Histogenesis of the cerebral cortex

Initially, the wall of the hemispheric vesicle consists of a single zone. In week 5, this develops into an inner cell-dense periventricular zone and an outer cell-poor marginal zone. In week 6, postmitotic proneurones leave the inner periventricular zone and form an intermediate (mantle) zone between the other two zones. By the end of week 6, the periventricular zone is subdivided into a cell-dense ventricular zone and a less cell-dense subventricular zone.

The most important event of cortical development occurs during week 8. A cortical plate between the marginal and intermediate zones is formed by proneurones which have migrated along radial glial cells from the ventricular and subventricular zones through the intermediate zone. (2,345. and6) A single radial glial cell can span the entire distance between the ventricular and pial surfaces. As the proneurones 'climb' to the cortical plate along the processes of the radial glial cell, they produce a vertically oriented cortical cell column. This radially guided migration is responsible for the architectonic organization of cortical layers II to VI. It is a prototype of the cortical map of the adult brain.(7) A further feature of cortical migration is the inside-to-outside layering, with the earliest proneurones being found in the deepest layers of the cortical plate and the latest in the most superficial layers. Thus layers V and VI of the adult cortex are generated before layer IV, and layer IV is generated before layers III and II. At the same time there is much tangential migration of cells.(8)

Regional differences in the development of the cortical plate subdivide the hemisphere into segments. The lateral segment, with a well-developed cortical plate and presubplate, develops into the neocortex. The mediodorsal segment, with a wide marginal zone and a thin folded cortical plate, develops into the archicortex, including the hippocampus. The mediobasal segment, with its inconspiciously developed cortical plate, is the precursor of the palaeocortex. The basolateral segment develops into the corpus striatum, the amygdala, and the septum.

During weeks 10 and 12, the axons of the serotoninergic and noradrenergic neurones contribute to the first synapses in the marginal and presubplate zones, where neurotrophin receptors (see below) are expressed. During the following 3 weeks, the subplate zone develops as axons grow in from the basal forebrain and thalamus, dendrites enlarge, and synapses form. From weeks 16 to 24, the cortical Anlage has a small marginal zone, a wide cell-dense cortical plate, and a very wide and less cell-dense subplate.

The transformation into the adult neocortical pattern starts between weeks 25 and 34 as the migration and proliferation of proneurones diminishes. Dendrites begin to differentiate and synapses begin to develop in the deepest cortical layers, progressing to the most superficial layer. (9) Before birth, six cortical layers can be recognized in all regions of the neocortex. In the postnatal period, layer IV (inner granular layer) disappears as part of the differentiation of the motor cortex, leaving the five-layered agranular neocortex of the motor region. (!°> Shortly before birth, the subplate, the subventricular zone, and most of the ventricular zone disappear, neuronal proliferation ceases, and the intermediate zone is transformed into the white matter of the pallium. The remaining ventricular zone contributes to the ependymal layer of the ventricular surface.

Dendritic and axonal differentiation continues after birth and into adult life. Synaptogenesis reaches a maximum during the first postnatal year, but continues at a lower rate during childhood. The myelination of the vestibular system is finished shortly before birth, that of the somatosensory, visual, auditory, pyramidal, and extrapyramidal fibre tracts is nearly complete by the end of the third postnatal year, and that of the associative fibre tracts in the cerebral hemispheres is continued until the second decade/lP The key change in synapses after birth is pruning; the density of synapses in the adult brain is half that in neonates. (!2>

The development of the neocortex is summarized in Fig 2.

Fig. 2 Development of the neocortex between 4 weeks of gestation and adulthood. Roman numerals indicate the six cytoarchitectonically defined layers of the neocortex, which originate from the marginal zone (layer I) and the cortical plate. CP, cortical plate; IZ, intermediate zone; MZ, marginal zone; SP, subplate; SVZ, subventricular zone; VZ, ventricular zone; WM, white matter.

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