1. NS development

Question 1

describe the process of neural tube formation

Answer 1

NEURULATION (starts in week 3, lasts 10 days):

1. The notochord (created during gastrulation) signals overlying ectoderm to convert to neuroectoderm and thicken into slipper-shaped neural plate.
2. Lateral edges of neural plate elevate into neural folds (around a neural groove) which fuse to form neural tube

Question 2

what are neuropores

Answer 2

Open ends of neural tube - cranial and caudal neuropores - which communicate with overlying amniotic cavity.

Cranial neuropore closes day 25 followed by caudal neuropore 3 days later.

Question 3

describe the 3 layers into which the neural tube organises into - what are the mature derivatives of these

Answer 3

1. inner NEUROEPITHELIAL layer
2. intermediate MANTLE layer:
   - consists of neuroblasts (primitive nerve cells) - later form grey matter of spinal cord
3. outer MARGINAL layer:
   - contains nerve fibres emerging from neuroblasts - are later myelinated to form white matter of spinal cord

Question 4

label this diagram of the primitive neural tube

Answer 4

Question 5

what are the basal and alar plates and what do these give rise to

Answer 5

Continuous addition of neuroblasts to mantle layer forms ventral and dorsal thickenings:

• ventral basal plates (contain motor horn cells)
• dorsal alar plates (contain sensory horn cells)

Question 6

what is the function of roof and floor plates

Answer 6

Are dorsal and ventral midline portions of neural tube that don’t contain neuroblasts - act to:

• regulate dorsal and ventral patterning
• guide neuronal positioning

Question 7

describe the formation of brain vesicles during dev. - what are their mature derivatives?

Answer 7

During neural fold formation, cephalic end of neural tube forms 3 dilations… become 3 primary brain vesicles after NT closure in week 4… then differentiate into 5 secondary vesicles by week 5

1. PROSENCEPHALON (forebrain) gives rise to:
   a) TELENCEPHALON = cerebral hemispheres
   b) DIENCEPHALON = optic vesicle, thalamus, hypothalamus & pituitary
2. MESENCEPHALON (midbrain) gives rise to:
   c) MESENCEPHALON = midbrain
3. RHOMBENCEPHALON (hindbrain) gives rise to:
   d) METENCEPHALON = pons and cerebellum
   e) MYELENCEPHALON = medulla oblongata

Question 8

describe the 2 flexures that form during brain dev.

Answer 8

Growth and dec. at cranial NT exceeds available space linearly… it must fold with neuroaxis not remaining straight:

• cephalic flexure: midbrain region
• cervical flexure: hindbrain-spinal cord junction

Question 9

what is the neural crest

Answer 9

Cells of the lateral border of neuroectoderm tube which become displaced and enter mesoderm - undergo epithelial-to-mesenchymal transition

Question 10

how do neural crest cells contribute to nervous system dev.?

Answer 10

Give rise to:

• cranial nerve and spinal (dorsal) root ganglia
• enteric ganglia
• sympathetic ganglia (chain and pre-aortic)
• schwann and glial cells
• leptomeninges (arachnoid and pia)

Question 11

how do neural crest cells contribute to head, neck and midline dev.?

Answer 11

Give rise to:

• connective tisue and bones of face and skull
• odontoblasts
• dermis
• C cells of thyroid gland

Question 12

what is hydrocephalus and how does it occur?

Answer 12

Obstruction to flow of CSF causing fluid accumulation and enlargement of ventricular system, with raised intracranial pressure.

Can be congenital (most common in neonates with spina bifida) or acquired (e.g. tumour or infection).

Question 13

how can hydrocephalus be treated

Answer 13

inserting ventriculoperitoneal shunt - allows drainage of CSF from one of cerebral ventricles to peritoneal cavity (important to prevent cognitive delay)

Question 14

what is rachischisis and why does it occur

Answer 14

Failure of neural fold elevation… splayed NT

(incompatible with life)

Question 15

what is anencephaly and why does it occur

Answer 15

absence of cranial structures, inc. brain, due to defect in cranial neural fold closure

(incompatible with life)

Question 16

what is spina bifida and what are the 3 types

Answer 16

Splitting of vertebral arches that can occur anywhere along length of spine (most common in lumbosacral region) due to caudal defect in neural fold closure.

• spina bifida occulta = vertebral arch defect covered by skin, not involving underlying neural tissue (usually asymptomatic)
• meningocoele = protrusion of fluid-filled meninges through vertebral arch defect. Results in neurological deficits but not usually associated with intellectual disability.
• myelomeningocoele = protrusion of fluid-filled meninges + neural tissue through vertebral arch defect. Results in neurological deficits but not usually associated with intellectual disability.

Question 17

why does hydrocephaly occur in 80-90% of severe NTDs?

Answer 17

Due to inappropriate tethering of spinal cord to vertebral column… vertebral column lengthens and tethered cord pulls cerebellum through foramen magnum… herniated cerebellum cuts off CSF flow

Question 18

how can spina bifida be diagnosed pre-natally

Answer 18

• ultrasound
• alpha-fetoprotein in maternal plasma