Patients requiring organs face lengthy waits for donors, but a medical breakthrough has potentially put us a step closer to using pigs and sheep instead.
Scientists in the US have successfully transplanted cells vital for fighting off diabetes from a rat to a mouse.
Mouse stem cells were injected into rat embryos, which were engineered to be unable to make their own pancreas. After they'd had developed into pancreatic islet cells, they were removed and put into diabetic mice.
Pancreatic islets create insulin, which is needed to maintain the body's glucose levels. Type 1 diabetes is when the body's immune system has destroyed the islets' beta cells, so they can't make insulin.
After only five days of medication to suppress the immune system's efforts to reject the implant, the mouse bodies accepted the rat-grown cells as their own. They were able to maintain blood glucose levels for over a year - about as long as mice are expected to live in the wild.
"This proof-of-principle study demonstrates how the organs of one species can be grown inside the body of another, a method that could one day aid the production of transplantable human tissue," the researchers, led by Hiromitsu Nakauchi, said in a statement.
"Many technical challenges and ethical and legal questions would first need to be addressed."
Rat stem cells grown in mice then transplanted back didn't work, because the cells only grew big enough to serve mice, which are smaller than rats.
"Organs would need to be generated in animals that are close to humans both in size and evolutionary distance, such as sheep, pigs or non-human primates," the researchers say.
Mice and rats look similar, but have evolved separately for the last 18 million years.
Similar experiments are underway at the University of California, where scientists are putting human stem cells into pig embryos.
So far, none have been transplanted back into humans - the embryos are routinely destroyed after a few weeks of gestation, to prevent the hybrids - or chimeras - from developing.
The average waiting time for an organ in New Zealand is about seven months, according to Ministry of Health figures collected in 2014.
Wait times vary depending on the patients' requirements. People needing kidneys can expect a 32-month wait, while those needing lungs only five months, and livers, only four.
The latest breakthrough was detailed on Thursday in the journal Nature.