Neural development

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The study of neural development draws on both neuroscience and developmental biology to describe the cellular and molecular mechanisms by which complex nervous systems emerge during embryonic development and throughout life.

Some landmarks of embryonic neural development include the birth and differentiation of neurons from stem cell precursors, the migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of axons from neurons and guidance of the motile growth cone through the embryo towards postsynaptic partners, the generation of synapses between these axons and their postsynaptic partners, and finally the lifelong changes in synapses which are thought to underlie learning and memory.

Typically, these neurodevelopmental processes can be broadly divided into two classes: activity-independent mechanisms and activity-dependent mechanisms. Activity-independent mechanisms are generally believed to occur as hardwired processes determined by genetic programs played out within individual neurons. These include differentiation, migration and axon guidance to their initial target areas. These processes are thought of as being independent of neural activity and sensory experience. Once axons reach their target areas, activity-dependent mechanisms come into play. Neural activity and sensory experience will mediate formation of new synapses, as well as synaptic plasticity, which will be responsible for refinement of the nascent neural circuits.

Developmental neuroscience uses a variety of animal models including the fruit fly Drosophila melanogaster , the zebrafish Danio rerio, Xenopus laevis tadpoles and the worm Caenorhabditis elegans, among others.

Contents 1 First stage: Neurulation 2 Human brain development 3 Neuronal Migration 3.1 Radial Migration 3.2 Tangential Migration 4 See also 5 References 6 External links

[edit] First stage: Neurulation

Main article: neurulation

See embryogenesis for understanding the animal development up to this stage.

Neurulation follows gastrulation in all vertebrates. During gastrulation cells migrate to the interior of embryo, forming three germ layers (endoderm, mesoderm and ectoderm) from which all tissues and organs will arise. In a simplified way, it can be said that the ectoderm gives rise to skin and nervous system, the endoderm to the guts and the mesoderm to the rest of the organs.

After gastrulation the notochord - a flexible, rod-shaped body that runs along the antero-posterior axis - has been formed (derived from mesoderm). The notochord sends signals to the overlying ectoderm, inducing it to become neuroectoderm, composed of neuronal precursor (or stem) cells. This is evidenced by a thickening of the ectoderm above the notochord, the neural plate. The neural plate will form the neural tube which then twists, turns and kinks to form the three primary brain vesicles and five secondary brain vesicles. The end result of this process is described in the article on the regions of the brain.

[edit] Human brain development

[edit] Neuronal Migration

Neuronal migration is the method by which neurons travel from their origin or birth place to their final position in the brain.

[edit] Radial Migration

Neuronal precursor cells proliferate in the ventricular zone of the developing neocortex. The first postmitotic cells to migrate form the preplate which are destined to become Cajal-Retzius cells and subplate neurons. These cells do so by somal translocation. Neurons migrating with this mode of locomotion are bipolar and attachs the leading edge of the process to the pia. The soma is then transported to the pial surface by nucleokenisis, a process by which a microtubule "cage" around the nucleus elongates and contracts in association with the centrosome to guide the nucleus to its final destination.^[1] Radial fibres (also known as radial glia) can translocate to the cortical plate and differentiate either into astrocytes or neurons.^{[citation needed]} Somal translocation can occur at any time during development.^[2]

Subsequent waves of neurons split the preplate by migrating along radial glial fibres to form the cortical plate. Each wave of migrating cells travel past their predesessors forming layers in an inside-out manner, meaning that the youngest neurons are the closest to the surface.^[3][4] It is estimated that glial guided migration represents 80-90% of migrating neurons.^{[citation needed]}

Multipolar migration...^[5][6] "<<expand this bit>>"

[edit] Tangential Migration

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[edit] See also

• Time lapse seqeunces of radial migration (also known as glial guidance) and somal translocation.^[2]

• Axon guidance

• Neural Darwinism

[edit] References

1. ^ Samuels B, Tsai L (2004). "Nucleokinesis illuminated". Nat Neurosci 7 (11): 1169-70. PMID 15508010. 
2. ^ ^{a b} Nadarajah B, Brunstrom J, Grutzendler J, Wong R, Pearlman A (2001). "Two modes of radial migration in early development of the cerebral cortex". Nat Neurosci 4 (2): 143-50. PMID 11175874. 
3. ^ Nadarajah B, Parnavelas J (2002). "Modes of neuronal migration in the developing cerebral cortex". Nat Rev Neurosci 3 (6): 423-32. PMID 12042877. 
4. ^ Rakic P (1972). "Mode of cell migration to the superficial layers of fetal monkey neocortex". J Comp Neurol 145 (1): 61-83. PMID 4624784. 
5. ^ Tabata H, Nakajima K (2003). "Multipolar migration: the third mode of radial neuronal migration in the developing cerebral cortex". J Neurosci 23 (31): 9996-10001. PMID 14602813.  Full text
6. ^ Nadarajah B, Alifragis P, Wong R, Parnavelas J (2003). "Neuronal migration in the developing cerebral cortex: observations based on real-time imaging". Cereb Cortex 13 (6): 607-11. PMID 12764035.  Full text

[edit] External links

• Myers, P.Z., 2004. "Neurulation in Zebrafish" in Pharyngula [1].