Bundled Three-Dimensional Human Axon Tracts Derived from Brain Organoids.

Reestablishing cerebral connectivity is a critical part of restoring neuronal network integrity and brain function after trauma, stroke, and neurodegenerative diseases. Creating transplantable axon tracts in the laboratory is an unexplored strategy for overcoming the common barriers limiting axon regeneration in vivo, including growth-inhibiting factors and the limited outgrowth capacity of mature neurons in the brain. We describe the generation, phenotype, and connectivity of constrained three-dimensional human axon tracts derived from brain organoids. These centimeter-long constructs are encased in an agarose shell that permits physical manipulation and are composed of discrete cellular regions spanned by axon tracts, mirroring the separation of cerebral gray and white matter. Features of cerebral cortex also are emulated, as evidenced by the presence of neurons with different cortical layer phenotypes. This engineered neural tissue represents a first step toward potentially reconstructing brain circuits by physically replacing neuronal populations and long-range axon tracts in the brain.