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_Sem 4. CNS NOTES > S1 L2 Development of CNS > Flashcards

S1 L2 Development of CNS Flashcards

1
Q

What is the first stage in the development of the CNS?

A

Gastrulation

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2
Q

What is gastrulation?

A

Formation of the trilaminar disc- endoderm, mesoderm and ectoderm

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3
Q

Describe the process of gastrulation?

A
  1. At about 17-19days, 0.61mm→ Primitive streak and primitive pit forms in the epiblast, starting at the caudal end and extending towards the cranial end
  2. Indentation appears along the surface of the primitive streak
  3. Epiblast cells approach the primitive streak and invaginate
  4. Inside they spread out forming the mesoderm and endoderm
  5. Epiblast layer becomes the ectoderm
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4
Q

What are the derivatives of the ectoderm?

A

Skin and neural tissue
Neural crest cells
(close relationship between skin and NS- many animals have the NS embedded in their skin)

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5
Q

What are the derivative of the mesoderm?

A

Heart
Vessels
Muscles …
Notochord→ develops cranially from the primitive streak, it inducts the overlying ectoderm to invaginate and form the neural tube

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6
Q

What are the derivatives of the endoderm?

A

Gut including accessory organs

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7
Q

What is neurulation?

A

Formation of the neural tube

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8
Q

How does neurulation take place?

A
  1. Day 19-21, 1.6mm→ Notochord inducts the overlying ectoderm to thicken
  2. Neural plates elevate forming the neural folds (ectoderm elevates and then invaginates)
  3. Fusion of the folds in the midline at the mid cervical level with reconstitution of the surface ectoderm
  4. Neural tube zips up rostrally and caudally
  5. When folds fuse, neural crest cells detach and migrate to their ultimate destination → found along the apices of the neural folds
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9
Q

What happens if the tube fails to close properly?

A

Rostral direction- anencephaly, no brain, incompatible with life
Caudal direction- spina bifida

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10
Q

What is the basic structure of the neural tube?

A

Day 23-26, 2.5mm
Fully invaginated and covered in ectoderm
Rostral part forms 3 swellings→
1- Prosencephalon- future forebrain
2- Mesencephalon- midbrain
3- Rhombencephalon- hindbrain
Flexures
- Cephalic flexure → between the forebrain and midbrain
- Cervical flexure → between hindbrain and spinal cord
Lateral outpouching on the forebrain become the optic nerves

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11
Q

How does the neural tube develop?

A

Day 35-38, 6.6mm
Swellings become further subdivided
1. Prosencephalon
→ Telencephalon- most of cerebral hemisphere (most cranial part of prosencephalon, remember→ telo- end like telomere)
→ Diencephalon- thalamus, hypothalamus, optic nerve and retina (remember di- two, paired structures)
2. Mesencephalon- stays the same
3. Rhombencephalon
→ Metencephalon- pons and cerebellum (met- behind or after, so behind the cerebrum)
→ Myelencephalon- forms the medulla (myelo- refers to spinal cord)

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12
Q

How does the ventricular system develop?

A
  • Develops from the lumen of the neural tube
  • Initially large lateral ventricles- brain tissue not developed so doesn’t fill out
  • Interventricular foramen
  • 3rd ventricles
  • Aqueduct of midbrain/ cerebral aqueduct
  • 4th ventricle
  • Medial and lateral aperture into subarachnoid space
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13
Q

What is the significance of the ventricular system developing from the lumen of the neural tube?

A

Developmental defects can lead to hydrocephalus - enlarged ventricles

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14
Q

How is development of the spinal cord related to development of the vertebrae?

A

Early in development there is a one to one growth between the spinal cord and vertebrae

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15
Q

How is the motor and sensory system arranged?

A

Pattern within the body - due to order events during development
Motor- anterior/ventral (remember motor forwards)
Sensory- posterior/dorsal

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16
Q

How does the arrangement of the sensory and motor system comes about?

A

Pattern exits because of development of alar (roof) and basal (floor) plates
Notochord induces the development of the ventral surface → basal plate via chemical signals etc… → motor neurones
Absence of influence form notochord it becomes the alar plate → sensory neurones

17
Q

How does the pattern of sensory and motor arrangement occur in the CNS?

A
  1. Spinal cord
    → Dorsal roots are sensory, ventral roots are motor
    → Dorsal horn sensory, ventral horn motor
    → Dorsal column (sensory tract) sits posteriorly, corticospinal tract (motor tract) sits anteriorly
  2. Medulla
    → Pyramids of the medulla (motor) sit anterior to the Lemnisci (sensory)
  3. Midbrain
    → Cerebral peduncles (motor) sit anteriorly to the colliuli (sensory)
  4. Cerebral cortex- shares the same pattern but for a different reason
    → Primary motor cortex anterior to the central sulcus
    → Primary sensory cortex posterior to the central sulcus
18
Q

Why does the cauda equina develop?

A

Initially→ one to one development of spinal cord levels and vertebral levels
Spinal root (dorsal and ventral root) tethered to the dorsal root ganglion
Vertebral column develops quicker than spinal cord- especially the lumbar levels
Lower portions of the cord are stretched drawing out the cauda equina

19
Q

What cauda equina defect can occur from neural tube defects?

A

Predisposed to hydrocephalus
Cord may become tethered to the VC
Therefore as it grows it takes the spinal cord with it
Causes the brainstem including 4th ventricle to be pulled downwards
Through the foramen magnum and become occluded
Impairs drainage out of lateral and 3rd ventricle into subarachnoid space
↑pressure in the ventricles leads to hydrocephalus

20
Q

What are neural tube defects?

A

Lots of different names for the same thing
- Spina bifida, spinal dysraphism, neural tube defects
Failure of the neural tube to ‘zip’ up during development

21
Q

What is the developmental basis for neural tube defects?

A

Neural tube fails to ‘zip up’ in the cranial or caudal direction
- Failure in cranial direction involves the brain - anencephaly
- Failure in caudal direction involves spine/spinal cord - classic spina bifida
All have failure of development of the posterior vertebral arches at one or more levels

22
Q

What does the vertebral column develop from?

A

Sclerotomes from somites
Appears neural tube is partly responsible for the migration of the sclerotome
Anomaly in the neural tube leads to disordered formation of the posterior arch

23
Q

What are the different types of neural tube disorders?

A 
Severe to least severe
- Craniorachischisis 
- Anencephaly 
- Myelocoele 
The following have normal neural tube but failure of posterior arch development
- Myelomeningocoele 
- Meningocoele 
- Spina bifida occulta
24
Q

What is Cranioarachischisis?

A

Entire neural tube remains open - failure of neural folds to come together and fuse
Failure of brain and spinal cord to develop
Incompatible with life

25
Q

What is Anencephaly?

A

Cranial neural tube fails to close
Failure of the brain to form
Might be born alive but don’t live for very long

26
Q

What is Myelocoele?

A

Spinal cord fails to develop i.e lumen of NT exposed to outside
Usually associated with a fluid filled cyst
Children frequently have neurological deficits and are susceptible to meningitis due to the presence of expose neural tube

27
Q

What are Myelomeningocoele, Meningocoele, Spina bifida occulta?

A

All have normal neural tube but fail to develop a posterior arch
1- Myelomeningocoele
→ CSF fluid filled cyst containing the spinal cord
→ Transillumination is poor because of tissue in the cyst
→ May have neurological deficits
→ Repair is necessary
2- Meningocoele
→ Presence of CSF fluid filled cyst
→ Cord situated within the vertebral canal
→ Transilluminable
→ Good neurological prognosis
→ Cyst needs repair- predisposed to infection
3- Spina Bifida occulta
→ Lack of posterior vertebral arch
→ Tuft of hair, large naevus over the defect
→ Not associated with significant neurological problems
→ Occurs in about 10% of the population

28
Q

How can neural tube defects be prevented?

A

Folic acid
→ Taken 3 months before conception
→ Taken until week 12 of pregnancy/ through out pregnancy
→ 400micrograms daily
→ Unknown mechanism but maybe important for fusion of epithelial sheets

29
Q

What are neural crest cells?

A

Highly specialised population of cells

From the apices of the neural folds- where they fuse and the ectoderm is reconstituted

30
Q

What are the derivatives of the neural crest cells?

A 
Cells derived from NC
1. All neurones who's cell bodies are in the PNS 
- Primary sensory neurones 
- Autonomic postganglionic neurones 
- Enteric neurones 
2. Schwann cells
3. Adrenal medulla cells 
4. Head mesenchyme 
5. Melanocytes in the skin 
6. The leptomeninges - arachnoid and pia mater 
7. Head mesenchyme → contributes to many tissues 
Tissue derived from NC
8. Thymus 
9. Thyroid
10. Parts of the heart 
11. Parts of the teeth
31
Q

What happens to the neural crest cells as they migrate?

A

Migrate from the dorsal part of the embryo- anterolateral direction
Populations of cell get left behind:
1. Dorsal root ganglia- sensory neurones
2. Sympathetic ganglia- sympathetic postganglionic neurones
3. Preaortic ganglia- sympathetic postganglionic neurones
4. Adrenal medulla- chromaffin cells, homologous to the sympathetic postganglionc neurones
5. Gut wall- enteric nervous system

32
Q

What disorders can arise from the neural crest cells?

A
  1. Di George syndrome
    - Immunodeficiency - due to involvement of the thymus
    - Facial anomalies - contribution of neural crest to facial development
    - Heart anomalies
    - Hypocalcaemia - involvement of parathyroids
  2. Hirschprung’s disease
    - Lack of enteric neurones in section of the large intestine
    - Leads to hypomobility and constipation

_Sem 4. CNS NOTES (21 decks)

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