Last week, Rusty Gage and colleagues at the Salk Institute announced that they had successfully transplanted lab-grown blobs of human brain tissue into mice. Gage’s team grew the blobs, known as brain organoids, from human stem cells. Once surgically implanted into rodent brains, the organoids continued growing, and their neurons formed connections with those of the surrounding brains. It was the first time such transplants had worked: Until now, organoids had only ever been grown in dishes.
To be clear, Gage’s mice weren’t running around with human brains, nor did they have a human mind trapped inside their heads. The biggest brain organoids are lentil-sized and contain 2 to 3 million cells; a human brain is 20,000 times bigger and contains around 172 billion cells. Even the biggest ones don’t have the full set of cells needed for a working brain. On their own, their neurons don’t form networks like those in our heads. They don’t sense, learn, or make memories. They are emphatically not brains in jars. They’re not mini-brains either, in the same way that a leaf is not a mini-plant and a doorknob is not a mini-building.
They do, however, capture some of the architectural features of parts of a brain, which is why they’re useful. Scientists can use them to understand how brains develop, and how they differ in disorders. For example, one group of researchers made organoids with a genetic mutation that’s linked to microcephaly—a condition where babies grow up with small brains. Those organoids were also smaller than usual, and the team could work out why. Another group, led by Hongjun Song at the University of Pennsylvania used organoids to confirm that the Zika virus can affect the fetal brain and cause microcephaly, and to pinpoint which cells it infects.