Embryology

Question 1

What is spina bifida? What are the different types?

Answer 1

Spina bifida cystica = failure of neural tube to close caudally —-> part of spinal cord/meninges herniates through the incomplete vertebral column and is exposed (cyst - no skin covering)

• can occur anywhere along the length of the spinal cord (most commonly in lumbo-sacral region)
• neurological deficits occur (but not cognitive delay - this would be caused by associated hydrocephalus)
• hydrocephalus nearly always occurs

Spina bifida occulta = failure of L5/S1 vertebral arches to close - covered by skin +/- tuft of hair (usually asymptomatic)
- 25% incidence

note: myelomeningocoele = cyst contains meninges and neural tissue (neural problems and life-threatening infections)

Question 2

What is anencephaly?

Answer 2

Failure of neural tube to close cranially

Absence of cranial structures, therefore incompatible with life

Question 3

What results from complete failure of the neural tube to close?

Answer 3

Rachischisis

Splayed open nervous system

Caused by failed neural fold elevation

Question 4

How are neural tube defects diagnosed? How can the risk of neural tube defects be reduced?

Answer 4

Increase in maternal serum alpha-fetoprotein (indicates any kind of open defect)

Ultrasound-guided sonography

Folic acid supplementation for 3 months pre- & post- conception reduces the incidence of neural tube defects by 70%

Question 5

Outline spinal cord development.

Answer 5

3rd month = spinal cord is same length as vertebral column

3 months + = vertebral column grows faster, pulling the spinal roots (exiting the vertebral column via intervertebral foramina) down —> cauda equina created (collection of spinal roots descending from the lower spinal cord and occupying the vertebral column below the spinal cord)

Question 6

Outline development of the brain.

Answer 6

4th week = dilations at cranial end of closed neural tube —> three primary brain vesicles (forebrain, midbrain, hindbrain)

5 weeks = primary brain vesicles become 5 secondary vesicles

• FOREBRAIN = telencephalon & diencephalon
• MIDBRAIN = mesencephalon
• HINDBRAIN = metencephalon & myelencephalon

Question 7

Outline the development of the neurological flexures.

Answer 7

Cranial neural tube development & growth exceeds available space, therefore must fold

CERVICAL FLEXURE = spinal cord-hindbrain junction

CEPHALIC FLEXURE = midbrain region

Question 8

Outline the development of the ventricular system.

Answer 8

Neural tube not occluded during development —> in newborn forms interconnected reservoirs of CSF

Question 9

What is hydrocephalus? What are the causes of hydrocephalus?

Answer 9

HYDROCEPHALUS = abnormal expansion of ventricles within the brain caused by accumulation of CSF

Congenital = obstruction of cerebral aqueduct (aqueductal stenosis)
Acquired: 
- spina bifida cystica 
- intraventricular haemorrhage 
- meningitis 
- tumours

Question 10

How is the neural tube organised?

Answer 10

Roof plate 
Alar plate (sensory ---> dorsal horn)
Sulcus limitans (boundary between alar & basal plates) 
Basal plate (motor ---> ventral horn)
Floor plate

Inner = neuroepithelial layer 
Middle = intermediate (mantle) layer of neuroblasts 
Outer = marginal layer (processes)

Question 11

Outline neural crest migration. Give some examples of disorders caused by disrupted neural crest migration.

Answer 11

Cells of lateral border of neuroectoderm tube displaced and enter the mesoderm —> become mesenchyme

Neural crest cell derivatives =

• dorsal root ganglion
• sympathetic ganglion
• suprarenal glands
• preaortic ganglia
• enteric ganglia

Foetal alcohol syndrome
Hirschsprung’s disease = aganglionic megacolon (single component affected)
DiGeorge syndrome = thyroid deficiency, immunodeficiency secondary to thymus defect, cardiac defects, abnormal facies (multiple components affected)

Question 12

What is the notochord? How is the neural plate induced to elevate its lateral edges?

Answer 12

NOTOCHORD = solid cord of cells formed by the prenotochord cells migrating through the primitive pit
- basis for midline, axial skeleton, & neuraltube

Notochord initiates induction of neural plate —> thickening of ecotderm & specialisation into neuroectoderm —> formation of neural folds —> neural folds fuse to form neural tube (cranial end fuses before caudal end)

Question 13

How is hydrocephalus corrected after birth?

Answer 13

Divert excess CSF into the peritoneum via ventriculo-peritoneal shunt

OR into atrium of the heart via ventriculo-atrial shunt

OR into jugular vein