Six months after surgery, a mouse scurries through a simple behavioural test. Nothing looks unusual. But inside its skull, the outer layer of its brain — the region responsible for memory, thought and the senses — is made almost entirely of human cells. This is not science fiction.

It happened in a laboratory at Stanford University, and it is already making neuroscientists rethink what a "model" of the human brain can be. Here is how the team, led by Prof Sergiu Pașca, pulled it off. First, they engineered mice to grow almost no cerebral cortex of their own — stripping out the grey matter that handles higher thinking. Then they took ordinary human skin cells, reprogrammed them into brain-like tissue called organoids, and implanted those structures into the empty space.

Something remarkable followed: the human cells divided, reorganised themselves, and wired into the mouse's existing circuits, connecting all the way down to the spinal cord. Dr Ilary Allodi noted that early brain scans looked "a bit messy." After about six months, though, the human tissue had settled into something that looked and behaved like a real cortex. Why go to all this trouble?

Because some human brain disorders — epilepsy, autism, cerebral palsy — simply do not appear in mice the way they do in people.