Scientists in the United States have injected human stem cells into pig embryos to produce human-pig embryos.  The goal is to grow human organs inside pigs to answer the worldwide shortage in donor organs for life-saving transplant.

The researchers at the University of California at Davis start with a newly fertilized pig embryo .  They use a technique called CRISPR to take out the gene sequence that would allow the pig to crow a pancreas.  Then, human induced pluripotent (iPS) stem cells are injected in the void. 

The pigs, known as "chimeras" to the UC Davis scientists, are hoped to look and behave like normal pigs once the experiment comes to fruition - except that one organ inside the beast will be composed of human cells.  In the latest phase of the test, the chimeric embryos are being allowed to develop in the sows for 28 days before the pregnancies are terminated and the tissue removed for analysis.

"Our hope is that this pig embryo will develop normally but the pancreas will be made almost exclusively out of human cells and could be compatible with a patient for transplantation," said UC David Professor Pablo Ross, a reproductive biologist who is leading the research.  

The ethics of these experiments are under scrutiny, and not everyone approves of the idea.  Last year, the US National Institutes of Health (NIH) banned government funding for such research. 

And there are some concerns that the human stem cells may migrate once inside the pig's body, impacting the development of other organs - even the brain - and creating a hybrid with uncomfortably human characteristics, such as in the H.G. Wells story "The Island Of Dr. Moreau".

"We think there is very low potential for a human brain to grow," said Professor Ross, "But this is something we will be investigating."