The myth of “Manbearpig” could be one step closer to reality, after researchers in California revealed details of their latest attempts to grow human organs inside pigs. While some may see this as “playing god,” the scientists believe their work could one day be used to generate life-saving donor organs.

Huge numbers of people die every year while waiting for an organ transplant, while many of those who do receive one often experience complications as their body rejects the foreign organ. When this occurs, the patient’s own immune system attacks the transplanted organ, and the only way to get around this is to take immunosuppressive drugs.

However, using this new technique, scientists from the University of California, Davis, hope to grow organs that are genetically identical to a patient’s own organs, thereby ensuring that it is accepted by their body.

To achieve this, the team used a gene-editing technology known as CRISPR to alter pig embryos by deleting the genes that code for the creation of a pancreas. They then injected these embryos with human induced pluripotent stem cells (iPS), which are unspecialized stem cells that have the capacity to develop into any type of cell.

The researchers believe that, since the pig embryos can’t grow a pancreas of their own, these iPSs will fill this gap and grow into a pancreas made entirely out of human cells, yet harbored inside a pig. In spite of this, the animals born out of these hybrid embryos – known as chimeras – should look and act just like any other normal pig.

Speaking to the BBC, lead researcher Pablo Ross explained that “our hope is that this pig embryo will develop normally but the pancreas will be made almost exclusively out of human cells and could be compatible with a patient for transplantation.” In theory, this same technique could also be used to grow a range of other human organs, such as hearts and livers.

However, the idea has created a fair bit of controversy, with some raising fears that the iPSs introduced to the embryos could invade the pigs’ brains, making them more human. In response, Ross says “there is very low potential for a human brain to grow, but this is something we will be investigating.”

Because of this and other ethical concerns, the researchers are advancing with caution, and are only allowing the embryos to grow for 28 days inside the womb of a sow before being destroyed and analyzed.