Scientists have successfully grown sheep embryos containing human cells, taking a step towards a method for growing human organs in animals for transplantation. 

The research team introduced human stem cells into early sheep embryos, creating hybrids which were allowed to develop for 28 days. The team are already working on genetically modifying animal embryos so they can't develop certain organs for themselves. If human stems cells are introduced into such embryos, the hope is that they will develop to replace the missing organ, for example producing a human pancreas inside a host sheep. 

This latest research, reported at last week's meeting of the American Association for the Advancement of Science in Austin, Texas, is part of a wider ongoing investigation into the possibility of animal-grown human organs. 

'We have already generated a mouse pancreas in rats and then transplanted those into a diabetic mouse and were able to show almost a complete cure without any immunosuppressants,' said Dr Hiro Nakauchi of Stanford University, California, one of the lead authors of the new study. 

These embryos are not the first human-animal hybrids: human-pig hybrids have already also been created, producing embryos with one human cell in every 100,000. This most recent advance has achieved a higher ratio of human to animal cells. 

'About one in 10,000 cells in these sheep embryos are human, [but] we think that that's still not probably enough to generate an organ,' said Dr Pablo Ross from the University of California, Davis, who is working with Dr Nakauchi on the project. The Guardian reports that around 1 percent of the embryo's cells may have to be human for an organ to grow successfully.

Sheep and pigs are suited to hosting developing human organs because their own are a similar size and function in a similar way, according to Dr Nakauchi. There are several advantages to using sheep embryos in particular. 'For a pig we typically transfer 50 embryos to one recipient [but] with the sheep we transfer four embryos to one recipient,' said Dr Ross. This means fewer embryos are needed for one experiment.

Using animals as hosts for developing human organs for transplantation could reduce both organ waiting lists – nearly 500 people died last year in the UK while waiting for a transplant, according to NHS figures – and organ rejection rates, as the patient's own cells could be used in the procedure, meaning the organs will be genetically compatible with the patient receiving them.

'Even today the best matched organs, except if they come from identical twins, don't last very long because with time the immune system continuously is attacking them,' said Dr Ross. 'We need to explore all possible alternatives to provide organs to ailing people.' 

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Reproduced from BioNews with permission, a web- and email-based source of news, information and comment on assisted reproduction and human genetics, published by Progress Educational Trust.