S3: Mechanisms of Neural Development

Question 1

What are the steps involved in turning an axon into a complex neural circuit?

Answer 1

1. Neurogenesis - the creating of the right number of nerve cells (and glial cells).
2. Migration and differentiation - to get the right cells to the right place and the right dendritic tree (and producing the right neurotransmitters).
3. Axon guidance - growing the axon to the right target area.
4. Synaptogenesis - making connections with potentially useful partners.
5. Activity dependent refinement - testing and perfecting the neural circuit.

Question 2

Describe neurogenesis

Answer 2

This starts off before the neural tube closes!

• At the beginning, the neural tube is composed on a single layer of replicating cell. They are called neuroepithelial cells/radialglial cells at this stage and they are long thin cells that attach to both the pial and luminal surfaces.
• Then the division process occurs where a cell will let go of the pial surface and drop down onto the luminal surface. Here they will split (perpendicular to luminal surface) to produce two daughter cells. These will then grow back up giving two new neuroepithelial cells so by doing this they are increasing their numbers and increasing the surface area of the neural tube.
• When the neural tube closes, the cells behave differently. The cell splits parallel to the luminal surface so the upper daughter cell will not be in contact with the lumen. This means the daughter cell will not contain a particular mixture of intracellular signalling molecules resulting on different genes being switched on. This daughter cell will turn into a neuroblast.

Question 3

What are the two surfaces of the neural tube?

Answer 3

• The pial surface which is the outer layer of the brain and spinal cord.
• The luminal surface which lines the inside. The inside will eventually become the ventricles so the luminal surface will become the ventricular surface.

Question 4

Why must the divisions of the neuroepithelial cells be precisely controlled?

Answer 4

They will determine the amount of nerve tissue that we have and therefore the amount of neural machinery available for the brain.

Question 5

How can mutations affecting the number of neuroepithelial cell divisions can affect brain size?

Answer 5

They can lead to microcephaly which is a small brain (due to loss of microcephalin protein). The individual will have severe learning difficulties but specific neurological defects (i.e. specific problem with visual or motor system etc.) are rare. This is because everything is there, there is just not enough to function optimally.

Question 6

What is migration and differentiation of neuroblasts?

Answer 6

The neuroblasts (which turn into neurones) crawl along using long processes allowing them to migrate to the right place.

Question 7

How do neuroblasts know what cells to become (differentiation)?

Answer 7

Throughout development, there are structures in the brain releasing substances that produce gradients.
- They can be medial, lateral, dorsal or ventral and examples include BMP, Wnt and Sonic Hedgehog.
These chemicals act as morphogens, they turn off and on genes to make sure a newborn cell has a particular gene profile and hence a particular phenotype (so they cause cell differentiation into different types of nerve cells). Morphogens tell the neuroblasts what cells to become and what guidance chemical signals to follow.

Question 8

How do neuroblasts know where to go (migration)?

Answer 8

The neuroblasts migrate in a particular way because they have inserted into their membrane particular receptors that attract them to specific regions in the brain.
- The thing they are attracted to is another class of extracellular chemicals called chemical guidance signals. Examples include neuregulins, reelin.
These guidance signal/chemicals create gradients in the tissue which cells will follow.

Question 9

What do cells in primitive cerebral cortex and ganglionic eminence differentiate into?

Answer 9

• In the primitive cerebral cortex, the cells here are destined to become excitatory cells of the cerebral cortex.
• The neuroblasts in the ganglionic eminence have two outcomes. They will either become ganglionic cells while other become the inhibitory interneurones of the cerebral cortex.

Question 10

Where do the cells in the primitive cerebral cortex, basal ganglia and ganglionic eminence migrate to?

Answer 10

• The cells born within the cerebral cortex tissue will migrate to the surface because they are attracted to reelin, found near the pia layer.
• Some cells from the ganglionic eminence will be attracted to neuregulins and then at a specific concentration they will decide to turn towards reelin and become the inhibitory neurones.
• Basal ganglia cells stay in the same area.

Question 11

Describe the early development of the cerebral cortex

Answer 11

• The neuroblasts migrates up towards the pia mater.
• The first set of cells reach the top, differentiate and set of residence becoming the marginal zone cells. It is these cells that produce reelin.
• The next load of cells crawl up the radial glia to arrive and become subplate cells.
• These two cell populations (marginal and subplate) will not last, they will die before birth. Their function is to help sheoherd the other cells to the right place.
• The third wave of cells will arrive, getting as close to the marginal zone as they can attracted by the reelin.
• These cells will move in between marginal zone and subplate to become the cortical plate.
• This is the outermost layer (near the pia).
• As more layers are being laid down, a switch occurs in the morphogens around telling the next set of cells that they are going to become a slightly different type. This means the adult cerebral cortex has 6 layers (although it does have sublayers) and these layers are laid down by cells that were given different instructions on what to become.
• As the outer layer cells are still being born and crawling to their destinations, the ones on the inside are starting to differentiate. This means they are starting to grow out their dendritic trees, release neurotransmitter and looking like normal cells.
• The last role of the radioglial cells is to drop down to the ventricular surface and become ependymal cells. Very few of them will remain as stem cells. They leave a large gap between the subplate and ependymal cells and this is full of axons running in this region to and from the cortex. This is white matter.
• Finally, the death of the subplate and marginal zone leaving behind the mature cortical structure.

Question 12

How does the cerebral cortex develop inside out?

Answer 12

The cortical plate is the outermost layer (near the pia) but they do not become the outermost layer of the cortex because each new wave of cells crawls past the ones that previously have laid down to get as close to the marginal zone before they set up residence. So the cortex develops from the inside out, with the earliest cells on the inside and last born cells on the outside.

Question 13

What will the outer layer, middle layer and deep layer of cells in cerebral cortex develop into? Describe the layers of cerebral cortex

Answer 13

• The deep layers (5+6) will turn into large pyramidal cells (triangular cell bodies) and their dendrites will go towards the surface. Here, the majority of axons will go down to subcortical structures, for example, projecting down to the thalamus.
• The middle layers will form stellate cells, they have local axons which will receive input from other cortical areas and send information around the local neural circuit.
• Eventually the outer layer cells (2+3) will differentiate into small pyramidial cells. Their axons go to other cortical areas.
• Then there are glial cells which are produced from neuroepithelial cells (once they have stopped producing nerve cells). They also produce astrocytes, oligodendrocytes etc.

Question 14

Describe how process of migration in brain can go wrong

Answer 14

There can be two reasons for this, the cell may not have the ability to crawl around, the cytoskeleton is used for this so mutations in the cytoskeleton can prevent cell mobility.
Another reason is the cell may not produce the receptors (properly) to detect the guidance signals or the guidance signals may not be produced.
- A severe loss of migratory ability in nerve cells leads to a brain that is very dysfunctional, it is unlikely to produce someone who is aware of what is going on around them.

Question 15

Describe the loss of doublecortin protein (DCX) in female

Answer 15

• This condition is X linked.
• In males they suffer from lissencephally.
• In females, with one mutated X gene they suffer from heterotopia.
• Some of the nerves will express the normal gene and others the mutated gene.
• The ones expressing good gene will produce a functioning cortex and the ones with the bad gene will produce a block of nerves that sits underneath the normal cortex.
• The girl may be appear normal if they have enough cortex, or could have severe learning difficulties. In either case severe epilepsy is likely. This is because normal cortical function requires inhibitory interneurons, the inhibition needs to be enough to stop nerve cells going crazy, if you have too little inhibition you get epilepsy as there is too much activity sweeping across the cortex.

Question 16

Describe how mutations affecting migration signals also disrupts the cortical organisation

Answer 16

• Loss of reelin means cortex will not develop from inside out.
• With the absence of reelin it means new cells just push up the previous ones. So the cortex develops the wrong way around, with the cells in the incorrect places.
• These cells end up wiring really badly and so with loss of reelin you get a quite a severe phenotype of severe learning difficulties, motor deficits and epilepsy (due to lissencephaly and cerebellar hypoplasia).

Question 17

When does axon guidance occur?

Answer 17

After migration and differentiation into nerve cells we need them to grow axons.

Question 18

Describe axon guidance (actin attracted and repulsed)

Answer 18

The lamellipodium is a spreading lamella structure at end of an axon and growing out of it are the spikey bits called filopodia. The filopodia are supported by actin bundles which extend the spike out, sense the environment to see what is there and can then come back in or stay.

• Actin attracted: The filopodia have receptors on their end looking for chemical guidance signals. Axons growing want to grow towards chemical guidance signals that are at the highest concentration. Once the receptors in the filopodia membrane bind the chemical guidance signal, this sets off intracellular signalling which results in growth of actin bundles.
• Actin repulsed: The filopodia membrane also has repceptors that causes the axon to be repulsed from other chemicals (breakdown signals). These substances cause the breakdown of the actin bundles so that the filopodia will shrink.

Question 19

How do types of surfaces affect how axons grow and how do axons grow between way points?

Answer 19

There are some surfaces axons can grow on and some they can’t.

• In order for a filopodia to grow out over a surface, it needs to have traction with the surface. To do this it must have proteins in its membrane to bind to proteins on the surface/extracellular matrix. This is useful as allows axons to be routed around properly so they don’t get in the way of other things so axons also grow between way points rather than straight towards one guidance signal.
• For example integrin in the filopodia membrane allowing binding to laminin.

Question 20

Describe sensory nerve cells growing between way points

Answer 20

• The sensory nerve cells that cross the midline, called commissural axons or nociceptor relay cells in the developing spinal cord.
• These start of by growing towards the floor plate because they contain proteins in their membrane that are attracted to the floor plate.
• Once they reach the floor plate, the signals at the location cause a change in gene expression so they change the switch the receptors in their growth cones. The filopodia get rid of the attractive receptors and replace them with repulsive receptors, that will be repulsed from the floor plate.
• Consequently the axons grow away from the floor plate and once they get to the white matter area they change again and grow up towards the head.
  This process is important, some people have been found lacking key receptors in this process so their axons can’t cross the midline.

Question 21

Describe synaptogenesis

Answer 21

Our nerve cells have grown out their axons to the right location and now need to connect to make the correct circuits with the right targets. We don’t have enough genes to tell the axons who they need to connect to so they do it by ‘trial and error.
- New friend request: Filopodia extend from dendrites seeking contact from passing axons in the enviroment. If the proteins on the axon surface are complementary to the proteins on the filopodia surface, then when the two touch they will bind to one another. With the two binding to one another it will trigger a primitive synapse to form, along with its other structures (e.g. receptors, neurotransmitter). The primitive synapse is not fully functional but it is a ‘test’ synapse. e.g. muscle spindle la afferent and its own muscle motor neurone.
- Failed friend request: If, the proteins on the filopodium and axon are not complementary, the molecules cannot bind to one another and a hence no synapse will form.
The filopodium will just retract. e.g. muscle spindle la afferent and antagonist motor neurone.

Question 22

What is activity dependent refinement - testing and perfecting the neural circuit?

Answer 22

There are a large amount of random connections made during synaptogenesis but only useful ones are kept and strengthened. There are many mechanisms by which this occurs.
e.g. long term potentiation that works for fast excitatory synapses

Question 23

Describe long term potentiation of fast excitatory synapses

Answer 23

Here are fast, glutamatergic synapse. New glutamatergic synapses have few working glutamate receptors. Some are AMPA receptors, which are straight LGIC. Others are called NMDA receptors, these open when they bind glutamate, however they have a magnesium ion in their channel which prevents anything flowing through the channel so long as it is there - NMDA receptors are only activated unless multiple synapses are activated. This is a weak synapse due to low numbers of ionotropic receptors.

1. If the synapse activates, the glutamate will be released and attach to the AMPA and NMDA receptor. The AMPA receptors will open allowing Na+ into the cell. If there is just one synapse like we can see activating then it will produce little postsynaptic depolarisation.
2. This is because the NMDA receptors will remain blocked/closed and this event will in essence be useless.
3. If this happens a lot then the synapse is probably not doing anything useful and may be eliminated!

• However, if this synapse is part of an effective circuit:
  1. Then many synapses (attached to the same dendrite) will activate simultaneously. This will be big enough to kick the Mg out off the NDMA channel and cause a strong postsynaptic depolarisation.
  2. So here the two things are occurring that allow the NDMA channel to open, number one it is binding glutamate and two the dendrite gets depolarised to a significant extent (because lots of excitatory synapses activating at same time).

The NMDA receptors therefore unblock and allow entry of Na+ but also Ca2+. This is only at the active synapses. Increased intracellular [Ca2+] triggers the process that strengthens the effective synapses. This is seen below:
- The calcium will cause increased number of AMPA receptors to be placed in the membrane and also cause phosphorylation of the AMPA receptors increasing their effectiveness.
- The calcium also has action on the cytoskeleton and this will increase the size of the dendritic spine (allowing insertion of more receptors)
- The dendritic spine may release retrograde signals telling the axon bouton to increase in size.
The result is that each action potential will produce a bigger depolarisation and this occurs over time and over again so the synapse gets stronger.

Question 24

Describe the loss of a weak synapse

Answer 24

If a synapse activates at a time when the dendrite isn’t depolarised, or the dendrite keeps depolarising strongly at a time when the particular synapse isn’t being activated. Both these events indicate that the synapse isn’t taking part in the circuit that the dendrite nerve cell is. This may result in the two getting smaller and weaker and in time may be deleted completely.

Question 25

What is synaptic plasticity?

Answer 25

Long term potentiation. It is very effective in the foetus because the inhibitory circuits are not fully formed thus there is weak inhibition and the NMDA channels are very effective. Plasticity is still high in babies, this allows their brain to adapt to abnormal situations. Children who suffer severe brain injury can recover a great deal of function. Plasticity does persist in adults, allowing learning but far weaker. Many of these plastic pathways become fixed and stable during early childhood.
Plasticity fades with age and wiring in the major pathways become permanent.
Each brain area/pathway has a unique “critical period” after which the neuronal circuits will be unable to rewire that much. It is very effective in the foetus and it is still high in babies allowing their brains to adapt to abnormal conditions.

Question 26

How can plasticity be maladaptive?

Answer 26

Maladaptive plasticity is thought to be a major cause of pathological pain.

Question 27

Describe how high plasticity in early life allows brains to adapt to abnormal situations (using squint in eye as example)

Answer 27

If one eye is compromised e.g. baby has a severe squint in right eye, then the brain will not be receiving much info from that eye during the developmental period. So the brain will not bother making much connections with it, the pattern of eye inputs will look like the lower image with one eye dominating.
This makes sense, because if you have a useless eye there is no point still dedicating half your visual cortex to it.
This is very important! If the eye damage (e.g. squint) was permanent then it doesn’t matter if it has few brain connections!
If we fix the squint in early development then the brain will re re-wire itself so the eye will be able to send info to the brain and be processed and the eye will function properly.
If we fix it later on (e.g. 5yrs), then the brain will stay in the structure with few connections to that eye because brain plasticity is now far lower and it can’t rewire itself back to normal. The child will permanently have very poor vision in this eye (will be amblyopic).