17. CNS Development

Question 1

What are the different steps of CNS development? (Brief)

Answer 1

• Neural induction
• Neurogenesis
• Cell differentiation
• Differentiation of connections
• Specialisation within the CNS
• Early spontaneous activity within the CNS
• Sensory connectivity patterns
• Plasticity

Question 2

What is neural induction? (Brief)

Answer 2

• This is where the CNS is developed from the neural plate
• The plate closes to form the neural tube
  
  • This tube is patterned in a longitudinal and ventral/dorsal pattern
  • Longitudinal patterning by Hox genes
  • Ventral/dorsal patterning by interactions from the surrounding mesoderm
• The rostral end of the neural tube forms the telencephalic vesicles, including the cerebral cortex (largest part of the brain)
• This process is linked to and occurs shortly after gastrulation

Question 3

When does early spontaneous activity occur within the CNS?

Answer 3

This can occur even before the sensory organs have been wired up

Question 4

How do human babies compare to those of most mammals?

Answer 4

• Compared to most mammals, humans babies are comparably more protracted in their development
• They still need to develop and refine a range of complex movements, including:
  
  • Somatosensory awareness
  • Thermal regulation
  • Motor control, particularly fine motor control
  • Taste
  • Postural control

Question 5

How do human and chimpanzee brain growths compare?

[EXTRA]

Answer 5

• Chimpanzee brains grow rapidly before birth
  
  • Growth levels off shortly after birth, completely doing so at approx. 2 years
• Human brains grow rapidly before birth through the first year and into childhood

Question 6

Give some examples of neurodevelopment disorders.

[EXTRA?]

Answer 6

• Spinal dysraphism
  
  • e.g. spina bifida, due to failure of closure of the neural tube
• Anencephaly
  
  • Rostral end of the neural tube fails to close properly, no telencephalic vesicles develop
• Holoprosencephaly
  
  • Only a few telencephalic vesicles develop
• Microcephaly
  
  • Brain is too small
• Lissencephaly
  
  • Smooth brain/not folded
• Band or nodular heterotopia
  
  • Failure of nervous system to migrate
• Conatal syphilis, toxoplasmosis, cytomegalovirus, ZIKA virus infection
  
  • These all effect metabolism and nutrients available, therefore affecting CNS development
• Disorders with more subtle anatomical influences (sometimes cannot be seen if imaging or viewing externally):
  
  • Childhood epilepsy (1:200)
  • Schizophrenia (1:100)
  • Autism (1:68)
  • ADHD (1:30)
  • Dyslexia (1:10)

Question 7

What occurs prior to neural induction?

Answer 7

• Fertilised egg divides and then invades the endometrium of the uterus
• The placenta, bilaminar and eventually trilaminar disc is formed
  
  • Trilaminar disc is formed during gastrulation

Question 8

What is gastrulation?

Answer 8

• This is the conversion of the bilaminar disc (made up of hypoblast and epiblast) into the trilaminar disc (mesoderm, endoderm and ectoderm)
• Cells migrate between the two primitive layers via the primitive streak - the cells that migrate become the mesoderm
  
  • Some of the primitive mesodermal cells migrate below the hypoblast to form the neural plate

Question 9

What is the neural plate and how does it develop?

Answer 9

• The neural plate is a thickening made up of ectoderm, and lies opposing the primitive streak (formed from migrating primitive mesodermal cells)
• Neural plate develops after the inducing effect of the primitive streak and is the basis of the nervous system
• Formation of nervous tissue involved complex interactions between mesoderm and ectoderm, mediated by:
  
  • Shh and noggin
  • BMPs, wnt, FGFs

Question 10

What is some experimental evidence to do with the primitive streak/node and neural induction?

[EXTRA]

Answer 10

• Spemann and Mangold (1924): used pigmented salamanders and suspected that the dorsal blastopore/lip of the gastrula had an organising effect
  
  • They showed this by dissecting the region from a pigmented salamander and introducing it to a host embryo that contained no pigment
  • A second body axis was formed, but the neural tube itself did not contain pigmented tissue
  • From these experiments they concluded that the transported blastopore induced the host cell to form a secondary body axis – we now know that this transported dorsal blastopore injected noggin or wnt RNA that formed the secondary body exis
    
    • The same effect can be seen by injecting or expressing noggin or wnt RNA in an ectopic location
    • Shows that the mesoderm has this effect on the induction of the neural plate and a secondary body axis

Question 11

What are some of the signalling molecules involved in neural induction?

[EXTRA]

Answer 11

Question 12

What is an example of when neural induction has gone wrong?

[EXTRA]

Answer 12

• Siamese twins
• Secondary body axis is formed within the same embryo
• Example: Abby and Brittany, dicephalic (x2 brains) parapagus (side by side, frequently share abdomen and pelvis) Siamese twins

Question 13

What germ layer forms the nervous tissue?

Answer 13

Ectoderm

Question 14

What happens to the neural plate, and what structures does it form?

Answer 14

• Edges of the neural plate (the neural folds) push the ends of the plate up and together, to form the neural tube (this is primary neurulation)
• A simple structure is formed, made up of:
  
  • Forebrain
  • Midbrain
  • Hindbrain
  • Spinal column

Question 15

After primary neurulation, what happens to the structures formed?

Answer 15

• The simple structures develop flexures along the whole tube:
  
  • Cephalic flexure
  • Cervical flexure
  • Pontine flexure
• These eventually lift the brain and face of the embryo so that it is no longer tucked into the chest

Question 16

How is the eye formed?

Answer 16

• As an outgrowth of the CNS (specifically the diencephalon)
  
  • This includes the retina, optic nerve and tract
• This means that the eyes are part of the CNS
• A cup is formed with inner and outer layers
  
  • Inner layer gives rise to the neuroretina
  • Outer layer gives rise to the pigment epithelium
  • [EXTRA] Retinal detachment occurs directly at this embryonic boundary
• The subarachnoid space extends to the optic disc
  
  • [CLINICAL] This means that intercranial pressure can cause the optic disc to protrude into the eye, so can be measured by looking at the papilla using an ophthalmoscope

Question 17

How does the ventricular system develop?

Answer 17

• These are formed from dilation of the space within the neural tube
• Start simple, but become more and more complex until their characteristic shape is obtained
  
  • Eventually forms two lateral ventricles, the third ventricle, the cerebral aqueduct and the fourth ventricle

Question 18

During eye development, what does the inner layer of the cup form?

Answer 18

Neuroretina

Question 19

During eye development, what does the outer layer of the cup form?

Answer 19

Pigment epithelium

Question 20

What are the different structures within the embryonic forebrain, midbrain and hindbrain?

Answer 20

• Forebrain
  
  • Telencephalon (cerebral hemispheres)
  • Diencephalon (anterior forebrain structures, including thalamus, hypothalamus, posterior pituitary, pineal gland)
  • Neural retina
  • Lens
  • (Lateral ventricle and 3rd ventricle)
• Midbrain
  
  • Mesencephalon (all midbrain structures, e.g. colliculi, tegmentum, cerebral peduncles)
  • (Cerebral aqueduct)
• Hindbrain
  
  • Metencephalon (pons and cerebellum)
  • Meyelencephalon (medulla)
  • (4th ventricle)

Question 21

What are the different structures within the embryonic forebrain?

Answer 21

• Telencephalon (cerebral hemispheres)
• Diencephalon (anterior forebrain structures, including thalamus, hypothalamus, posterior pituitary, pineal gland)
• Neural retina
• Lens
• (Lateral ventricle and 3rd ventricle)

Question 22

What are the different structures withint the embryonic midbrain?

Answer 22

• Mesencephalon (all midbrain structures, e.g. colliculi, tegmentum, cerebral peduncles)
• (Cerebral aqueduct)

Question 23

What are the different structures withint the embryonic hindbrain?

Answer 23

• Metencephalon (pons and cerebellum)
• Meyelencephalon (medulla)
• (4th ventricle)

Question 24

What are the requirements of neural tube closure, and what significance does this have?

Answer 24

• Proper neurogenesis
• Proper movement of tissue
• Many complex changes throughout the structure
• This means that there are many things that can go wrong

NB Picture is [EXTRA]

Question 25

What is spinal dysraphism?

Answer 25

• These are defects that occur when the neural tube does not close properly rostrally, and can include the bone, nerves, spinal cord and fluid coverings
• Spina bifida comes in 3 forms:
  
  • Myelomeningocele - defect includes spinal cord contents in a sac outside of the body
    
    • Babies typically have weakness below the sac
  • Meningocele - defect only contains spinal fluid/no nerves in a sac outside of the body, only the meninges
    
    • Spine normally otherwise develops normally, so can often be solved using surgery
  • Spina bifida occulta - common bony deficit (5-10% of population), 1 or more vertebrae do not form properly but the gap formed is very small, so there are frequently no effects
    
    • People often do not realise they have the issue, and is often associated with lumbrosacral abnormalities, e.g. tuft of hair, dimple, sinus or ‘port wine stain’
    • [EXTRA] DO NOT confuse with Mongolian blue spots (common in Asian populations), there are bluish/grey skin markings that appear at/shortly after birth, are transient and are not associated with illnesses

Question 26

What are some statistics for spina bifida in the UK?

[EXTRA]

Answer 26

Question 27

What are some steps that have been taken by the UK government to reduce occurrence of spina bifida?

[EXTRA]

Answer 27

• Only ~50% of pregnancies in the UK are planned, therefore it was suggested that the fortification of flour with folic acid would be necessary to aid foetal development
• The UK has now fortified flour since 2018
• There was no evidence that the fortification of flour resulting in an ingestion of >1mg of folic acid per day would result in neural defects/toxicity in the nervous system.
• Folic acid is not the answer for everything, it only covers around 50% - inositol must also be considered, and even then things can still go wrong, as a single molecule cannot cure everything within such a complex process.

Question 28

What is the signalling centre for ventro-dorsal patterning? What is established by this patterning?

Answer 28

• Notochord - this is a signalling centre outside of the CNS, and is of mesodermal origin, using Shh as a signalling molecule
• Notochord induces the floor plate, which induces further differentiation along the neural tube
• Patterning also allows the establishment of motor neuron pools (somatic motor and visceral)

Question 29

What structure does the notochord induce in the neural tube?

Answer 29

Floor plate (FP), which lies ventrally, using Shh

Question 30

What does the floor plate do?

Answer 30

• Induces further differentiation throughout the neural tube
  
  • This is not limited to the spinal cord but also extends to the brainstem and base of the telencephalic vesicle

Question 31

What is the roof plate (RP)?

Answer 31

• Thickening on the dorsal side of the neural tube
• Expresses BMP4 to effect development of neurons
  
  • Specifically induces the formation of commissural interneurons (decussating interneurons)
• Dorsal side contains sensory (somatic and visceral) nerve fibres

Question 32

Is embryonic patterning of the spinal cord retained in the adult?

Answer 32

Yes, roughly

Question 33

What are neural crest cells?

Answer 33

• Cells released from the dorsal lip when the neural tube is closing
  
  • Neural tube cells undergo epithelial-to-mesenchymal transition (EMT), delaminate and migrate to the periphery
• These cells are essentially a sea of stem cells that can then migrate and differentiate to form a wide range of cells in the periphery
  
  • The PNS is derived from neural crest cells

Question 34

Give an example of abnormality of neural crest cell migration.

[EXTRA]

Answer 34

Hirschsprung’s disease, congenital megacolon

• This is the congenital absence of Meissner’s and Auerbach’s autonomic plexuses in the bowel wall due to a failure in migration by the neural crest cells, resulting in a failure to colonise the abdomen and form the enteric nervous system
• This is usually limited to the colon, most commonly anal and only very rarely involving the entire GI tract
• Peristalsis is absent or abnormal, with spasms and obstructions within the bowel – dilation of more proximal, normally innervated segment
• Correct diagnosis is needed ASAP, including biopsy, or else it can result in toxic enterocolitis
• Treatment is a colostomy: resection of the aganglionic bowel (this region is determined via biopsy) and then the two ends are reconnected, to form a somewhat functioning and complete GI tract

Question 35

What are rhombomeres?

Answer 35

• Regional differences in the rostro-caudal axis
• Hindbrain segments contain cranial nerve nuclei and are patterned using rhombomeres
  
  • Different cranial nerves are associated with different rhombomeres
  • Different regions specified in part by the activation of Hox genes and segmental Hox gene patterning (Hox signals come from within the CNS, there are partially overlapping patterns along the structure)

Question 36

How can CNS nuclei be knocked out?

[EXTRA]

Answer 36

• Altering gene expression patterns in the rhombomeres
• For example, mutation or KO in Gbx2 causes a failure to develop the trigeminal nerve

Question 37

What are some features and experimental evidence on Hox genes?

[EXTRA]

Answer 37

• Hox gene patterning is highly conserved, even between organisms (e.g. humans and Drosophila)
• Experimental example: antennapedia, mutation in homeotic Hox gene
  
  • Homeotic gene = gene which, if mutated, causes the conversion of one body part into another
  • In this case causes antennas to be substituted for legs

Question 38

What does rostro-caudal patterning depend on?

Answer 38

Depends on the dimensions and signals from the rhombomeres, and the location of the nulcei

Question 39

What is neurogenesis?

Answer 39

The process by which new neurons are produced in the brain (essential for development, also continues in some regions throughout life e.g. olfactory bulb)

Question 40

What cells facilitate neurogenesis?

Answer 40

• Neuroepithelium generates neurons at a very high rate
• Progenitor cells (therefore partially restricted) are found within the neuroectoderm

Question 41

Where do most of the divisions in neurogenesis occur?

Answer 41

Next to the ventricular zones/CSF-filled ventricles

Question 42

What happens after neurogenesis?

Answer 42

• Cells then migrate out towards the pial surface/towards the skull
• The first post-mitotic cells are found at this surface, which is known as the primordial plexiform zone or ‘pre-plate’
• This patterning method is the same within all mammals, with projenitors in the ventricular zone/subventricular zone and mature post-mitotic cells lying close to the pial membrane/pre-plate region
• This migration route starts short but eventually grows longer

Question 43

What is the patterning of neurogenesis?

Answer 43

• This method of patterning is the same within all mammals, with progenitors in the VZ (ventricular zone) and SVZ (subVZ), with mature post-mitotic cells being observed close to the pial membrane/in the pre-plate region – the route starts short and then increases in length
• This is initially split into two regions (marginal zone and the sub-plate), then the cortical plate develops from an inner- to outermost fashion between the two, largely transient cell layers.
• [EXTRA] To discover where the divisions occur, a very simple immunological stain for phospho-histone H3 (anti-phospho histone H3 binds at the epitope), which is only exposed in dividing cells.

Question 44

What are the progenitor cells of the cerebral cortex?

Answer 44

• Radial (glia) progenitor cells, with extended, long projections
  
  • There are also other progenitors, including intermediate, short neural and outer radial glial progenitors
  • These classifications depend on the shape and connections

Question 45

How can different progenitor cells produce different neurons?

Answer 45

• Different progenitors can produce different numbers of neurons and in different combinations
• Some of the radial progenitors can generate neurons directly (direct neurogenesis)
• Some produce intermediate progenitors, which then cycle and produce more neurons – in species with very large brains, there are more intermediate neurons as this helps to generate more neurons and therefore a bigger brain.

Question 46

What is inter-kinetic nuclear migration?

[EXTRA]

Answer 46

• This is the process by which nuclei from radial glia progenitors migrate through the cytoplasm as they divide
  
  • The nuclei descend to the subventricular zone to meet the centrosomes

Question 47

What are some abnormalities associated with nuclei-moving machinery?

[EXTRA]

Answer 47

• If abnormal spindle-like microcephaly-associated protein is present, APSM microcephaly occurs as the progenitor cells cannot divide – the head circumference will be two standard deviations below average (definition of microcephaly)
  
  • This is a breakdown of mitotic machinery, resulting in a devastating output from the germ layers and a far, far smaller brain
• If there is an alteration in the expression of transcription factors within the telencephalic vesicle, for example those involved with the SVZ, again the result is a smaller brain – if there is a Tbr2 mutation, for example, a smaller brain with fewer neurons is produced due to the reduced number of progenitor cells
  
  • In humans, Tbr2 knockout is known to cause silencing of T-box transcription factor EOMES, leading to microcephaly with polymicrogyria (smaller gyri and sulci) and agenesis of the corpus callosum (which usually communicates between the two hemispheres)
• ZIKA virus is an example of an environmental influence that affects neurogenesis – the virus has a devastating effect on pregnant mothers, especially if the mother became infected during the first trimester, causing cases of microcephaly (study showed strong temporal clusters relating the condition to the ZIKA virus)
  
  • About 80% of the population had no symptoms and the other 20% only had mild symptoms (e.g. swelling, mild oedema, facial rash)
  • In about 1% of infections during the first trimester, the baby would be born with a microcephaly condition, including microcephaly, corpus callosum agenesis, lissencephaly, ventriculomegaly, arthrogryposis (curving of joints)
  • ZIKA virus kills off some of the progenitor cells, also effecting the retina and joints, making this virus result in a complex congenital syndrome
  • Microcephaly cannot be ‘grown out of’, so the children born with the condition just have to be managed
  • The risk of adverse effects from ZIKA were relatively low (1%), but the effect was seen to such a huge extent due to the extremely high proportion of cases within the population

Question 48

What is the effect of Zika virus?

[EXTRA]

Answer 48

• ZIKA virus is an example of an environmental influence that affects neurogenesis – the virus has a devastating effect on pregnant mothers, especially if the mother became infected during the first trimester, causing cases of microcephaly (study showed strong temporal clusters relating the condition to the ZIKA virus)
• About 80% of the population had no symptoms and the other 20% only had mild symptoms (e.g. swelling, mild oedema, facial rash)
• In about 1% of infections during the first trimester, the baby would be born with a microcephaly condition, including microcephaly, corpus callosum agenesis, lissencephaly, ventriculomegaly, arthrogryposis (curving of joints)
• ZIKA virus killed off some of the progenitor cells, also effecting the retina and joints, making this virus result in a complex congenital syndrome
• Microcephaly cannot be ‘grown out of’, so the children born with the condition just have to be managed
• The risk of adverse effects from ZIKA were relatively low (1%), but the effect was seen to such a huge extent due to the extremely high proportion of cases within the population

Question 49

What is direct and indirect neurogenesis?

Answer 49

• Direct neurogenesis is asymmetric mitosis, the radial glia progenitors (subtype I) give rise to one neuron with each round of division
  
  • Produces comparatively fewer neurons
• Indirect neurogenesis is where radial glia progenitors (subtype II) give rise to intermediate progenitors upon mitosis, and it is these secondary cells that divide within the sub-ventricular zone and give rise to neurons
  
  • Intermediate progenitor cells do not have the same long, radial projections
  • This process gives rise to comparatively more neurons

Question 50

Are all neurons produced by neurogenesis retained in the adult brain?

Answer 50

No, there is a lot of apoptosis

Question 51

What is some experimental evidence showing apoptosis in the developing CNS?

[EXTRA]

Answer 51

• Caspase-9 KO mice caused a lack of cell death among progenitor cells, resulting in a protruding brain that cannot be contained within the skull

Question 52

What does the subplate/marginal zone contain?

Answer 52

• Transient cells, which die off after the cortical plate develops
• Subplate allows for connections between different regions in the developing brain, but these cells die off in large numbers
  
  • Preferential death is shown in the subplate as opposed to cortical regions, for example
• The white matter below the cortical plate initially contains cells, but these die off with only some interstitial cells remaining

Question 53

What is some clinical and experimental evidence related to the subplate/marginal zone?

Answer 53

• The sub-plate is a very important platform in which connections are made within the developing brain, but then these cells will die off in very large numbers
• This has been shown through the use of birth-dating studies – to show that a cell was born at a particular time, a thymidine analogue is injected and then incorporated into the genetic information of new-born cells which can then be followed
• Hypoxia can cause ischaemia within these cells even before they begin to function, preventing connections from being made between different developmental regions

Question 54

Why do neurons need to migrate in the developing CNS?

Answer 54

Most neurons are generated at a site distant from their final destination, so then have to migrate to their proper location. This is a very vulnerable process.

Question 55

What determines the cell fate of the developing neurons?

Answer 55

The last S phase of neurogenesis

Question 56

How do cells reach the germinal zone?

Answer 56

• Cells are destined for a particular region upon leaving the germinal zone
  
  • If there is an abnormality in migration, the cells simply do not make it and function is lost
• When cells have to migrate, they must first extend a leading process (growth cone)
  
  • These cells are thought to follow radially orientated radial glia progenitor cells (using their projections as a guide)
  • Once the cells reach their proper destination, they then dissociate and begin to differentiate

Question 57

How do GABA-ergic cells migrate?

Answer 57

• These migrate and development in a different method to other cells of the cortex
• These are generated away from the cerebral cortex and migrate in a tangential manner, to then end up residing within the cortex
• They have to make it into the cerebral cortex via several channels

Question 58

Why is it likely that neurons are generated at distant sites from their final destination, and undergo differentiation at such a late stage?

Answer 58

• This is likely because an enourmous number and variety of both neuronal and supportive cell types must be generated, therefore neurogenesis must be spatially and temporally seperated
  
  • This allows different combinations of transcription factors to be available to different progenitor cells, creating huge variety
• For example, in direct neurogenesis within the VZ, there is different transcriptional control to indirect neurogenesis occurring in the SVZ
  
  • Waves of transcription factor expression in different combinations results in the development of different cells
• Different layers within the brain form vastly different interconnections, with various circuits forming very different computational functions depending on the circuitry present

Question 59

When does the majority of neurogenesis occur?

Answer 59

• During development (prior to birth/in utero), with very little neurogenesis occurring in later life
• In adulthood, there are only limited sites at which neurogenesis occurs:
  
  • Sub-ventricular/subepyndymal zone
    
    • In the angle of the lateral ventricle, produce neurons that then migrate out to the olfactory bulb (site of continued cell replacement)
  • Dentate gyrus
    
    • Part of the hippocampus, involved in learning and memory - [EXTRA] Cab driver experiment, hippocampus increased in size compared to age-matched controls, also larger hippocampus relative to time spent on the job

Question 60

What is the dogma of adult neurogenesis?

Answer 60

There is no mitosis of mature neurons, with very limited sites of adult neurogenesis (SVZ, dentate gyrus)

• There is no mitosis of mature neurons
• There is no or very limited neurogenesis in the cerebral cortex
• There are limited sites of adult neurogenesis after birth, but it is debated for how long this process continues (SVZ, dentate gyrus)
• In injured brain, there may be potential for action of dormant progenitors, which is thought to be able to restore some function

Question 61

What is some experimental evidence that may contradict the dogma of adult neurogenesis?

[EXTRA]

Answer 61

• There is debate as to whether there are other sites of neurogenesis, and it has been suggested that humans have little adult neurogenesis and are slightly different to other mammals
• In experimental animals, thymidine analogues can be injected into neuronal cells and then incorporation of these labelled cells can be measured to see whether there is any neurogenesis taking place
  
  • This will not reflect human actions, however
  • Researchers believe that positive results may be due to injection of analogues into satellite and glial cells as opposed to neurons, leading to debate
• In the biotope, there has been a recent drastic increase in the number of ¹⁴C isotopes due to surface testing of nuclear weapons – after 1963, the limited nuclear test ban treaty was signed, stopping the surface testing of nuclear weapons and decreasing the concentration of ¹⁴C levels in the atmosphere
  
  • These isotopes will have been incorporated into neurons and glia, so could use someone born around 1963/1964 and test the ¹⁴C content of their brain to see whether levels indicate that they were born at this time – human carbon dating!
  • If the result and the DOB don’t match up, this could indicate neurogenesis – however, it was indicated that the neurons were the same age as the individuals, but the glial cells were approx. 10 years younger
  • Debate concerning dentate gyrus and SVZ is still going on, as it has only been confirmed in animal models
  • Some papers suggest that there is very limited neurogenesis occurring from a few years after birth

Question 62

Do neurons have a predetermined fate after leaving their site of neurogenesis? What effect does this have?

Answer 62

• Yes, their fate is predetermined
• This means that restriction of migration can have a massive effect on function
  
  • Neuronal migration defects are relatively common, as fates of neurons are likely to be determined in the last S phase before final division

Question 63

Compare migration of glutaminergic and GABAergic neurons.

Answer 63

• Glutamatergic neurons migrate radially (VZ to CP – cortical plate cells mature inside first, then outside)
• In GABA-ergic cells there is a tangential migration to travel from the ganglionic eminence to the cortex.

Question 64

What is a historical clinical example of the effects of defects in neuronal migration?

[EXTRA]

Answer 64

High levels of intellectual defects seen in children after the atomic bombs were dropped in Japan at the end of WW2

Question 65

What is needed to move developing neurons throughout the cortex/to their final destination? How does this work?

Answer 65

Intact molecular machinery, including:

• Filamin
• Actin
• Small GTPases
• Microtubules (essential for establishing a perinuclear cage that then drags the nucleus after the leading edge extension)

Neurons extend a leading edge and then drag the nucleus behind themselves - this process needs to be repeated several times in order for the neuron to reach its target destination.

Question 66

What defects can occur is there is no leading edge extension?

[EXTRA]

Answer 66

• If there is no leading-edge extension, then cells do not really leave the site of generation and instead form disorganised clusters of cells
  
  • This can lead to heterotopias, which means ‘altered location’
  • The cells next to the ventricular zone were never able to leave the site of neurogenesis and fail to develop properly or reach the cortex, resulting in altered neuronal activity
  • For example, filamin mutations can lead to X-linked periventricular heterotopias – failures in the role of actin microfilaments or small GTPases can also lead to this sort of malformation
  • Can also be observed as smaller, periventricular nodules with potential ectopic activation
• Lissencephaly (smooth brain, issues with the folding of the brain) is another type of migration defect
  
  • There will be an only partially developed cortex due to a failure of the more superficial layers to migrate through those that are already established – this results in a failure of the brain to fold properly
  • There are different degrees of lissencephaly with varying degrees of smoothness from a condition where there are no gyri (agyria), where only certain regions of the brain have no folding (e.g. posterior agyria) or just different degrees of smoothness
    
    • E.g. Type 2 lissencephaly is where cells breach the pial surface as they do not stop in their migration, forming heterotopias at the surface of the brain - this gives the brain a cobblestone-like appearance

Question 67

What happens when there are defects in the separation of the top plate and marginal zone?

[EXTRA]

Answer 67

• This results in the formation of a super-plate
  
  • This causes the cortical plate to develop in an inverse manner (outside to inside)
• This is caused by a defect in the Reelin pathway, causing a cerebellum and cerebral-cortical defect
  
  • The name is derived from the characteristic movements seen in this condition

Question 68

Describe some migrational defects using diagrams.

[EXTRA]

Answer 68

Question 69

How have the factors that regulate brain folding been discovered?

Answer 69

• Through finding the determinants of the lissencephaly spectrum

Question 70

Give some experimental evidence that indicates what regulates brain folding.

[EXTRA]

Answer 70

• Tallinen, 2016: made 3D-printed brain models of early brains from scans, then using it to produce a master moulds and replicated a brain using specific gels that would expand after absorbing specific solutions
  
  • This was achieved in two stages: there was a core and then a separate outer layer with different properties
  • This was then put into solution and observed the way in which their ‘brain’ was expanding
  • It was shown that as the brain expanded, folds occurred, due to differential growth between the core and the outer layer
  • The jelly brain showed extremely similar structural changes to an actual brain as it expands during development – the pattern of folds etc was very similar
• The major sulci and gyri are the same between individuals – genetic or epigenetic alterations are what will cause major changes in the folding of the brain

Question 71

Which brains are thought to receive more sulci and gyri?

Answer 71

• The brains which have more layers of progenitor cells (esp. OSVZ/outer subventricular zone cells) are suggested to receive more sulci and gyri
  
  • This is not a complete association, however, as these cells are found in less folded mammals and in individuals suffering from lissencephaly
  • You must look at a huge variety of species to understand this concept

Question 72

What is the effect of alcohol and drug exposure during pregnancy?

[EXTRA]

Answer 72

• Foetal alcohol syndrome results in a devastating neurological pathology
• It is a lifetime disability – children do not just ‘grow out of it’, but early diagnosis and intensive (and appropriate) can make an enormous difference in the prognosis for that child
  
  • Prevention, however, is still the most important factor
• In the US and Europe, 1 in 100 babies being born has some degree of FAS, and up to 30% of pregnant women are taking alcohol during pregnancy

Question 73

What are some of the societal consequences of foetal alcohol syndrome?

[EXTRA]

Answer 73

Question 74

What is a good analogy for the effect of transcription factors of cell differentiation?

Answer 74

Rubix cubes - there are only a limited number of transcription factors, but their order and arrangement (both temporally and spatially) can result in the huge amount of cell variety seen within the cortex

Question 75

Is the genetic programme for neurogenesis preserved?

Answer 75

Yes, both during embryogenesis and in adulthood

Question 76

How do some transcription factors interact during neurogenesis?

[EXTRA]

Answer 76

• Some transcription factors have a cross-suppressing effect on each other, allowing the gene expression to be regional and keeping proper development
  
  • It is important to regulate expression in order to retain the correct proportion of cells
  • For example, COUP-TFI KO mice have specific impairment in corticospinal tract-mediated fine motor skills due to imbalances in molecular messaging during development – this means that the mice are unable to retrieve a piece of food from between a small gap where a wild type mouse would be able to with ease (KO can see and smell the food pellet, but is unable to produce the correct motor control to retrieve it)

Question 77

What are ‘cerebral organoids’ and what potential use do they have?

[EXTRA]

Answer 77

• ‘Cerebral organoids’ – induced pluripotent stem (iPS) cells as models
  
  • The organoids have very similar early development processes as to those seen in the early developing brain of humans – they are able to model some human brain disorders and the earliest stages of brain development
  • The timing of the molecular waves is preserved and is very similar to that seen in human brains, working on the same time period

Question 78

Where is neural cell death seen during development?

Answer 78

• Some die in the germinal zone
• Some cells are destined to die, for example subplate/subcortical neurons that are the earliest generated cells in the brain and lie adjacent to white matter

Question 79

What are subplate/subcortical neurons and what is their fate?

Answer 79

• Earliest generated cells
• Lie adjacent to the white matter
• These cells are frequently destined to die
• They are very susceptible to hypoxic ischaemic brain damage, whilst also retaining an important role in the development of the cortex
• Transient/only present during development before dying off
  
  • [EXTRA] This pattern has been observed using birth dating
• These cells act as a dynamic, transient scaffold essential for brain development
  
  • Removal of subplate cells means that the connectivity of the brain cannot be established and therefore cortical circuits cannot be modified

Question 80

What are some effects of preterm white matter injury?

[EXTRA]

Answer 80

• They can have different levels of severity
• Removal of the sub-plate cells means that the connectivity of the brain cannot be established, and the cortical circuits are not able to be modified
• When the insult occurs has a large effect on the severity of the condition and development of the baby – earlier tends to be more devastating

Question 81

How can inflammation due to hypoxia have an effect on the developing brain?

[EXTRA]

Answer 81

• Inflammation hypoxia can have an impact on the physiology of the oxygenation of the brain, and it can also disrupt the BBB function which will in turn lead to secondary effects
  
  • It can change the developing axons, the grey matter, then cascading down to all sorts of complex reduced brain functions

Question 82

How are neural connections established?

Answer 82

• If there are lots of projections extending from one region of the brain to another, they have to interconnect in a topographic way/organised manner
  
  • Some rely upon molecular cues to reach their final destination, whereas a small proportion are able to find their way using neuronal activity
• The axons and cells extend long neurites that are able to navigate within the environment through the use of growth cones
• Connections are usually established when the distance between cells is minimal
• The growth cone exists on the tip of the advancing neuron, using filopodia to explore the immediate environment (look like flickering fingers)
  
  • There is actin polymerisation occurring within these filopodia (can be visualised using GFP)
• There is local attraction and repulsion, the insertion of new cytoskeleton and autonomous action
• Growth cones respond to various positive and negative cues (e.g. contact, distance, diffusible chemicals)

Question 83

How are axons thought to be guided to their correct connections?

Answer 83

• Using molecular cues - growth cones are either attracted or repelled by these
  
  • These cues can be local (substrate-bound) or long-range (diffusible)
• A given molecule can be both attractive and repulsive, depending on the interaction

Question 84

What are some actions of netrin?

[EXTRA]

Answer 84

• Netrin is a diffusible molecular guidance cue, which is expressed by the notochord (after initial expression of Shh)
  
  • If a dorsal neural tube explant is put next to a floor plate, then the explants will grow towards the floor plate
  • Netrin is also able to repel some neuronal populations (it does not attract all neurons) – for example, in the brainstem, netrin is repulsive to the developing cranial nerves, causing them to exit the brainstem in a dorsal direction (opposite side to netrin expression)
  • The action of netrin is able to switch from attraction to repulsion if they go through the region

Question 85

Why is synaptic refinement needed and how is it achieved?

Answer 85

• Initial connections can be imprecise, with incorrect target nuclei or incorrect locations within the target nuclei
• Axons connect to more targets and targets are contacted by more axons that required during development (therefore refinement/pruning is necessary)
• Regulation is achieved through:
  
  • Neuronal death
  • Axonal retraction
  • Synapse elimination/pruning

Question 86

What is commonly seen in developing neurons (and a reason for specific elimination)?

Answer 86

• Primary axons typically extend way beyond their target, causing their target to instead by innervated by de novo collateral branches that form interstitially along the primary axon
• Excess or inappropriate collaterals, alongside the considerable length of the primary axon that overshoots the distal target are eliminated by a process that appears to rely primarily on degeneration

Question 87

What is an example of axonal overshoot?

[EXTRA]

Answer 87

• Best studied example is of cortico-optical connectivity development
• Layer 5 neurons (largest of the brain) send out projections all the way into the spinal cord
  
  • All cortical areas send projections into the spinal cord, even the visual cortex (although these are eventually lost)
  • The same is not true for the motor areas, their projections are maintained throughout development

Question 88

What is Hebbian synapse theory?

Answer 88

• ‘Cells that fire together, wire together’
• Donald Hebb, 1904-1985, suggested that neuronal connections that are co-activated have a stronger coactivity
• Connections are made within the nervous system from a very young age – neuronal activity is possible from the very beginning of development and even has some action in deciding which connections are retained
  
  • The brain is essentially put together by itself
• This can lead to situations where being efficient at a task can lead to loss of circuits, as travel of impulses within the brain will affect the structure
  
  • This causes better use of what is available, but also reduces the ability to adapt due to a lack of connections

Question 89

When does cortical thickness peak?

Answer 89

Age 11 where there is the highest number of neurons, after this point there is a general decline due to cell death

Question 90

What is synaptic plasticity?

Answer 90

The ability to change synapses - once neuronal cells are generated, they can interconnect in many different ways and are able to change their synapses/are plastic throughout their lifetime

Question 91

How does input into the CNS change throughout development?

Answer 91

• Initially there is no peripheral input, only spontaneous activity within the brain
  
  • Innate process
• At a later stage in development, neuronal activity is driven by sensory input as they are exposed to the outside world
  
  • This has a major effect on organisation and circuit development within the brain
  • Determines response to the environment

Question 92

What is some experimental evidence indicating the effect of sensory input on cortical development after birth?

[EXTRA]

Answer 92

• If the eye is removed in embryonic stages (as has been done in monkeys), the shape of the brain is changes and the development of the primary visual cortex is limited
  
  • Rakic, 1988, indicated that borders between cortical areas were not fixed using the above method
• If a similar manipulation is done after birth, such changes are not seen - this removal has to occur during a critical stage of development to see the effect
• This experiment indicates that it is a combination of both nature (genetics/embryonic development) and nurture (environmental influences) that affects the patterning of the brain

Question 93

In what way are both nature and nurture involved in shaping the developing brain?

Answer 93

• Initial production of neurons is achieved through gradients of molecules within the brain
• These gradients are then modified by external (i.e. sensory) influences, carving out specific regions that are optimised for specific functions

Question 94

What are the most important extrinsic signals influencing cortical functional localisation during development?

Answer 94

All visual, audition and somatic-sensory inputs mediate through the thalamus, and the thalamocortical connections mediate extrinsic signals, transmitting them to the cortex

Question 95

What is some experimental evidence indicating the effect of extrinsic signals on cortical functional localisation during development?

[EXTRA]

Answer 95

• Experiments using barrel fields in mice - these are huge areas devoted to the representation of whiskers in the upper lip of mice
• If a whisker is stimulated, then some of the contralateral cortical region is stimulated where rings of cells are found (rough shape of a barrel in coronal sections)
• If the sensory input is changed early on by removing some of the whiskers in a new-born mouse, the cortical representation is changed
• If the infraorbital nerve is cut (carried sensory information to the trigeminal nucleus), then no barrel patterns are formed
• The cortex has not been touched – the alterations occur purely through alterations to methods of input
• If a similar manipulation occurs later on, there will be no changes – the plasticity is only available for a limited amount of time

Question 96

How are topographic maps established within cortical areas?

Answer 96

• To establish connections between neurons, a certain level of activity needs to occur and be delivered to the cerebral cortex
• For example, the connections of the eyes to the cerebral cortex are initially mixed but segregate during development
  
  • This process is dependent upon a certain level of activity occurring within the visual cortex
  • There is a very specific pattern that develops over time, and is not found in young samples
  • If the connection between the eye and the thalamus is blocked in very early stages, blocking the activity as if the eye is unopened, then the patterns and distinctions between the connections of the two eyes are not seen
  • Early inputs into the brain are mediated by cholinergic mechanisms, before glutaminergic mechanisms begin to dominate
• Spontaneous activity also has an important role in the development of early sensory maps

Question 97

What is the effect of closing one eye at certain time periods after birth?

[EXTRA]

Answer 97

• If one eye is blocked, then the other will take over but this has to occur in the very early stages of development
  
  • This indicates the critical period immediately after birth upon which full eye development is dependent on
• If one eye is blocked at birth, then the other eye takes over/dominates the connections within the visual cortex
• If one eye is blocked after 3 weeks, then the unblocked eye will become dominant but both will still have representations within the cortex
• After 6 weeks, the closing of one eye has no effect on development/representation in the visual cortex

Question 98

What are some examples of plasticity in adult and mammal brains?

[EXTRA]

Answer 98

• Use of different body parts in different ways will affect representation of these actions within the brain – for example, the middle three digits on a monkey’s hand were trained for 1 hour a day and there was a noticeable change in representation; after 3 months the 3b area associated with this movement was noticeably larger
  
  • This type of plasticity is even present in adults
  • If a human subject is trained to do a rapid sequence of finger movements for 3 weeks (10-20 mins per day), then a huge change in the region of activation was seen during an MRI scan – the change persisted for several months
• It is clear that nerve connections can reorganise, even in the adult
• Example: the sensory pathways for the hand and the face run separately through the cortex, but if the arm is amputated, then the pathways can join
  
  • So if asked to close your eyes after this injury, someone touching your face can feel like someone is touching your non-existent arm, due to plasticity of the connections
    
    • ‘Facial remapping’, which can be explained by the plasticity of the system
  • Touching of the face can be perceived as touching of the non-existent hand
    
    • E.g. after a mastectomy, the sensory innervation of the breast can re-wire to be connected to the ear, due to plasticity and the close proximity of both of these circuits

Question 99

What is the basal plate in CNS development?

Answer 99

• The basal plate is the region of the neural tube ventral to the sulcus limitans (found in the 4th ventricle, separates the cranial nerve motor nuclei (medial) from the sensory nuclei (lateral))
• It extends from the rostral mesencephalon to the end of the spinal cord and contains primarily motor neurons
• Is split into left and right basal plates by the floor plate (spinal cord is symmetrical)

Question 100

What is the alar plate in CNS development?

Answer 100

• The alar plate is a dorsal region of the neural tube
• Associated with sensory neurons, including the sensory nuclei of some cranial nerves

Question 101

What does the telencephalon become and how is this achieved?

Answer 101

• The telencephalon is a swelling at the rostral end of the neural tube
• These begin to expand into two symmetrical structures that sit alongside each other at the end of the neural tube
• These will become the cerebral hemispheres