Scientists have identified the cellular origins of the corpus callosum, the 200 million nerve fibers that connect the two hemispheres of the brain. A study of mice and human brains shows that during development, astroglia, the main supporting cells of the brain, weave themselves between the right and left lobes, and form the bridge for axons to grow across the gap. Without these astroglia, the corpus callosum doesn't form correctly, causing a condition called callosal agenesis -- which affects 1 out of 4,000 people -- and a range of developmental disorders.