A.1 Neural Development

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A.1 Neural Development
The neural tube of embryonic chordates is formed by in-folding of ectoderm followed by
elongation of the tube.
Neurons are initially produced by differentiation in the neural tube
Immature neurons migrate to a final location
An axon grows from each immature neuron in response to chemical stimuli
Some axons extend beyond the neural tube to reach other parts of the body
A developing neuron forms multiple synapses
Synapses that are nut used do not persist
Neural pruning involves the loss of unused neurons
The plasticity of the nervous system allows it to change
Incomplete closure of the embryonic neural tube can cause spina bifida
Events such as strokes may promote reorganization of brain function
Annotation of a diagram of embryonic tissues of Xenopus, used as an animal model, during
Xenopus – frog embryonic tissue
Neural fold
Notochord – causes formation of
neural plate
Ectoderm: brain and nervous system
Mesoderm: skeletal, reproductive,
circulatory, excretory, muscular
Endoderm: lining of gut/organs
Archenteron: primitive gut
Neural plate: folds in,
closes  neural tube 
elongates into
brain/spinal cord
Neural tube closure: starts from head – proceeds to
caudal area
Failure to close caudal area by day 27 causes
Field of Study
Characteristics needed for model
Suitable species
Large numbers and short generation
Fruit fly
Baker’s yeast
Nematode worm
Robust embryos that are easily
African clawed frog (Xenopus)
Genomic studies,
such as genes that
cause diseases
60% of human genetic diseases
studied have a counterpart in the
fruit fly and nematode
Fruit fly
The mouse genome is similarly
organized to the human genome
Neurogenesis - process of differentiation from
neuroblasts when specific brain parts form
Carry messages
Most new neurons in cortex formed between 5th week
and 5th month
Do not carry messages – 90% brain cells
Function – physical and nutritional support of neuron
Provide scaffolding for immature neuron migration
During growth – one axon extends
Guides direction of growth
CAM (cell adhesion molecule) signals growth cone
Located on target cell
CAM specific receptor
Upon reaching target cell:
Synaptic connections must be made
Via chemical message sent (on surface or excreted extracellular)
Neuron synapses with cell
Chemotrophic factors
Creates growth and attachment of
Multiple synapses occur during neurogenesis
Single nerve  myriad of synapses to neighboring nerve cells  best fit wins, others die off
Strengthening communication in
that single connection
Controlled by IgCAM (neural adhesion molecule)
Motor neurons extend beyond CNS
Gives mammal voluntary control over
During embryogenesis  neurons follow same
pathways to synapse using CAMs
Neuromuscular junction
Neural pruning eliminates axons not being used – completed via Microglia cells
Remove simple connections – replace with complex ones
Ability to ‘rewire’ differs with age, but always possible
Structural plasticity:
brain can change its
physical structure as a
result of learning
Example the
hippocampus of
London taxi drivers is
larger than other
people’s because of
their learnt knowledge
Functional plasticity:
ability of the brain to
move functions from
damaged area
undamaged area
Example: stroke patients
learn how to use their
arms and legs again.
Stroke patients rewired
Axon sprouting
Post-stroke neurogenesis
Differentiation of glial cells
New association with neurons and blood vessels
MEG (brain mapping)

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