Other scientists had already figured out how to grow the pancreatic tissue of a rat inside a mouse. On Wednesday, that team announced that mouse pancreases grown inside rats successfully treated diabetes when parts of the healthy organs were transplanted into diseased mice.
The Salk-led group took the concept one step further, using the genome editing tool called CRISPR to hack into mouse blastocyststhe precursors of embryos. There, they deleted genes that mice need to grow certain organs. When they introduced rat stem cells capable of producing those organs, those cells flourished.
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In all, the team created 186 later-stage chimeric embryos that survived, says Wu, and we estimate [each had] about one in 100,000 human cells.
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The human tissue appears to slow the growth of the embryo, notes Cheng, and organs grown from such embryos as they develop now would likely be rejected by humans, since they would contain so much pig tissue.
The next big step, says Cheng, is to figure out whether it's possible to increase the number of human cells the embryos can tolerate. The current method is a start, but it still isn't clear if that hurdle can be overcome.
Belmonte agrees, noting that it could take years to use the process to create functioning human organs.