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WIREs Cogn Sci
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Construction of the human forebrain

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The adult human brain is arguably the most complex of biological systems. It contains 86 billion neurons (the information processing cells of the brain) and many more support cells. The neurons, with the assistance of the support cells, form trillions of connections creating complex, interconnected neural networks that support all human thought, feeling, and action. A challenge for modern neuroscience is to provide a model that accounts for this exquisitely complex and dynamic system. One fundamental part of this model is an account of how the human brain develops. This essay describes two important aspects of this developmental story. The first part of the story focuses on the remarkable and dynamic set of events that unfold during the prenatal period to give rise to cell lineage that form the essential substance of the brain, particularly the structures of the cerebral hemispheres. The second part of the story focuses on the formation of the major brain pathways of the cerebrum, the intricate fiber bundles that connect different populations of neurons to form the information processing systems that support all human thought and action. These two aspects of early brain development provide an essential foundation for understanding how the structure, organization, and functioning of the human brain emerge. WIREs Cogn Sci 2017, 8:e1409. doi: 10.1002/wcs.1409

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  • Neuroscience > Development
Diagram of a prototypical projection neuron illustrating the cell body, and the neural processes, specifically the axon which can extend over substantial distances to make contact with other neurons and the dendrites which are a major site of contact with the incoming axons of other neurons. (Blausen.com staff. Blausen gallery 2014. Wikiversity Journal of Medicine. doi: 10.15347/wjm/2014.010. ISSN 20018762. Creative Commons Attribution 2.5 Generic license)
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Diffusion tensors. (a) Illustration of tensor from region with high isotropic diffusivity, as in cerebrospinal fluid. (b) Tensor exhibiting isotropic, but lower diffusivity, as in gray matter. (c) Elongated tensor exhibiting anisotropy, as in fiber tracts. (d) Illustration of tractography of the superior longitudinal fasciculus (a major fiber tract connecting posterior with frontal parts of the cortex) shown in red.
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Annualized rate of apparent thinning of the cortex on MRI. Maps above show different views of the cerebral cortex and the color codes for the estimated rate of thinning. Note that in some areas of the cortex of 4‐ to 10‐year olds, the cortex appears to think at a rate above 1.5% per year. In older children, age‐related thinning continues but at a more modest rate. Measured in the PING sample and described in Ref .
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Annualized rate of cortical expansion at different ages. Maps above show different views of the cerebral cortex and the color codes for the estimated rate of expansion (warm colors) or contraction (blue). Note that in some areas of the cortex of 4‐ to 6‐year olds, the cortical surface is expanding at a rate above 3.5% per year. In older children, expansion decelerates and gives way to modest levels of surface area contraction. Measured in the PING sample and described in Ref .
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This drawing shows the regions of the human cerebral cortex as delineated by Korvinian Brodmann on the basis of cytoarchitecture. (Creative Commons Attribution 2.5 Generic license, modified by User:Looie496, original artist unknown but probably Brodmann—this image was made by modifying a scan of p. 288 of the book ‘Anatomy of the Nervous System,’ by Stephen Walter Ranson, W. B. Saunders, 1920)
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Cortical plate formation. A schematic representation of human neocortical development is shown. (a) The VZ prior to the onset of neuronal migration contains symmetrically dividing progenitor cells. (b) The first neurons to leave the VZ form a sparse preplate layer at the edge of the VZ (GW < 7). (c) Axons of the preplate neurons along with the axons projecting from subcortical areas, form the intermediate zone. (d) The preplate is split into an outer MZ and a deeper subplate zone by the developing cortical plate (beginning GW7‐8). (e) In the mature cortex, only the cortical layers and the underlying white matter pathways are evident. VZ, ventricular zone; MZ, marginal zone; PP, preplate; SP, subplate; SZ, subventricular zone.
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(a) Coronal section (see reference of slice location on whole brain insert) of an adult human brain showing the gray and white matter compartments as well as the fluid filled ventricular cavities. The neocortex is the thin layer of cells covering the surface of the brain. White matter pathways run beneath the cortical surface. Deep gray matter nuclei serve as relay stations. (John A. Beal, PhD Department of Cellular Biology & Anatomy, Louisiana State University Health Sciences Center Shreveport. Creative Commons Attribution 2.5 Generic license)
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Migration patterns from the two primary neural proliferative zones. (a) The dorsal proliferative system consists of the ventricular (VZ) and subventricular zones. Radial glial cells in the VZ are a major population of neural progenitor cells; they also extend a cellular process from the VZ to the cortical surface that serves as a kind of scaffold for migrating neurons. The ganglionic eminences (GE) make up the ventral proliferative zone. Interneurons (green) migrating into the cerebral wall from the GE interact with radial glia (red) and can exhibit changes in direction of migration after contacting radial glia. Interneurons can use radial glia as a scaffold upon which to migrate as they ascend to the cortical plate (CP) or descend in the direction of the ventricular zone (VZ). Particular orientation and morphological dynamics of migration may be associated with particular subsets of interneurons (Creative Commons Attribution 2.5 Generic. (b) Most of the projection neurons in the brain are produced in the VZ and an adjacent proliferative region called the SVZ (not shown). Radial glia progenitors produce neurons that then migrate to the neocortex via the radial glial scaffold.
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The four major classes of glial cells. The macroglial cells are the oligodendrocytes which form myelin sheaths around the axons in the brain, astrocytes which perform a range of functions in the formation and maintenance of neural pathways as well as housekeeping functions and the ependymal cells which play an important role in the production of cerebral spinal fluid (CSF). Macroglial cells are derived from the neural progenitor cell line. Microglia are phagocytes that serve to clear waste but they also play important roles in the maturation of neural circuits. (Blausen.com staff. Blausen gallery 2014. Wikiversity Journal of Medicine. doi: 10.15347/wjm/2014.010. ISSN 20018762. Creative Commons Attribution 2.5 Generic license)
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Coronal section of the neocortex containing projection neurons (visualized by green GFP staining) and interneurons (visualized by antibody staining in red). Scale bar: 100 µm (Ref , Figure 6(f)), slightly altered (plus scale bar, minus letter ‘f’). (Creative Commons Attribution 2.5 Generic license)
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