[Histogenesis of the corpus callosum]

The corpus callosum results from neocortical commissural axon fasciculation. Its development reflects the interhemispheric circuitry and then follows the successive steps of synaptogenesis. The first stage consists of callosal neuron differentiation, which allows the extention of the future callosal axon; this is an early event that occurs while neuronal migration to the cortical plate is still ongoing. Callosal axon guidance towards its specific target is the second step which includes reaching and crossing the midline and further target recognition with formation of initial synapses. This period extends from 12 to 22 post-conceptional weeks and corresponds to the following histological features: i) progressive invasion by callosal growth cones of the dorsal part of lamina reuniens through a preformed glial pathway; ii) appearence of the three parts of corpus callosum, namely truncus, rostrum and lastly the splenium. Both these stages are genetically controlled either directly by developmental gene expression (neurogenesis genes) or indirectly by the establishment of cue maps (spatial expression of extra-cellular matrix proteins). The third step is that of synapse remodeling by synaptic activity, giving rise to axonal elimination, macroscopically revealed by a transitory thinning of corpus callosum. This perinatal event contributes to the corpus callosum acquiring a mature topography. Finally, analysis of corpus callosum ontogenesis appears as a striking model of synaptogenesis study and provides physiopathological assumptions for a understanding of the corpus callosum agenesis.