Pig embryos could provide a future solution to the insufficient supply of human organs for transplant.Pig embryos could provide sources of new organ and tissue transplants for people, thanks to new findings by Israeli researchers.

For more than two decades, scientists have pointed to the potential of embryonic pig tissues as a source for organ transplantation. But studies in the past have had little success when tissue has been taken relatively late in a pig embryo’s development. The new Israeli study shows that, for the transplantation to be successful, the stem cells need to come from specific stages of an embryo’s development.

Scientists at the Weizmann Institute of Science have determined distinct gestational time windows for the growth of transplanted pig embryonic liver, pancreas and lung precursor tissue into functioning organs in mice. They found that if cells were taken from pig embryos at precisely the right time, they grew into liver, pancreas and lung tissues in mice. These findings – which appeared in the Proceedings of the National Academy of Sciences – could help enhance the chances for successful implementation of embryonic pig tissue in the treatment of a wide spectrum of human diseases.

The study, led by Prof. Yair Reisner of the Institute’s Immunology Department, involved the extraction of embryos from sows at various stages of pregnancy and implantation of organ-committed cell tissue into immunodeficient mice.

“By implantation of pig embryonic tissues into immune-deficient mice, we have now determined for the first time distinct gestational time windows for the growth of transplanted pig embryonic liver, pancreas, and lung precursor tissue into functioning tissue,” Reisner told National Geographic.

Many researchers are working on the possibility of animal to human transplants, also called xenotransplants. Pigs are a popular choice because they are about the same size as humans and have a similar physiology. But there are big hurdles, not the least of which is massive rejection by the human body of animal tissue.

Reisner noted that other researchers are working to genetically engineer pigs that do not carry the protein responsible for the immune response. And embryos may have less of it, he added.

“We anticipate that immune suppression required might be less intense compared to that employed in the context of adult tissue transplantation,” he said.

Reisner and colleagues said their findings also help explain why earlier experiments did not work. If transplanted at the wrong time, they found the pig embryo cells grew into tumors called teratomas instead of the desired tissue.

“Disappointing results in past transplantation trials may be explained, at least in part, by these results,” said Reisner.

To identify when to harvest the embryonic cells, the scientists took embryonic pig tissue that had begun to form particular organs at various developmental stages and transplanted them into mice.

They studied three types of organs – liver, pancreas, and lung – and found optimal time windows for transplantation for each organ.

“What he has shown is that there’s a window of opportunity,” said Bernhard Hering at the Diabetes Institute for Immunology and Transplantation at the University of Minnesota.

Researchers say the supply of human organs will always be insufficient to satisfy demands, making xenotransplantation – the act of transplanting organs or tissue between two species – an attractive alternative.

Not only is there a shortage of whole organs for transplant, but tissue transplants could be used to treat diseases such as diabetes, Parkinson’s and liver failure.

“Considering the ethical issues associated with human embryonic stem cells or with precursor tissue obtained from human abortions, we believe that the use of embryonic pig tissue could afford a more simple solution to the shortage of organ donors,” Reisner told Reuters.

In previous studies, Reisner’s group demonstrated that transplanted human and pig kidney embryonic tissue could grow into miniature, functioning human or pig kidneys inside a mouse. His novel approach was a matter of timing: gestational age proved to be the key to successful kidney growth from transplanted embryonic tissue.