Transient patterns of cortical lamination during prenatal life: do they have implications for treatment?
Transient laminae containing circuitry elements (synapses, postsynaptic neurons and presynaptic axons) appear in the cerebral wall from the eighth postconceptional week (PCW) and disappear with the resolution of the subplate zone after the sixth postnatal month. The first endogeneous synaptic circuitry develops in two laminae, above and below the cortical plate. Mid- and late fetal period (15-23PCW) shows lamination pattern with a thick subplate zone containing GABAergic, glutamatergic and peptidergic neurons, synapses and thalamocortical afferents which are waiting and accumulating in the superficial subplate zone between 21 and 23PCW and these mark regional boundaries. In preterm infants, some thalamocortical fibers relocate to the cortical plate in visual, somatosensory, auditory and associative cortices, forming a framework for sensory-driven connectivity, while other remain engaged in the endogeneous subplate zone circuitry. Corticocortical pathways continue to grow. In the neonatal period, there is a major reorganization of callosal projections and development of short corticocortical connections, dendritic spines and synapses. In conclusion, transient neuronal circuitry underlies transient functions during the fetal, perinatal and early postnatal life and determines developmental plasticity of the cerebral cortex and moderates effects of lesion of the developing brain.