One approach to organ engineering is to create lineages of chimeric pigs, a species with organs of a similar enough size and shape for transplant into humans, in which the organs of interest are made up of human cells rather than pig cells. This may or may not turn out to be harder than the alternative of taking ordinary pig organs, decellularizing them, stripping them of harmful remaining proteins, and then repopulating the remaining structure with human cells. There is some degree of irrational hysteria surrounding the creation of chimeras, which only makes the real challenges harder to surmount. Still, it seems to me that these are stopgap technologies that will have a short practical life span. They will be overtaken in cost and efficiency by the ability to generate organs from a patient's own cells in a bioreactor:
Braving a funding ban put in place by the NIH, some U.S. research centers are moving ahead with attempts to grow human tissue inside pigs and sheep with the goal of creating hearts, livers, or other organs needed for transplants. Based on interviews with three teams, two in California and one in Minnesota, it is estimated that about 20 pregnancies of pig-human or sheep-human chimeras have been established during the last 12 months in the U.S., though so far no scientific paper describing the work has been published, and none of the animals were brought to term.
The extent of the research was disclosed in part during presentations made to the NIH at the agency's request. One researcher showed unpublished data on more than a dozen pig embryos containing human cells. Another provided photographs of a 62-day-old pig fetus in which the addition of human cells appeared to have reversed a congenital eye defect. The experiments rely on a cutting-edge fusion of technologies, including recent breakthroughs in stem-cell biology and gene-editing techniques. By modifying genes, scientists can now easily change the DNA in pig or sheep embryos so that they are genetically incapable of forming a specific tissue. Then, by adding stem cells from a person, they hope the human cells will take over the job of forming the missing organ, which could then be harvested from the animal for use in a transplant operation.
Other kinds of human-animal chimeras are already widely used in scientific research, including "humanized" mice endowed with a human immune system. Such animals are created by adding bits of liver and thymus from a human fetus to a mouse after it is born. The new line of research goes further because it involves placing human cells into an animal embryo at the very earliest stage, when it is a sphere of just a dozen cells in a laboratory dish. This process, called "embryo complementation," is significant because the human cells can multiply, specialize, and potentially contribute to any part of the animal's body as it develops. In 2010 researchers used the embryo complementation method to show that they could generate mice with a pancreas made entirely of rat cells. "If it works as it does in rodents, we should be able have a pig with a human organ."