About 123,000 Americans are on a wait list to receive an organ donation. To meet the demand for livers, hearts and other organs, some scientists at research labs are experimenting with growing them inside animals by adding human stem cells to pig and sheep embryos, reports Anthony Regalado.

As you might imagine, creating so-called human-animal hybrids — also called chimeras — is controversial. Not much is known about what can happen when human cells are grown inside animal embryos.

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But the worry is that the human stem cells could multiply and specialize, as stem cells are wont to do, and start to give the animal human characteristics. Those features could range from physical features to perhaps intelligence, wrote Regalado in Technology Review.

Despite the controversy, some labs are moving forward with research in this area, which as of late has become much easier, thanks to advances in stem-cell biology and gene-editing techniques.

These techniques make it possible to genetically engineer pigs and sheep that can't develop certain tissue and organs. Here's the idea: scientists would inject human stem cells into the embryos of such animals and allow the stem cells to grow into the missing organ, which would then be harvested.

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Between the labs working in this area, scientists have produced at least 20 pregnancies of pig-human or sheep-human chimeras, Regalado reported. These early efforts are not meant to grow human organs, not yet, but rather to create methods toward that end.

I can't help recall H.G. Wells’ story, “The Island of Doctor Moreau,” where a mad scientist creates human-animal hybrids. But the chances that scientists would produce an overwhelmingly human-like animal are low.

Nonetheless, the National Institutes of Health said last September it wouldn’t fund studies of this nature until it had looked at the science more closely.