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. [...] 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. 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. [...]
In Platt's experiments, the human stem cells were injected into the pig fetuses about a third of the way through gestation. The injections must be given after the body plan of the fetus has developed, but before the immune system is active. The former ensures the animals look like normal pigs and sheep. The latter prevents the human stem cells being rejected.
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