The development of cerebral connections during the first 20-45 weeks' gestation.

We have correlated data on neuroanatomical organization and magnetic resonance imaging of transient fetal zones shown to contain connectivity elements (growing axons, synapses, dendrites). In the fetal phase, afferent fibres 'wait' within the subplate zone which is the most prominent lamina on histological and magnetic resonance images and is a substrate of endogenous neuronal activity. In early preterm the thalamocortical afferents accumulate within the superficial subplate and grow into cortical plate developing synapses. In late preterm, the resolution of the subplate and growth of cortico-cortical fibres into the cortical plate occur simultaneously with gyration. Both preterm phases characterize the coexistence of endogenous and sensory-driven circuitries and occurrence of the transient electrical phenomena. In neonates, the long cortico-cortical pathways stop growth, and the main histogenetic events are an elaboration of intracortical circuitry and synaptogenesis. In conclusion, the growth of the axonal pathways preterm explains their vulnerability and plasticity. In neonates the vulnerability is related to the intracortical circuitry.