An international research team led by The University of Tokyo modeled the growth of cerebral tracts. Using neurons derived from stem cells, they grew cortical-like spheroids. In a microdevice, the spheroids extended bundles of axons toward each other, forming a physical and electrical connection. Fascicles grew less efficiently when one spheroid was absent, and when a gene relevant to cerebral tract formation was knocked-down. The study further illuminates brain growth and developmental disorders.