Scientists suggest it might one day be possible to grow an animal designed to provide replacement organs for individual patients.
'Chimaera' pigs were created
Injecting bone marrow cells into a sheep foetus produces a "chimaera" - with a blend of human and sheep cells.
US researchers say that, for example, human liver cells could be harvested later to boost a failing organ.
Whole organs taken from these animals might be less likely to be rejected by the body, New Scientist reports.
There is an urgent need for transplant organs, and organs from animals are one potential source.
So far, this has been ruled out, mainly because of safety concerns about animal viruses which might have unknown harmful effects in humans if transferred along with the transplant organ.
Researchers also have to overcome the problem of rejection - the immune system is likely to attack the new organ, meaning that patients would need to spend a lifetime on powerful anti-rejection drugs.
At the University of Nevada in Reno, scientists believe they have found a way to at least partly overcome the rejection issue.
Their idea is to take "stem cells" found in the bone marrow of the human patient and inject it into an animal foetus growing inside the womb.
The idea is that these human cells would be incorporated into the growing foetus, forming part of virtually every tissue, and be present in greater numbers once the animal was fully grown.
As these are human cells, there would be no problem with rejection.
The Nevada team, led by Dr Esmail Zanjani, produced sheep human chimaeras with a high proportion of human cells in some organs.
In some cases between 7% and 15% of all the cells in the sheep's livers are human.
In the liver, for example, the team believes that it might be possible to extract clusters of human cells and use them in transplantation operations.
Their sheep human hybrids also contain high proportions of human heart cells.
The sheep appear biologically normal from the outside, despite their cell make-up internally.
Even organs which are not exclusively composed might be suitable for transplantation - and produce fewer problems with rejection.
Dr Alan Flake, from the Children's Hospital in Philadelphia, told New Scientist that the technique could work.
He said: "I don't think that in 10 to 15 years that's out of the question."