-benefit assessment, potentially in favor of pursuing research and away from those concerns that are more philosophical and conceptual.” 
 
 [NIH may allow funding for human-animal stem cell research] 
 
 In an effort to address the world's growing organ shortage — an estimated 22 people a day die waiting for transplants, according to the U.S. Department of Health and Human Services — scientists have been trying to grow organs outside the human body. But organs developed in petri dishes are not identical to the ones that grow inside a living thing. 
 
 “That's where the rationale of this kind of experiment comes in,” said Juan Carlos Izpisua Belmonte, a developmental biologist at the Salk Institute and the senior author on the study of the human-pig chimera. “What if we let nature do the work for us? What if we just put human cells inside the embryo and the embryo knows what do to?” 
 
 
 
 An illustration of a potential process for harvesting human organs from pigs using chimera embryos. (Hiro Nakauchi) 
 
 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 — “emptying a niche” 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 — the region of the pancreas that produces hormones like insulin — 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 — 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. 
 
 “This is a form of transplantation we could do in the clinic with human patients someday,” he said. 
 
 [Stem-cell clinics face new scrutiny from federal regulators] 
 
 Nakauchi also conducts research on human-chimera embryos, but his efforts to inject human stem cells into sheep embryos have largely been unsuccessful — the evolutionary distance between humans and livestock may be making it difficult to get human stem cells to take hold in those animals. Other researchers have achieved human-mouse chimeras that developed to full size and grew to adulthood, but there is debate about how substantially human cells can contribute to mice, which are much more distantly related. 
 
 He said he was cheered to read the Cell study, which represents the most significant progress on human-animal chimeras yet, though the technique is still nowhere near ready for an experiment like the one performed in Nakauchi's mice. 
 
 “If you read the paper, the contribution of human cells is very limited, is very, very minor, and only in the early embryonic phase, so we’re still not sure if we can make human chimeras,” he cautioned. “But I'm glad that they're doing this research.” 
 
 
 
 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. 
 
 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. 
 
 “You go from one cell [at] fertilization to 200 pounds, the average size of an adult [pig], in nine months,” Ross said. “I think that's very reasonable, when you think about the fact that the average wait for a kidney transplant is about three years.” 
 
 [Scientists turned mouse skin cells into egg cells — 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: “naive” cells that were at the very earliest stages of development, “primed” cells that have developed further (but are still pluripotent), and “intermediate” 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. 
 
 “We were just trying to answer the yes or no question of, can human cells contribute at all?” he said. “And the answer to that question is yes.” 
 
 
 
 Salk Institute scientists Jun Wu (seated) and Juan Carlos Izpisua Belmonte, authors of the new Cell paper. (Salk Institute) 
 
 The Cell study researchers also discussed progress with rat-mouse chimeras. Though they have not performed an interspecies organ transfer, they were able to grow hearts, eyes and pancreases in chimeric embryos. They also grew a rat gall bladder inside a mouse embryo, even though rats don't grow gall bladders during normal development — suggesting that rats have the genetic coding for gall bladders but those genes are suppressed by their developmental environment. 
 
 That's another important aspect of chimera embryo research, Izpisua Belmonte said, one that is sometimes overlooked in the focus on organ transplants. Chimera embryos can be used to understand development, examine genetic diseases and test drugs without risking the health of humans. 
 
 In August, NIH released a draft of a policy that would change the guidelines to allow funding of certain human animal chimeras. Under the proposed new rule, the taxpayer funds could be used for experiments that introduced human stem cells to early stage embryos of all animals except other primates. Some nonhuman primate research would also be allowed, but only using embryos at later stages of development and only after an extra layer of review by a special NIH committee. But the policy change is still under review. 
 
 Neither Nakauchi's nor Izpisua Belmonte's study was funded by NIH grants. Nakauchi said he hoped that recent progress in the field might garner support for easing the ban. 
 
 “Finally we’re able to provide a proof of principle that ... this approach of making organs … is possible and also safe and efficient,” he said. “So I hope people will understand this.” 
 
 He continued, “Many people think this is a kind of science fiction story. But this is becoming reality.” 
 
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– For the first time, scientists have successfully implanted living bone grown in a laboratory, Live Science reports. And while those implants were in miniature pigs, not humans, it's pretty impressive nonetheless. Scientists removed part of the jaw bones of 14 pigs, carved cow thighbones into the right shape, removed all the cells from the cow bones, saturated the cow bones with stem cells from the pigs, and left the bones to grow in the lab. When the new, living bones were implanted in the pigs, they showed impressive regrowth and returned the jaw to full strength, according to Science Magazine. “The pigs woke up, and a half-hour later they were eating,” researcher Gordana Vunjak-Novakovic tells the New York Times. “We thought they would be in pain. But no, they’re doing great.” The researchers published their findings this month in Science Translational Medicine. They want to try the process in humans in the next few years. At the moment, no great options exist for people who need replacement bones due to injuries, birth defects, or diseases. There are titanium replacements or donated bones, but those come with the risk of rejection. Doctors replacing facial bones currently take the bone from elsewhere in the patient's body, but that causes a new injury, and it can be hard to find a large enough chunk of bone to borrow. Prior methods of bone regrowth have involved letting the new bone grow inside the body, but researchers say growing it first in a lab could have a number of benefits. (Scientists hope to grow human organs in farm animals.)