At the moment, there is a shortfall of organ availability and, according to NHS Blood and Transplant, nearly 460 people died waiting for organs in the United Kingdom in 2016 - not including those who died after the transplant, because their bodies rejected the organ. Robin Lovell-Badge, head of the division of stem cell biology and developmental genetics at the Francis Crick Institute in London, said there is a chance the human body could reject a transplant organ grown in an animal as it might contain blood vessels of the host, according to a report in The Guardian. The initial goal is for the sheep's cells to be 1% human.
Pig-human hybrid embryo from earlier research, in early 2017.
Researchers say that this could lead to the first transplant in humans within five years, which would be a major development for many people on the waiting list for potentially life-saving transplants.
By introducing human stem cells into sheep embryos, the Stanford scientists have grown a hybrid creature that is more than 99 per cent sheep.
Do you agree that chimeras like the sheep-human hybrids must be used to produce organs for people?
Xenotransplantation of human tumor cells into immunocompromised mice is a research technique frequently used in pre-clinical oncology research. In time, as scientists breed hybrids with a higher and higher fraction of human cells, such chimeras could have their organs harvested for human transplants.
"I have the same concerns", said Ross, noting that the team is looking at where the human cells end up in the chimera.
There are a number of advantages in using pigs and sheep to grow human organs. They can be easily produced by in vitro fertilization (IVF) and require fewer embryos to be transplanted into an adult than pigs.
While the small percentage of human in the sheep was destroyed after 28 days, the slippery slope arguments still remain. The team has shown that they are capable of focusing the human cells to be able to construct specific organs.
The scientific breakthrough was announced at the American Association for the Advancement of Science meeting in Texas this week. But meanwhile, with each new iteration, the ethical debate will only intensify.
The authors emphasize that while it remains unclear exactly how human cells can affect the appearance of the sheep, and in their nervous system.
"The contribution of human cells so far is very small", stem cell biologist, Hiro Nakauchi from Stanford University, said at the press briefing.
Breeding animals for the sole objective of harvesting their organs may seem cruel and vile.
"Growing an organ in a lab has proven to be a very hard task but animals grow their organs every time and they always do it right". Certainly, this is no easy discussion.
"All of these approaches are controversial and none of them are flawless, but they offer hope to people who are dying on a daily basis", said Ross. "We need to explore all possible alternatives to provide organs to ailing people".