Are rats with human brain cells still just rats?

This week I wrote about a fascinating experiment that involved implanting human brain cells into rats’ brains. The brain cells from both species were able to form connections and work together. The human cells became part of the rats’ brains.

The idea is to get a better sense of what happens in the brains of living people—something that is notoriously difficult to do. For the last decade or so, scientists have been studying lab-grown clumps of brain cells called organoids. The new study shows that these organoids start to look much more like functional human brain cells when they are implanted into the brain of a baby rat.

A few months after they’d been implanted, the human cells made up around a sixth of the rats’ brains and appeared to have a role in controlling the animals’ behavior. Which invites the question: Are these animals still 100% rat?

It’s a tricky one. The scientists behind the work argue that there’s nothing really human about these rats. Throughout the study, the team examined the rats to see if those with...