Scientists in Japan will develop creatures with both animal and human cells after the government agreed to the controversial research.
The research by a University of Tokyo team led by Hiromitsu Nakauchi will grow human cells in rat and mouse embryos.
Opponents of the research fear that human cells could grow in the ‘wrong’ parts of the animals, or create a creature which is part human, part animal.
Supporters hope that the research could lead to transplant organs for human patients being grown in animal bodies.
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Nakauchi told the Asahi Shimbun newspaper, ‘We don't expect to create human organs immediately, but this allows us to advance our research based upon the know-how we have gained up to this point.’
The team will not bring any embryos to term immediately, Nakauchi said - instead brining mice embroys to 14.5 days, and rat embryos to 15.5 days.
Earlier this year, Nakauchi said, ‘We are trying to ensure that the human cells contribute only to the generation of certain organs.
‘With our new, targeted organ generation, we don't need to worry about human cells integrating where we don't want them, so there should be many fewer ethical concerns.’