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Neural Prenatal Development

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  1. Introduction
  2. Neurogenesis and neural identity
    1. The first postmitotic neurons
    2. The microenvironment
  3. Neuronal migration
  4. Establishing connections
    1. The leading edge of the axon
    2. Illustrating the complexity of pathfinding
    3. The molecules guiding pathway selection
  5. Cell death
    1. Apoptosis
  6. Neuronal differentiation and neurotransmitter selection
  7. Implications for psychiatry
  8. Localized abnormalities
  9. Neurodevelopmental hypothesis of schizophrenia
  10. Neuroimaging
  11. Neuropathology
  12. Consequences of abnormal cytoarchitecture
  13. Conclusion
  14. Suggested readings

The wonder of development is that a structure as complex as the human brain originates from a flat sheet of embryologic ectoderm. The final, formed brain shows remarkable order in its predictable cortical layering, its diversity of cortical areas, and the numerous networks linking specific cortical areas and subcortical structures. To have cells choosing to become a certain neuronal type, attaining the correct laminar position, finding the correct target, and expressing the correct neurotransmitters at first seems overwhelmingly difficult. However, the final, breathtakingly complex set of connections in the human brain depends on a series of much simpler decisions as neurons become progressively more restricted in the choices they make. These decisions require the subtle interplay of genetic and environmental factors; much has been learned at a molecular level about these processes. At first glance this information seems most relevant to mental retardation or autistic disorder, in which abnormal brain development results in lifelong disability. However, even schizophrenia is believed to originate in subtle aberrant brain development, and understanding it requires an understanding of its etiology.

[...] Aberrations in the formation of neural ectoderm or in the formation of neuroblasts are likely to result in gross abnormalities like anencephaly. A generalized failure of the migration of daughter neurons into cortical layers is seen in a lissencephalic brain, characterized by an agyric (smooth) cerebral surface. Mutations in certain cell adhesion molecules affect neural migration or axonal outgrowth, and are associated with inherited hydrocephalus. Such gross pathology is unlikely to result in an illness appreciated as psychiatric because children with these disorders have severe mental retardation and neurological syndromes. [...]

[...] Neuronal Migration Once neurons are born in the ventricular zone, they migrate past earlier born neurons to assume their final laminar position. The formation of the six cortical layers is complete between gestational age 26 and 29 weeks. To reach their laminar location, neurons migrate along radial glial fibers that stretch from the ventricular to superficial surface, a journey that may take place over tens of millimeters. Neurons must travel through a complex, rapidly expanding zone containing afferents from the thalamus and other cortical areas. [...]

[...] One of the first discovered was the neural cell adhesion molecule (NCAM). NCAM is one of a family of molecules that mediates cell-cell or cell-substrate adhesion, and is found in many parts of the developing nervous system. Deficits in NCAM expression result in subtle cytoarchitectural abnormalities in specific brain areas. Similarly, growth-associated protein (GAP-43) is a molecule that plays a key role in guiding axon growth and modulating new connections. If GAP-43 is overexpressed, aberrant extra connections are formed in the hippocampus and other areas of the central nervous system and the peripheral nervous system. [...]

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