Week 5

Question 1

What are the 3 main types of cytoskeletal filaments?

Answer 1

1. Microtubules:
   - Composed of alpha and beta tubulin (forms heterodimers)
   - The heterodimers form chains known as protofilaments which associate to create a long hollow cylinder
   - Make up centrioles, flagella and cilia and provide a pathway for the intracellular movement of organelles, vesicles and proteins
   - Important for axonal migration of neurons
   - MAPs regulate microtubule dynamics
2. Intermediate Filaments:
   - Have no unique structural basis
   - Assembly occurs as homo or heterodimers from a large variety of proteins
   - provide mechanical stability to the cell
   - take part in assembly of the nuclear envelope
   - supports the plasma membrane when it is in contact with other cells/ECM
   - The organisation of intermediate filaments is cell-type dependent
   - An example of an intermediate filament is keratin found in epithelial cells, desmin found in muscle cells and lamin in the nucleus of cells
3. Microfilaments (actin):
   - Helical filamentous polymers made up of 2 intertwined chain of actin monomers (G actin monomers intertwine to form F actin- the polymer)
   - Humans have 6 actin genes:
   4 encode alpha actin (muscle contraction)
   1 encodes beta actin (occurs at the front of moving cells)
   1 encodes gamma actin (acts as a stress fibre)
   - Mainly found in the actin cortex (under plasma membrane) and as actin stress fibres (spanning through cytoskeleton)
   - Form filopodia and lamellipodia on the mobile edge of migrating cells

Question 2

What are the major functions of cytoskeletal components?

Answer 2

1. Movement (migration) of cells
2. Maintenance of cell shape
3. Movement of organelles within cells
4. Movements of cilia and microvilli
5. Cell contraction
6. Endocytosis
7. Stabilisation of cell attachments
8. Mitosis (cytoskeleton creates the contractile ring full of actin filaments that allows for cytokinesis

Question 3

Describe the cytoskeletal composition of non-dividing cells:

Answer 3

1. Plasma membrane is supported by a thin, dense actin cortex (red)
2. Actin stress fibres span through the cell (red)
3. Microtubules (green) radiate from the microtubule organising centres throughout the cytoplasm and are associated with the intermediate filaments

Question 4

List the cytoskeletal components from most stiff to least stiff

Answer 4

Microtubules >actin filaments > intermediate filaments

Question 5

List the cytoskeletal components from largest to smallest

Answer 5

Microtubules > intermediate filaments > actin filaments

Question 6

Describe the microtubule structure of cilia and flagella:

Answer 6

• Have microtubules arranged in a 9 + 2 conformation

- The two unfused microtubules in the centre serve as molecular motors (dynein “arms”)

Question 7

What is individual vs collective cell migration?

Answer 7

• The migration of cells either individually or collectively is based on structural determinants of both tissue environment and cell behaviour

1. Individual cell migration:
   - Allows cells to cover local distances and intergrate into tissues
   - Allows cells to move to different locations in the body to fulfill effector functions e.g. immune cells
   - can contribute to cancer progression (metastasis)
2. Collective cell migration:
   - builds, shapes and remodels cells into complex tissue compartments such as ducts, glands and vessels

Question 8

Describe the progress of cell migration:

Answer 8

• Actin microfilaments are the cytoskeletal element of primary importance for cell migration

1. The actin microfilaments form a cortex just under the cell membrane
2. The actin microfilaments form the structures of the lamellipodium and the filopodia
3. Actin polymerisation extends the protusion of the lamellipodium forming focal contacts that contain integrins
4. The actin cortex is under tension and causes the contraction of the other end of the cell

Question 9

How do microtubules allow for the movement of organelles, chromosomes and other intracellular components?

Answer 9

1. Assists with vesicle movement between organelles and cell surface
2. Movement of chromosomes during mitosis and meiosis
3. Movement of pigment vesicles

Question 10

What is the difference between oligodendrocytes and Schwann cells?

Answer 10

1. Oligodendrocytes: can myelinate multiple axons from different neurons and are in the CNS only
2. Schwann Cells: can myelinate only one axons and are in the PNS only

Question 11

What is the function of astrocytes?

Answer 11

• A type of glial (supporting cell) in the nervous system
• Form an intermediate between the blood vessels in the neurons and in doing so generate the blood-brain barrier
• Remove excess neurotransmitter from synapses after APs
• They can phagocytose cell debris
• Help keep nervous tissue together

Question 12

What is the process of neurulation?

Answer 12

1. The neural plate differentiates from the rest of the ectoderm giving rise to the neuroectoderm
2. With cell proliferation the neural plate expands to form a neural groove
3. The neural folds on either side of the groove are pushed towards each other 4. The neural folds fuse transforming the neural plate into a neural tube
4. The superficial ectoderm detaches to form skin and cells at the junction form neural crest cells
5. The neural crest cells delaminate to form bone structures of the face and cranium, smooth muscles, melanocytes etc.

Question 13

How is the neural tube patterned along the dorso-ventral axis?

Answer 13

Ventral:
BMPs are at the highest concentration on the ventral side and establish the roof plate

• Sonic hedgehog from the notochord is at the highest concentration on the dorsal side and establishes the floor plate
• Shh acts on the patched receptor and inhibits it so smoothened can become active

Question 14

Describe the anterior-posterior patterning of the neural tube:

Answer 14

• The anterior end (develops into brain) has the highest concentration of Otx-2
• The posterior end (develops into spinal cord) has the highest expression of Gbx-2
• The midbrain-hindbrain boundary secretes Wnt-1 anteriorly and Fgf-8 posteriorly

Question 15

Describe the formation of the brain compartments:

Answer 15

• The anterior most region forms the telencephalon which will grow to cover most of the diencephalon and form the neocortex
• The diencephalon is the next most anterior and forms the thalamus
• The mesencephalon forms the midbrain
• The metencephalon forms the hind brain including the cerebellum

Question 16

Describe neurogenesis:

Answer 16

1. Progenitor cells are located in the ventricular zone- these cells send out long processes that attach to the outside of the neural tube
2. The cell body moves up and down the process and when it reaches the ventricular zone it divides either symmetrically to increase the pool of progenitor cells or asymmetrically to form 1 progenitor and 1 neuron
3. Any neurons that are formed migrate along the glial processes up to the cortical plate and settle there in a layer
4. The next wave of neurons formed move through the already pre-established cortical plate layer and settle on top of them
   - This process called cortical lamination means the outer-most layer of neurons in the brain were the last born

• After the process of neurogenesis is completed, the progenitor cells will generate oligodendrocytes and then astrocytes

Question 17

Where does adult neurogenesis occur?

Answer 17

• Adult neurogenesis is very limited but it does occur to some degree in:
  1. The subventricular zone of the lateral ventricle: to generate new neurons that migrate to the olfactory bulb
  2. Hippocampus

Question 18

How is the neocortex patterned?

Answer 18

• Pax-2 is strongly expressed in the anterior end of the brain
• Emx-2 is strongly expressed in the posterior end of the brain

Question 19

Give an overview of the neural tube defects:

Answer 19

1. Cranioachischisis: completely open brain and spin cord
2. Anencephaly: open brain and lack of skull vault (no closure of neural tube at anterior end)
3. Ecephalocele: herniation of the meninges
4. Iniencephaly: occipital skull and spine defects with extreme retroflexion of the head
5. Spina bfida occulta: closed asymptomatic NTD in which some of the vertebrae are not fully closed
6. Cloed spinal dysraphism: deficiency of at leads two vertebral arches
7. Meningocele: protrusion of the meninges (filled with CSF) through a defect in the skull of spine)
8. Myelomeningocele: open spinal cord

Question 20

What is open vs closed spina bfida:

Answer 20

Open Spina Bfida:

• Occurs when the closure of the neural tube stops before the edges of the neural tube meet
• The CSF flows out of the open tube
  e. g. myelomeningocele

Closed Spina Bfida:

• The neural tube is formed and closed but there is a defect with the meninges and/or vertebrae
• Can have obvious bulging or no bulging (spina bfida occulta)

Question 21

How is Spina Bfida diagnosed?

Answer 21

Prenatally:

• A protruding mass may be seen via ultrasound
• In cases of open spina bfida fetal surgery may be attempted (but success rates are poor)

Postnatally:
- Closed spina bfida can be identified by a bulging of tisue at the base of the spine or in the base of spina bfida occulta- a tuft of hair at the base of the spine

Question 22

How does folate help prevent spina bfida?

Answer 22

• Folate is a substate for the synthesis of important amino acids that are essential for cell metabolism and proliferation

Question 23

What is microencephaly?

Answer 23

• Small brain
• The brain fails to achieve normal growth
• Suggests defects in progenitor cell proliferation
• Associated with mental retardation, cerebral palsy and/or epilepsy
• Can occur prenatally, postnatally or perinatally
• The severity of symptoms varies greatly

Question 24

What is lissencephaly?

Answer 24

• Smooth brain
• Absence of gyri and sulci
• Caused by neuronal migration defects
• Type 1 classic lissencephaly: thicker than normal neocortex with impaired laminar structure
• Type 2 cobblestone complex lissencephaly: comprised pia integrity leading to over-migration of neurons to the cortical surface which form into nodules

Question 25

Describe the formation of excitatory and inhibatory neurons:

Answer 25

1. Excitatory: born locally and migrate radially in an inside-out pattern 2. Inhibitory (inter) neurons: born in subcortical compartments e.g. POA and migrate tangentially to reach cortex - These neurons migrate with the formation of solubule membrane bound ligands and receptors, the interaction can be attractive or repulsive

Question 26

Describe how schizophrenia is linked to brain development:

Answer 26

- Networks of neurons required to process sensory information are affected - Volume reduction of thalamus - DISC1 is a common mutation that affects neuronal migration

Question 27

Explain how ASD is linked to brain development:

Answer 27

- ASD is due to the disorganisation of the laminar structure of the neocortex - Patterns of disorganisation can be located in different areas between children - Ephrin-A2 is an important factor in controlling migration of interneurones - Ephin-A2 acts as a promoter of neuron division so a knockout leads to a lack of neurons being produced - KO Ephrin A2 mice show the same patches of neuronal disorganisation as children with ASD