What general principles are essential for understanding the developmental dynamics of fetal and baby brain?

• Developmental events occur in specific architectonic compartments, such as embryonic and fetal zones. 
• The compartments are transient, but can be visualized in historical sections and MR images.
• To be able to understand the development of functional connectivity, the nature and timing of development of basic connectivity elements and their molecular properties need to be analyzed. 

What are important events in brain development in the early fetal period (eight to fifteen postconceptional weeks)?

• There are processes of proliferation, migration, and cell aggregation. 
• All embryonic brain divisions and their major subdivisions are clearly visible on coronal sections at the end of the embryonic period.
• The formation of the cortical plate is an important cytoarchitectonic event. After its formation, the cerebral wall of the lateral neocortex consists of the marginal zone, the cortical plate, the presubplate, the intermediate zone, the subventricular zone, and the ventricular zone. 
• According to the radial unit hypothesis, the cortical neurons are generated in proliferative units of the ventricular zone, migrate along radial glial guides and settle in vertical ontogenetic columns within the cortical plate. 
• With regards to the growth of early afferents to the human cerebral cortex, thalamocortical fibers pass through the cerebral stalk, cross the diencephalo-telencephalic and subpallio-pallial border and fan out within the intermediate zone on their way to the cortical anlage. Basal forebrain fibers reach the neocortical cerebral wall through the external capsule. Corticospinal and corticopontine pathways are located medial to thalamic radiation and are partily intermingled with it. 
• The first synapses in the neocortical anlage appear. 
• There is a trilaminar pattern of organization consisting of the cortical plate, the intermediate zone, and periventricular proliferative zone. 
• Changes in cell aggregation (cytoarchitectonics), proliferation and migration, neuronal and dendritic differentiation, and axonal growth.

What are important events in brain development in the midfetal period (fifteen to twenty three postconceptional weeks)?

• Four histogenetic-neurogenetic events are most important during this period: neuronal aggregation and cytoarchitectural development, axonal outgrowth and ingrowth, dendritic differentiation, and molecular specification.
• The formation of synapses continues in the subplate and marginal zone. 
• Molecular specification of cerebral cortex can be divided in two processes, namely the areal specification and the specification of subsets of cortical neurons. 
• Major protection and commissural pathways are still growing (think of corticostriatal, corticospinal, thalamocortical, corticopontine, and corpus callosum).
• Associative pathways are not well developed, except for associative fibers connecting frontal cortex with cingulate neocortical portion of the limbic lobe.
• The presence of synapses in the subplate and the dense distribution of synapses in the marginal zone indicate circuitry development.

What are important events in brain development in the late fetal period (twenty four to thirty four postconceptional weeks)?

• Three histogenic and neurogenetic processes are most important during the beginning of the late fetal period: ingrowth of axons, synaptogenesis, and dendritic differentiation of pyramidal neurons.
• Rapid development of primary sulci and gyri. The central, precentral, and postcentral sulcus delineate the developing precentral and postcentral gyrus. Superior and inferior temporal sulcus appear in the temporal lobe. Superior and inferior frontal sulci mark the position of future superior, medial, and inferior frontal gyrus in the frontal lobe.
• On the medial hemispheric surface, there is deepening of the parieto-occipital and calcarine fissure and the appearance of the cingulate sulcus.
• Gradual decrease in the intensity of neuronal proliferation in ventricular and subventricular zone.
• At the end of the late fetal period secondary sulci develop rapidly, there is an increase in the volume of the cerebral wall, and there is a decline in proliferative zones.
• During the end of the late fetal period, the most intensive histogenetic events are neuronal aggregation, cytoarchitectonic changes in laminar pattern, axonal ingrowth and outgrowth, dendritic differentiation, and synaptogenesis in the cortical plate.

What are important events in brain development in the neonatal period?

• The main event is the formation of tertiary gyri.
• There are advances in neuronal aggregation and cytoarchitecture, with parallel establishment of tangential and radial patterns. There is gradual resolution of layer IV in the premotor cortex and the disappearance of this layer in the motor cortex, resolution of the voluminous subplate and its transformation into a characteristic thin band at the interface between layer VI and the gyral white matter, and an increase in size of pyramidal cell bodies.
• There is growth of short corticocortical fibers.
• Dendritic differentiation.
• Synaptogenesis.
• Myelination and increase in compactness of axonal pathways.
• Cell death and axonal pruning.
• The proliferation and migration of neurons have ceased, while the proliferation of astrocytes and oligodendrocytes is continuing.

What are important events in brain development in early infancy?

There is a rapid and massive increase in the total brain volume during the first year. The elaboration of cortical gyrification continues. Some histogenetic processes rapidly increase in intensity (such as synaptogenesis and dendritic differentiation), while others follow a steady pace (such as cytoarchitectonig development, neurochemical maturation, and myelination). There is a decline in the growth of axonal pathways.

What are important events in brain development in late infancy?

The cerebral hemispheres continue to grow. The most intense histogenetic and neurogenetic events during this period are morphological differentiation of neurons and dendrites, synaptogenesis, myelination, and changes in cortical cytoarchitectonics.