A team of Salk Institute researchers have generated human cells and tissues in the embryos of pigs and cattle. The findings have been reported in the journal Cell on January 26. The researchers assert that this is just the first step. They are still far away from their goal.

“The ultimate goal is to grow functional and transplantable tissue or organs, but we are far away from that,” adds Juan Carlos Izpisua Belmonte, study lead investigator, and a professor, the Salk Institute of Biological Studies’ Gene Expression Laboratory, Science Daily reports. “This is an important first step.”

Initially, the researchers introduced rat cells into mouse embryos, creating a rat/mouse chimera. They used CRISPR genome editing tools to delete genes in fertilized mouse egg cells and swap it with rat cells. They found that the rat cells filled in the space of the deleted mouse cells. As a result, the rat cells formed the chimera’s heart, eyes, and pancreas.

The mouse also grew gall bladder from rat cells. The rats themselves cannot form gall bladder after separating from a mouse in the evolutionary tree in 18 million years. The researchers decided to replicate this in other animals. So they thought of introducing human cells into cow and pig embryos since these animals have organs that closely resemble humans’.

Using cow embryos were difficult and expensive. So they focused on pigs. For four years, they studied 1,500 pig embryos and genes from 40 people. Consequently, the human cells introduced actually survived, forming a human/pig chimera embryo. The embryos were only allowed to develop between three and four weeks, which long enough to provide insights without raising ethical concerns.

The researchers assert that they made sure they did not use human cells that are precursors to the formation of brain cells. They said that they only used cells that would develop into muscle cells and other organs.