L2 Development

Question 1

Development of the nervous system

Answer 1

One of the first to start developing

Last to fully develop so still susceptible to teratogenic insult in the foetal period

Question 2

Primitive streak

Answer 2

In week 3, the primitive streak appears which marks the onset of gastrulation

Question 3

Gastrulation

Answer 3

Creation of the endoderm, mesoderm and ectoderm germ layers

Question 4

Formation of the neural tube

Answer 4

1. Gastrulation produces the notochord
2. Notochord sends neurulation signals to the ectoderm that converts the ectoderm to nueroectoderm
3. Formation of the neural plate
4. The neural plate thickens laterally causing the edges to elevate and fold
5. Depressed mid region = neural groove
6. The neural folds fold into the midline and fuse forming the neural tube

Question 5

Notochord

Answer 5

Solid rod of cells running in the midline which release signalling molecules for neurulation

Question 6

Neural plate

Answer 6

The signalling molecules are limited by diffusion therefore only the ectoderm in close vicinity to the notochord with signalling can receive the signalling molecules and undergo neurulation to form the neural plate

Occurs from the midline outwards with the caudal end the last to fuse and form the neural tube

Question 7

Neurulation

Answer 7

Converts ectoderm to neuroectoderm to produce the neural tube

Takes 10 days

By 28-32 days the neural tube is completely closed

Question 8

Neuropores

Answer 8

The holes present at the caudal and cranial end when the neural tube hasn’t fully closed yet

If there are closing errors, it can lead to birth defects

Question 9

Cranial neural tube defect

Answer 9

Anencephaly - absence of brain and cranial structures (not viable for life)

Question 10

Caudal neural tube defect

Answer 10

Spina bifida - split spine

Question 11

Spina bifida

Answer 11

Commonly occurs in the lumbosacral spine

Meningocoele - meningeal cyst

Myelomeningocoele - the arachnoid and spinal cord protrude through the defective arch of the vertebrae [severe]

Can cause neurological deficits but not cognitive deficits

Hydrocephalus commonly occurs which can cause cognitive delay

Question 12

Rachischisis

Answer 12

The neural tube does not form

Failure of neural folds elevation

Question 13

Diagnosis of neural tube defects

Answer 13

Blood test - raised maternal alpha fetoprotein

- not specific but identifies a closure failure as normally retained in the neural tissue

Question 14

Prevention of neural tube defects

Answer 14

Folic acid pre-conceptually and first trimester

Question 15

Development of the spinal cord

Answer 15

Most of the neural tube becomes the spinal cord

3rd month: spinal cord and vertebral column are the same length

After the 3rd month: vertebral column grows faster

Therefore the spinal roots must elongate to exit at the intervertebral foramen, forming the cauda equina at L2

Question 16

3 primary divisions of the brain during development

Answer 16

Dilation at the cranial end when the neural tube closes in the 4th week

Forebrain - prosencephalon
Midbrain - mesencephalon
Hindbrain - rhombencephalon

Question 17

5 secondary divisions

Answer 17

Forebrain (prosencephalon):
Telencephalon - cerebral hemispheres
Diencephalon - thalamus and hypothalamus

Midbrain (mesencephalon):
Mesencephalon - midbrain

Hindbrain (rhombencephalon):
Metencephalon - pons and cerebellum
Myencephalon - medulla oblongata

Question 18

Flexures

Answer 18

Growth and development at cranial neural tube exceeeeds available space linearly therefore it folds

• cervical flexure - between the spinal cord and hindbrain
• cephalic flexure - midbrain folding

Question 19

Ventral aspect of brain and spinal cord

Answer 19

Brain - inferior

Spinal cord - anterior

Question 20

Dorsal aspect of brain and spinal cord

Answer 20

Brain - superior

Spinal cord - posterior

Question 21

Ventricular system development

Answer 21

Telencephalon - lateral ventricles and intervertebral foramen
Diencephalon - third ventricle

Mesencephalon - cerebral aqueduct

Metencephalon and myencephalon - fourth ventricle

Question 22

Hydrocephalus

Answer 22

Dilation of ventricles due to excessive accumulation of CSF

Common in newborns with spina bifida
Treatable by using shunt

Occurs due to blockage e.g. tumour or infection

Question 23

Layers of the neural tube

Answer 23

Neuroepithelial - makes CSF
Intermediate (mantle) - contains neuroblasts
Marginal layer - processes of neuroblasts

Question 24

Roof plate

Answer 24

Posterior point in neural plate - dorsal

Alar plate formation - afferent sensory

Question 25

Floor plate

Answer 25

Ventral point in neural plate - anterior

Regulates the formation of the basal plate - efferent motor

Question 26

Sulcus limitans

Answer 26

Boundary between the alar and basal plate on the neural plate

Question 27

Neural crest cells

Answer 27

Cells of the lateral border of the neuroectoderm tube

Before the neural tube fuses, some neural crest cells migrate and displace into the mesoderm and undergo epithelial to mesenchymal transition

Question 28

Examples of neural crest cells around body

Answer 28

Nervous system:
Dorsal root ganglion
Sympathetic ganglion 
Cranial nerves ganglia 
Schwann cells 
Glial cells 
Leptomeninges 

Head and neck structures

• C cells of thyroid gland
• dermis

Other:
Melanocytes 
Suprarenal gland - chromaffin cells 
Preaortic ganglia
Enteric ganglia

Question 29

Neural crest cell migration defects

Answer 29

Very susceptible to insult e.g. alcohol

• Hirschsprung’s disease - aganglionic megacolon
• DiGeorge’s syndrome - thyroid and thymus deficiency - immunodeficiency