Scientists are supposed to be injecting human stem cells into pig and sheep embryos in an attempt to grow human organs in the animals which then might be harvested for transplant into people who need them. Not much is known about the technology so it is controversial. The scientists are proceeding with it experimentally anyway. Per the article:
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....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....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....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....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.