Corpus callosum is the largest commisure system in the brain and responsible for the integration of the information between two hemispheres. Cllosal axons originate primarily from pyramidal neurons in layers II/III and V, cross the midline and project to the contralateral cortex with specific patterns. However, it is unclear about how the exact projection patterns is formed and what the mechanisms underlying the development of this projection are. Here we used in utero electroporation to label a subpopulation of layer II/III callosal neurons in the mouse somatosensory cortex. We found that callosal axons displayed region- and layer-specific projection patterns within the first 2 weeks postnatally and both electrical activity and synaptic activity of callosal neurons are critical for the formation of this specific projection.
In the second part of the experiments, we disrupted sensory input by the unilateral peripheral nerve transection to explore whether sensory input is required for the development of callosal projections. By unilateral transection of infraorbital nerve on either side, we found that bilateral sensory inputs are essential for the cortical invasion of callosal axons and the formation of region-specific projection pattern. Interestingly, blocking sensory input bilaterally, by simultaneously transecting both IONs, has no effect on target selection. Non-simultaneous bilateral transection, however, has the same effect as unilateral transection, regardless of which side is lesioned first. These findings reveal that not only bilateral peripheral sensory inputs but their balance are critical for the development of callosal connections.