How does the human brain get its walnut-like wrinkles?

In a first, Israeli researchers have developed a "brain-on-a-chip" model to explain why the brain has wrinkles resembling a walnut. The team grew tiny brains called organoids in the lab from embryonic stem cells and limited their growth in the vertical axis. By the second week, wrinkles began to appear and deepen on the flat organoids, explaining the brain folds.