2 - Embryology of the Nervous System

Question 1

How does the neural tube form?

Answer 1

• 3rd week gastrulation produces notochord in mesoderm
• Notochord sends signals to overlying ectoderm to differentiate to form neuroectoderm
• The neural plate thickens and the lateral edges elevate and come together to form tube
• Neural crest cells are released during this

Question 2

What issues can arise during neural tube closure? Difference between anterior and posterior?

Answer 2

• Neural tube fuses in the middle first and then the cranial and caudal ends close. If it doesn’t close then neural tube defects
• Failure of neuropores to close can cause defects
• Occurs early in pregnancy

Question 3

What is anencephaly?

Answer 3

• Absence of a major portion of the brain, skull, and scalp that occurs due to the anterior neuropore not closing
• Incompatible with life

Question 4

What is Rachischisis?

Answer 4

• Failure of the neuro folds to elevate so the neural tube doesn’t form so just flat primitive nervous tissue
• Incompatible with life

Question 5

How can we detect neural tube defects in utero and how do we prevent these from occuring?

Answer 5

• Measure for raised alpha fetoprotein in the mother’s blood as this can **leak from neural tube **into amniotic fluid
  - Ultrasound
• Take folic acid supplements three months before conception and throughout first trimester, as this helps epithelial folds to fuse together

Question 6

What is spina bifida and the different types?

Answer 6

• Usually at L5/S1 level and is due to the neural tube not fully closing so spinal cord issues and vertebral arches fail to fuse

- Occult: no protrusion or lumps, only missing vertebral arch

- Meningocele: meninges herniate through posterior vertebral arch defect but cord still in tact

- Myelomeningocele: meninges, CSF, bones, cartilage and spinal cord herniate through the defect and have neurological consequences

Question 7

How does the spinal cord develop embryologically?

Answer 7

• Most of the neural tube is the spinal cord. At first the cord grows at the same rate as the vertebral column but at third month column grows faster.
• Therefore spinal roots have to elongate to exit at their intervetebral foramen, forming the cauda equina at L1

Question 8

How does the brain form embryologically?

Answer 8

• At the top of the neural tube 3 dilations form: forebrain (prosencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon)
• Neural tube closes in 4th week and these are the three primary brain vesicles
• 3 turn to 5 secondary vesicles at 5 weeks

Question 9

What are the flexures that form during the embryological development of the brain?

Answer 9

• Folds to fit

- Cephalic flexure: bend in the midbrain

- Cervical flexure: bend at the hindbrain-spinal cord junction

Question 10

How does the ventricular system from embryologically?

Answer 10

Starts as lumen of the neural tube, CSF coming from the choroid plexus cells. Mainly draining back into the superior sagittal sinus or 1/3 apertures (2 lateral and 1 medial)

Question 11

What is hydrocephalus and how is it treated? What’s the main occurrence?

Answer 11

- Build up of CSF in the ventricles which can increase pressure in the skull leading to midline shift or compression symptoms
- Blockage can be by stenosis, tumour, infection. Mainly in spina bifida children
- Use ventricular peritoneal shunt, with length for growth, where shunt is connected to dilated ventricle so CSF reabsorbed into abdomen

Question 12

What are the layers of the neural tube?

Answer 12

• Tube lined by neuroectoderm layer
• Intermediate layer of neuroblasts outside this and these develop into alar (sensory) and basal (motor) plate.
• Plates divided by sulcus limitans and the outer marginal layer
• Roof and floor plate help regulate organisation

Question 13

In overview, what 5 things have to happen embryologically to form a nervous system?

Answer 13

Question 14

Where do neural crest cells originate from?

Answer 14

• Lateral border of the neuroectoderm tube
• Get displaced and enter mesoderm and undergo EMT

Question 15

What are some of the derivatives of neural crest cells?

Answer 15

• Endocardial cushions
• Dorsal root ganglion
• Sympathetic ganglion
• Schwann cells

Sensitive to alcohol, easily injured

Question 16

Wht is Hirschsprung’s disease?

Answer 16

• One structure affected by defect in neural crest cells, enteric ganglion leading to aganglionic megacolon

normally nerves tell colon, rectum and anus to work together to push waste, but without nerves waste backs up

Question 17

What is DiGeorge syndrome?

Answer 17

Multiple structures affected by defect in neural crest cells, e.g thyrod deficiency, secondary immunodeficiency as no thymus, cardiac defects, abnromal face

Question 18

Answer the following SAQ.

Answer 18

A. Falx Cerebri

B. Right lower limb due to decussation and cortical homunculus

C. Superior and Inferior sagittal sinuses, anterior cerebral artery