9.3

• True or False: Cells migrated in an orderly fashion, progressing from superficial to more deeper layers.

  False, deeper to superficial
• Because each wave of cortical cells migrates through the already formed lower layers of cortex before reaching its destination, this radial pattern of cortical development is referred to as an inside-out pattern.
• The neural crest is a structure situated just dorsal to the neural tube. It is formed from cells that break off from the neural tube as it is being formed.
• Neural crest cells develop into the neurons and glial cells of the peripheral nervous system as well as many other cell types in the body.
• Once developing neurons have migrated, they must align themselves with other developing neurons that have migrated to the same area to form the structures of the nervous system. This process is called __________.

  Neural aggregation
• Aggregation is thought to be mediated by at least three non-exclusive mechanisms:

  1. cell-adhesion molecules
  2. gap junctions
  3. interactions between glial cells and neurons
• Cell-adhesion molecules (CAMs), which are located on the surfaces of neurons and other cells have the ability to recognize molecules on other cells and adhere to them.
• True or False: Elimination of just one type of CAM in a knockout mouse have positive effect on brain development.

  False, Negative effect
• Once neurons have migrated to their appropriate positions and aggregated into neural structures, axons and dendrites begin to grow from them. This process is what we call the _________.

  Axon Growth
• At each growing tip of an axon or dendrite is an amoebalike structure called a growth cone, which extends and retracts fingerlike cytoplasmic extensions called filopodia.
• The filopodia behave as though they are searching for the correct route.
• This theory states that each postsynaptic surface in the nervous system releases a specific chemical label and that each growing axon is attracted by the label to its postsynaptic target during both neural development and regeneration.

  chemoaffinity hypothesis of axonal development
• What is the limitation of Roger Sperry's chemoaffinity hypothesis of axonal development?

  It had failed to account for the discovery that some growing axons follow the same circuitous route to reach their target in every member of a species rather than growing directly to it.
• Pioneer growth cones —the first growth cones to travel along a particular route in a developing nervous system— are believed to follow the correct trail by interacting with guidance molecules along the route.
• The tendency of developing axons to grow along the paths established by preceding axons is called fasciculation.
• According to this hypothesis, axons growing from one topographic surface (e.g., the retina) to another (e.g., the optic tectum) are guided to specific targets that are arranged on the terminal surface in the same way as the axons’ cell bodies are arranged on the original surface.

  topographic gradient hypothesis
• The key part of this hypothesis is that the growing axons are guided to their destinations by two intersecting signal gradients.

  topographic gradient hypothesis
• Once axons have reached their intended sites, they must establish an appropriate pattern of synapses. A process called __________.

  Synapse Formation
• True or False: It takes coordinated activity in at least one neuron to create a synapse between them.

  False, two neurons
• The formation of new synapses.

  synaptogenesis