"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," said Yair Reisner of the Weizmann Institute of Science, who led the study.

Writing in the Proceedings of the National Academy of Sciences, 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.

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. Experiments have been done using cells from human fetuses as well as from animals.

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 in an e-mail.

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

It may be easier to work with embryonic cells than with more mature tissue, Reisner added.

"The great advantage of embryonic tissue implantation is that it does not require any manipulation ex-vivo (outside of the body). The precursor tissue is simply implanted and grows in the recipient," he wrote.

The other hurdle involves porcine endogenous retroviruses, found in pig cells. While they are likely to be found in embryonic tissue he said, he believes they are unlikely to infect a human transplant recipient.