Brain Development

• Neural tube


  Develops into the CNS (brain & spinal cord)

  
• Development of Precursor Structure


  1. Neural tube forms

  2. Main divisions of the CNS appear

  3. Form ventricles and surrounding brain structures

  
• Retina & optic nerve


  Part of the CNS
• Producing Neurons


  1. Neurons & glia develop from neural stem cells at the ventricular surface

  2. Cells travel to their eventual destination

  
• Cell Proliferation


  1. Undifferentiated stem cell grows extension

  2. Cell nucleus moves up and duplicates DNA

  3. Nucleus moves down

  4. Cell retracts extension and divides in two
• Cell division


  • Vertical split: Both daughter cells repeat the process
  • Horizontal split: One daughter cell repeats, the other moves away and will not divide again
• Cell proliferation stops almost completely weeks before birth
• Cell Migration


  1. Pyramidal cells & astrocytes originate from dorsal areas, migrate vertically

  2. Inhibitory interneurons & oligodendroglia originate from ventral areas, migrate laterally
• Cortex


  • Not a single, homogenous sheet
  • Many different types of neurons
  • Organised in structured layers
• Cell Migration (continued)


  1. Radial glia cells extend processes in ordered pattern

  2. Developing neurons crawl along processes towards destination

  3. First cells take up subplate layer, then migrate to cortical plate in "inside out" order
• Different cortical areas have different cyto-architecture ("Brodmann areas")
• Axon guidance


  • Growing axons are guided by chemical signals in the environment
  • Neurons have different chemical affinities based on location
• Chemo-affinity hypothesis


  Axons from a specific 'source' structure are only attracted to chemicals from a specific 'target' structure
• Sensory & motor cortices


  • Form 'maps' (retinotopic map etc.)
  • Important for action control - input and output must be systematically linked
• Forming Pathways


  1. Overabundance of synapses initially

  2. Pruning: Supported synapses strengthened, unsupported ones disappear

  3. Correlated activity strengthens connections, uncorrelated activity weakens them

  4. Neurons that lose too many connections die
• Prenatally, correlated activity is established through spontaneous 'waves' or retinal activity
• To coordinate both eyes, actual visual input is needed - newborn babies have to learn to see