Pigs grown from fetuses into which human stem cells were injected have surprised scientists by having cells in which the DNA from the two species is mixed at the most intimate level.
It is the first time such fused cells have been seen in living creatures. The discovery could have serious implications for xenotransplantation - the use of animal tissue and organs in humans - and even the origin of diseases such as HIV.
The adult pigs that had received human stem cells as fetuses were found to have pig cells, human cells and the hybrid cells in their blood and organs.
"What we found was completely unexpected. We found that the human and pig cells had totally fused in the animals' bodies," said Jeffrey Platt, director of the Mayo Clinic Transplantation Biology Program.
The hybrid cells had both human and pig surface markers. But, most surprisingly, the hybrid cell nuclei were found to have chromosomal DNA that contained both human and pig genes.
The researchers found that about 60 per cent of the animals' non-pig cells were hybrids, with the remainder being fully human.
Importantly, the team also found that porcine endogenous retrovirus (PERV), which is present in almost all pigs, was also present in the hybrid cells. Previous laboratory work has shown that while PERVs in pig cells cannot infect human cells, those in hybrid cells can. The discovery therefore suggests a serious potential problem for xenotransplantation.