"Researchers have previously created hybrid brains by injecting neurons or transplanting pea-sized brain organoids from one species into either a developing brain or a fully formed one, either a mouse or rat.
"These experiments have told us that we are somewhat limited in when and how we can add brain cells to an existing brain," Baldwin says. "If the brain has developed to a certain point, the transplanted cells don't necessarily wire together appropriately."
"Instead, Baldwin's team introduced rat stem cells into mouse blastocysts, an early stage in development that occurs just hours after fertilization, so that the rat and mouse cells could grow together and integrate on their own.
"The technique, called blastocyst complementation, is similar to a technique used to create mice with human immune systems, which have proven to be powerful research tools. But until this study, the technique had not been successful in creating hybrid brains of two different species."