For the first time, scientists have demonstrated that growing humans organs in other species could be possible.

In a study published in the journal Cell this month, researchers successfully injected human stem cells into a pig embryo. While the developing embryo was "highly inefficient," the stem cells developed into the precursors of heart and liver cells. A portion of the developing embryo was comprised of human cells.

Even though that embryo — dubbed a "chimera" would not have survived, it's the most successful combination of two large, distinct animals to date.

"The ultimate goal is to grow functional and transplantable tissue or organs, but we are far away from that. This is an important first step," said Juan Carlos Izpisua Belmonte who led the experiment at the Salk Institute for Biological Studies in La Jolla, California.

Pigs are good candidate for human chimera experimentation because their organs are the roughly the same size. Even so, the problem with growing human cells inside the embryos of other animals is the gestational timeline.

Pig cells develop much quicker than those of humans. In future experiments, researchers will try to "turn off" pig genes to give targeted human cells (such as the heart) an advantage to grow into fully developed organs.

However, the ethical implications of such a process are the subject of intense debate. The National Institutes for Health (NIH) decided last year to stop funding research into human-animal chimeras while questions about the ethical ramifications are considered.

One of the arguments against such research is the potential to create hybrid animals, such as one with a human brain or reproductive organs. Others say animals and humans should never be crossed no matter how good the intent.

Izpisua Belmonte says any lab-created chimeras would be highly controlled products rather than "mythological" creations. Even so, he acknowledged the feelings of uncertainty such research can elicit.

"The idea of having an animal being born composing of human cells creates some feelings that need to be addressed," Belmonte said.

That sentiment was echoed by Dr. Daniel Garry, a cardiologist who leads a different chimera project at the University of Minnesota. Garry said the mere possibility of human chimeras raise ethical questions like:

  • Will the resulting progeny look human or pig?
  • Could human cells overtake pig cells, resulting in mostly human, but slightly piggish offspring?
  • Could a pig with human thoughts be created?

"These more fantastical possibilities are not a problem in reality," he said.

Vardit Ravitsky, a bioethicist at the University of Montreal's School of Public Health, said the research could be furthered if the potential benefits are clearly highlighted.

"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. Ravtisky said. "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."