What is a xenotransplant?
A xenotransplant is a transplant between species. Transplanted organs are called grafts, hence a xenograft is an organ transplanted from one species to another.
What is a species?
The barrier that defines a species is whether reproduction is possible. A dog and a pig cannot mate and succesfully produce offspring, therefore they are a different species and a transplant from one to the other would be called a xenotransplant.
A collie and a labrador retriever can mate and produce offspring, therefore they are the same species and a transplant from one to the other would not be called a xenotransplant.
A transplant between two genetically different members of the same species is called an allotransplant. A transplant between members of the same species that are genetically identical (inbred animals or identical twins) is called an isotransplant. And a transplant from one person to themselves (for example moving bone from the hip to the back to fix a broken vertebra) is called an autotransplant.
What research is being done in the area?
Extensive research into xenografting is ongoing throughout the world. TheNIH gopher search engine will allow you to find out the specifics about all xenograft projects that are currently funded by the National Institute of Health. There were two xenograft projects funded in 1994, and four the year before. In addition there is work being done overseas, especially in Great Britain, and at private corporations in the United States.
What xenotransplants have been done?
There have only been a few attempts at human xenografting over the years, but no human solid organ xenograft projects are currently approved by the FDA. "Baby Fae", a child born with a malformed heart survived for a short period of time with a baboon heart. Two men were transplanted with livers from baboons at the University of Pittsburgh. These patients lived for several weeks. And in late 1995 a man with AIDS was transplanted with the bone marrow of a babboon. He is still alive and doing well as of April 1, 1996, but there is not yet evidence that the baboon cells are helping his immune system.
Do xenografts work?
Currently xenografts are not very succesfsul compared to allografts. However, the shortage of organs has prompted continued research into this area. Thousands of people die each year due to the shortage of human organs.
What are the problems with xenografts?
As with allografts, the major problem is with rejection. Humans have "natural antibodies" that circulate in the blood and cause immediate graft failure when organs from some species are transplanted (for example pig organs). Also, a system of proteins in the body called "complement" are activated whenever pig organs are transplanted into primates, leading to severe systemic toxicity. This may be because the primate complement regulatory systems do not recognize the porcine complement. Genetic engineers are trying to get around this problem by genetically modifying pigs so that they have some complement regulatory proteins on their cells.
Will xenografts ever be commonplace?
There is no way to answer this question since there are too many scientific barriers right not to know if xenotransplants can ever be made to work well. Some scientists and transplant surgeons are very optimistic that xenografts are "just around the corner", while others think they will never be a succesful technique.
What are the ethical aspects of using animal organs to save human lives?
Animal tissues have been used as grafts to humans for decades. Heart valves from pigs have been used succesfully to replace damaged heart valves in humans with great success. Blood vessels from cows were used to replaced damaged blood vessels in humans prior to the development of synthetic materials such as Dacron and Goretex.
There are two major ethical concerns regarding xenografts: the concern that xenotransplantation will introduce viruses into the human population and the concern that animals have the same "rights" as humans do and therefore should not be put to death, even if it saves human lives.
Regarding the first question, this area is currently being hotly debated. No one knows whether transplanting animal organs into humans can possibly result in the outbreak of an animal virus in the human species. Noted authorities in virology are concerned about the possibility, but no one really knows if it is likely or not,and many scientists feel it is very unlikely. The National Institutes of Health is currently studying the subject and should issue guidelines or a report at some point. The chief concern is the use of primate organs, pig organs should pose much less of a threat.
Regarding the animal "rights" concern: although small groups of people have annoyed serious researchers in this country and Europe for decades, this opposition is generally taken to be extroidinarily naive. The fact is that our society values human life and we have customs and laws to protect humans. However, we eat animals for food and use their skins and other products for our shoes and warmth. It seems perfectly consistant with these customs and beliefs to use a pig liver to save a human's life. The issue with primates is more heatedly debated since the intelligence level of some monkeys rivals that of some humans. However, primates are not ideal as donors anyway for other reasons including the small litter size, the long latency before pregnancy is possible, and the high frequency of viral infection among primates. Thus, most xenotransplant researchers are concentrating on animals other than primates.
Where can I learn more about Xenotransplants?
"The Why Files" web site has an informative article on xenotransplantation entitled "An answer to the Transplant-Organ Shortage".
See also the FDA (Federal Drug Administration, USA) 9/20/96 Fact Sheet on Xenotransplantation, and "Organ Transplants from Animals: Examining the Possibilities".