Cortical Development

Question 1

What is the neural tube made off?

Answer 1

This neural tube is made from a single layer of embryonic neural stem cells (neural epithelial cells)

Question 2

What do the neural stems form?

Answer 2

The neural stems generate the CNS, forming the brain at the anterior end of the neural tube and the spinal cord dorsally

Question 3

What will the central canal of the neural tube form?

Answer 3

The central canal of the neural tube will become the central canal of the spinal cord which is continuous with the ventricles of the brain. These cavities are filled with cerebrospinal fluid

Question 4

What protein is expressed by neural epithelial cells forming the neural tube?

Answer 4

Sox2

Question 5

What can the neural epithelial cells form?

Answer 5

These neural epithelial cells (ESCs) have the capacity to generate all of the different cell types of the central nervous system: neurones, astrocytes and oligodendrocytes

Question 6

What will the neural epithelial cells of the anterior and posterior neural tube form?

Answer 6

• Neural stem cells in the anterior neural tube will form the cell types needed to form the forebrain, while those cells in the posterior region of the neural tube will generate cell types required to form the spinal cord

Question 7

Are the neural epithelial cells non-specialised?

Answer 7

These cells are non-specialised, that can self-renew to produce more stem cells, and differentiate into the appropriate specialised cell types. These cells need to make more of themselves, otherwise they would run out. This is important to form all the cell types required to form the CNS. Differentiation generates all these cell types

Question 8

Describe the structure of the cerebral cortex:

1. Dark region next to the ventricle
2. What is interconnected nuclei migration?

Answer 8

The dark region next to the ventricle is neuroepithelium also called the ventricular zone abbreviated to VZ. This is composed of neuroepithelial cells (embryonic neural stem cells). This region looks like it is composed of multiple layers (it is not), it consists of a single layer of cells and looks multi-layered because the embryonic neural stem cells undergo a process called interconnected nuclei migration as depicted by a cartoon in the VZ. This means the cell nucleus moves in phase with the cell cycle. The nucleus moves to the outer (basal) side of the ventricular zone where DNA synthesis occurs and then to ventricular surface (apical) VZ where mitosis occurs

Question 9

What can Impairment of kinetic neural migration cause?

Answer 9

• Impairment of kinetic neural migration can lead to a loss of progenitor cells and to a small cortex, this is due to abnormal mitosis occurring away from the apical surface of the VZ, this is followed by death of exit from the cell cycle. Interconnected neural migration also allows more cells to be packed into a limited space.

Question 10

How do neurones generated in the VZ migrate to the developing cortical plate

Answer 10

Neurones generated in the VZ migrate from there point of origin to the developing cortical plate (this is an embryonic precursor of the cerebral cortex). In order to do this they use radial glia cells as guides (a type of progenitor cell that have there cell body within the VZ and there processes that span the entire cortical wall from ventricle to the pial surface. So newly born neurones can use the base or processes of these cells to get to the cortical plate

Question 11

What is the intermediate zone?

Answer 11

• In the diagram you can also see the IZ (intermediate zone) which is composed of young migrating neurones and axons that enter and leave the developing cortex. In the adult this will become the white matter

Question 12

What is the marginal plate and sub plate?

Answer 12

• The MZ (marginal zone) and SP (sub plate) are cell layers that are generated during cortical plate development – come back to in next slide

Question 13

GO over

Answer 13

• In the adult the cortex is a six legged structure, where each layer is composed of a specific subtype of excitatory projection neurones (neurones that use the neurotransmitter glutamate) and inhibitory neural circuit neurones (neurones that use the neurotransmitter GABA)
• These local circuit neurones regulate projection neurone activity. For example, the deep layers of the cortex (5 and 6) are composed of neurones that project to sub-cortical areas such as the thalamus, brain stem and spinal cord. While the superficial layers of the cortex (2-4) are composed of intra cortical neurones that project locally or to the contralateral hemispheres of the cortex.
• However, neural stem cells and neural progenitor cells of the developing cortex only generate pyramidal glutamatergic neurones. Inhibitory GABAergic neurones are not generated within the developing cortex but migrate into the cortex during development
• Some neurones are generated within the cortex, but the bulk of neurone generation occurs within the subcortical regions.

Question 14

In cortical development how are neurones generated?

Answer 14

During cortical development neurons are generated in waves with each wave generating a different cortical layer

They divide asymmetrically to generate a cohort of neurons that migrate out of the VZ and form the preplate,(abbreviated PP) between the VZ and the meniniges of the brain.

Question 15

What is the next group of neurones that migrate from the VZ?

What is also happening at this time?

Answer 15

• The next cohort of neurons migrate from VZ and from a layer of neurons within the preplate. These will form layer 6 of the developing cortical plate and in doing so will split the preplate to form a marginal zone (under the pial surface) and subplate (that will come to lie underneath the cortical plate).
• During this time the neuroepithelial cells are also generating those progenitor cells called radial glial cell whose processes span the developing cortex from the VZ to the pial surface

Question 16

What is the next group of neurones generated?

Answer 16

• The next cohort of neurons are thought to be generated by both neuroepithelial cells and radial glial cells. These neurons will leave the ventricular zone and migrate past the previously generated layer 6 neurons and form a new layer above or, more superficial to, them. This is layer 5 of the cortical plate.

Question 17

How is the subventricular zone then formed?

Answer 17

• At the same time as layer 5 cells are being generated the neuroepithelial cells and the radial glial cells are together generating another type of progenitor cell called the intermediate progenitor cell (also called the basal cell). These cells form a new germinal zone called the subventricular zone more superficial to the ventricular zone.

Question 18

What does the SVC generate

Answer 18

• The SVZ cells will generate the other layers of the cortex in an inside out manner - layer 4 first, then layer 3 and finally layer 2, with each successive wave of neurons migrating past those made earlier.

Question 19

What neurones are generated by the developing cortex and what neurones migrate in?

Answer 19

• BUT, as I have just told you the neurons generated by the cortical germinal zone glutamatergic neurons, GABAergic neurons are generated elsewhere and migrate into the cortex during development

Question 20

Describe the positions of the apical and basal surface of the neuroepithelial cells in relation to the cortex

Answer 20

The apical surface is the surface nearest to the ventricle and the basal surface is nearest to the pial surface

Question 21

Where does nuclear migration occur?

Answer 21

Venricular zone

Question 22

What characteristics are shared between the radial glial cells, neuroepithelial cells and astrocytes?

Answer 22

• Radial glial cells share characteristics with neuroepithelial cells and some astrocytes. They express the intermediate filament nesting which is a characteristic of neuroepithelial cells. They also express the glutamate transporter – GLAST, the calcium transporter binding protein – S100 beta, the intermediate filament vimentin and brain lipid binding protein – BMBP, which are proteins also expressed by astrocytes
• These cells appear after neurogenesis

Question 23

How are the • The intermediate progenitor cells (basal cells) generate ?

Answer 23

• The intermediate progenitor cells (basal cells), they do not undergo kinetic nuclear migration. They retract there apical process and undergo mitosis at the basal surface of the ventricular zone, eventually forming the sub particular zone. Intermediate progenitor cells are characterised by the expression of the TFS TBR2, Cux1/2 and Svet1
• As intermediate progenitor cells are generated by neuroepithelial cells and radial glial cells it is no surprise they appear after the onset of mutagenesis

Question 24

Describe the relationship between the different progenitor cells?

Answer 24

• Panal A – neuroepithelial originally divide asymmetrically to expand the neuroepithelial cell pool. As development proceeds, they begin to divide asymmetrically to generate neurones or radial glial cells as shown in panel B, or IPC as shown in panel C
• Radial glial cells can also self-renew by symmetric divisions that primary undergo asymmetric divisions as shown here producing a new radial glial cell and a neurone as shown in panel B or a radial glial cell and IPC shown in panel C
• The IPC divide symmetrically to generate more IPCs or neurones as shown in panel C.
• As development proceeds, neuroepithelial cells disappear

Question 25

Why do we need so many cell types to generate different neurones?

What do radial glial cells also form?

What do radial glial cells function to do?

Answer 25

We need neuralepithelial cells because they are neural stem cells – the found cells of the CNS. They generate all the different cell types within the CNS, mostly by multi-potential progenitor cells e.g radial glial cells and dedicated progenitor cells (meaning they only generate a specirfic cell type such as the IPC)

NB: Radial glial cells also generate astrocytes and oligodendrocytes

Radial glial cells also act as guides for migrating neurones so they have a very specific function in addition to there role in generating neurones, intermediate progenitor cells and radial glial

Question 26

Why do we need IPCs?

Answer 26

• These cells divide symmetrically and self-renew before undergoing a terminal division that gives rise to 2 neurones.
• So IPC enable us to generate neurones in a given time
• So we have IPC to enable sufficient producion of neurones and in primate brain IPC undergo more divisions before differentiating to enable the generation of the number of neurones required to form such complex brains

Question 27

What is the purpose of Wnt?

Answer 27

• But early in cortical development Wnt also promotes the division of stem and progenitor cells

Question 28

What happens in the absence of Wnt?

Answer 28

• In the absence of Wnt, a destruction complex consisting of Axin, GSK-3 and APC reside in the cytoplasm where it binds and phosphorylates B-catenin (the signal transducer for Wnt signalling)
• Phosphorylated B-catenin gets degraded by the proteosome

Question 29

What happens in the presence of Wnt?

Answer 29

• However, when Wnt is present it interacts with the frizzled receptor and the low density lipoprotein related protein called LRP. This leads to a phosphorylation of Dsh, which activates GSK3 beta and leads to the association of axin with LRP. This results in the destruction complex falling apart and the release and stabilisation of Beta-catenin. Beta catenin can now translocate to the nucleus and associate with TFs such as T-cell factor and lymphoid enhancer binding factor leads to the transcription of genes involved in promoting division such as cyclic D1 and engraled.

Question 30

How is Wnt also involved in later stages of neurogenesis?

Answer 30

• Wnt also promotes neurogenesis at later stages of development of cortical development through promoting the synthesis of pro-neural genes such as the neurogenins. These pro-neural genes are transcription factors that promote the differentiation of stem and progenitor cells into neurones

Question 31

What does Notch do ?

Answer 31

1. Notch plays a role in maintaining STEM and progenitor pool during neurogenesis, making sure that sufficient progenitor cells are retained to generate the neurones and glial cells that are needed
2. Notch receptors delta and jagged are expressed on the cell surface of neurones and dedicated neuronal progenitor cells (such as the IPC)
3. When Notch binds to its receptor the Notch intracellular domain is released by proteolytic cleavage allowing each translocate to the nucleus and interact with other transcription factors at the promoter regions of Notch target genes
4. Two genes are of specific interest for progenitor cell maintenance: Hes1 and Hes5. These are essential for keeping neuroepithelial cells and radial glial cells undifferentiated. They do this through repression of target such as Neurogenin 1 and 2 and mash 1, so its stop neural differentiation.

Question 32

How do neurons migrate from the VZ/SVZ to the cortical plate?

Answer 32

• After neurones are generated what mechanisms do they use to migrate from there point of origin in the ventricular zone/ subventricular zone to a destination in the cortical plate?
• There are two forms of migration somal translocation and glial guided translocation

Question 33

Describe somal translocation

Answer 33

• This is a rapid form of migration and occurs when the leading process of the new neurone can reach and attach to the pial surface
• The NE is generating a new neurone in red that is extended to the pial surface
• During somal translocation, the nucleus and soma of the neurone are displaced into the leading process and rapidly translocated towards a pial surface whole the trailing or aplical process is retracted

Question 34

Describe glial guided translocation

Answer 34

Glial guided migration
• This uses radial glial cells and fibres as a scaffold for migrating neurones. The neurones develop a multipolar morphology and then a bipolar morphology as they attach to the radial glial fibre, characterised by a leading unchaining process that neurones maintain due to the migration
• When neural migration is terminated, the leading process is assumed to transform into the apical dendrite and a trailing process into the axon
• So cortical neurones acquire there characteristic bipolar axal-dendritic polarity during radial migration to the cortical plate
• As migrating neurones near the pial surface they switch to using somal translocation, now referred to terminal translocation

Question 35

Compare somal to glial guided translocation

Answer 35

• In summary, early in cortical development neurones migrate using somal translocation and this applies to pre-plate neurones and the neurones of layer 6.
• Neurones are populate from 5-2 and migrate to the cortical plate using glial guided migration. But when there leading process reaches and can be anchored in the marginal zone (layer 1 of the cortex) migration changes from glial guided to terminal translocation

Question 36

Read over and understand the recap

Answer 36

On image

Question 37

How are neurones, oligodendrocytes and astrocytes formed?

Answer 37

Our NE cells or neural stem cells initially divide symmetrically to expand the progenitor pool, they then divide asymmetrically to generate neurons and also radial glial cells. Both neuroepithelial cells and radial glial cells then divide asymmetrically to generate an intermediate progenitor cell, and these cells divide symmetrically to generate neurons. Radial glial cells can also generate neurons directly. These cells have the capacity to also generate oligodendrocytes and astrocytes via specific, dedicated progenitor cells. They also generate the astrocyte-like neural progenitor cells that exist in the adult brain.

Question 38

Where are GABAergic neurones generated?

Answer 38

• GABAergic neurones are generated from the lateral and medial ganglionic eminences abbreviated to LG and MGE shown in green

Question 39

Where do acetylcholine neurones develop?

Answer 39

• Neurons that use acetylcholine as a neurotransmitter develop from the anterior endopunduclolar area – AEP. This is shown in blue

Question 40

Along the Dorsal Ventral axis describe the concentrations of

Answer 40

• For example lets consider the position of a progenitor cell along the dorsal/ventral axis and the actions of the morphogens: BMP and Shh that is secreted by specialised cells in the dorsal and ventral midline.
• Dorsally there is high concentrations of BMP that decreases ventrally. Ventrally there is high concentrations of Shh that decreases dorsally.
• At any position along the dorsal ventral axis, a cell will se a specific concentration of BMP and Shh
• The concentration that each cell sees will determine the TFs are expressed by the cell

Question 41

What is required for differentiation into GABAergic neurones

Answer 41

Transcription factors required for differentiation into GABAergic neurones are expressed in the rodent telencephalon in the ganglionic eminences (there are 3 – the lateral, medial and caudal).

They express TFs that cause the differentiation of GABAergic neurones subtypes

Question 42

Where are GABAergic neurones destined for the developing cortex made?

Answer 42

GABAergic neurones destined for the developing cortex are generated mainly at MGE where Nkx2.1 is expressed but also from the CGE.

Question 43

What NEURONE TYPES does the MGE and CGE form?

Answer 43

• The MGE generates parvalbumin and somatostatin expressing into neurones
• The CGE generates vaso active intestinal peptide and calretinin expressing neurones and also breathing expressing neurones (derived from radial glial cells and IPCs)

Question 44

What factors stimulate the migration of the neuornes out the germinal zones

Answer 44

• Mobility factors such as hepatocyte growth factor (HGF) and Skat factor (SF) stimulate the migration of neurones out of the germinal zones

Question 45

When neurones leave the MG, why do they migrate to the cortex rather than the striatim?

Answer 45

• This is because of interactions between ligands called semaphorins and receptors called neuropilins
• Progentor cells within the MGE express the TF nkx2.1, but this blocks the expression of neuropilins (1 and 2). So neurones destined for the cortex turn off nkx2.1 expression to allow these cells to express neuropilins.
• So they express neuropilin 1 and 2, the ligands for these receptors: semaphorin 3A and 3F are expressed and secreted by cells in the striatum
• When semaphorin is secreted by the striatal cells it interacts with neuropilin receptors expressed by the migrating neurones
• A repulsive cue is sent to the neurones preventing them from migrating into the region. Passive highways are thought to be provided by the Corticofugal fibre system, fibres that project the cerebral cortex to the brainstem and spinal cord

Question 46

What chemoattractants attract the migrating

Answer 46

• Signals secreted by the meninges and within the intermediate zone and subventricualr zone of the developing cortex attract the migrating neurones. Two such chemoattractants are the chemokines CXCL12 that interacts with CDCR4 receptors expressed by mugrating neurones. Neuruglin 1 interacts with ErbB4 receptors expressed by migrating neurones
• The migration of young neurones from the MGE to the developing cortex is controlled by positive cues such as chemoattractants, negatives cues such as replusion and passive such cues such as the fibres mentioned.

Question 47

Where are oligodendrocytes precursor cells generated?

Answer 47

• Oligodendrocytes precursor cells generated by cells within the LGE, MGE and CGE
• TFs required for oligodendrocytes precursor cell development are expressed in these domains

Question 48

What 3 mice models are used to study the reelin pathway?

Answer 48

• Reeler – Reelin
• Scrambler – Dab1
• VLDL receptor/ApoE receptor 2, double knockout mice

Question 49

Cue

Answer 49

Cue

Question 50

How does the relin pathway work?

Answer 50

1. Reelin is a large secreted glycoprotein that is synthesised by calretinin cells (CR) within the marginal zone – marginal zone neurones are some of the first neurones to be generated during cortical development
2. CR have multiple origins: the cortical ventricular zone, the caudal ganglionic eminence and the cortical hem (found at the border of the developing cortex and the core plexus). These multiple generation sites are required to provide the developing cortex with a sufficient number of CR during a short developmental period
3. Secreted Reelin binds and clusters on VLDLR and ApoER2 on the surface of migrating neurones. When reelin binds to these receptors it induces tyrosine phosphorylation of the cytoplasmic adaptor protein Dab1 (that interacts with the cytoplasmic tail of these receptors)
4. NB: prevention of Dab1 phosphorylation also results in a reelin phenotype – phosphorylation is a key step
5. Reelin binds to ApoER2 and VLDL receptors clustering them, Dab1 binds to these receptors as well
6. This results in phosphorylation of Dab1. Neither VLDLR or APoER2 have kinase activity (phosphorylation activity). Reelin also binds to Ephrin B and CNRs that are expressed by neurones, these can recruit non-receptor tyrosine kinases such as Src which can phosphorylate Dab1

Question 51

How does relin work?

Answer 51

How does reelin work?
• By modifying the neuronal cytoskeleton by changing the adhesive properties of neurons
• By functioning as a stop signal for neuronal migration at the interface between the MZ and the developing CP.
• By acting as a permissive or enabling factor for migration (at lower concentrations than found in MZ)

Question 52

How does reelin work?

Answer 52

• By modifying the neuronal cytoskeleton by changing the adhesive properties of neurons
• By functioning as a stop signal for neuronal migration at the interface between the MZ and the developing CP.
• By acting as a permissive or enabling factor for migration (at lower concentrations than found in MZ)

Question 53

Go over Dab1 signalling

Answer 53

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