Development of the CNS

Question 1

When does neurulation begin

Answer 1

Gastrulation begins in week 3 and produces the notochord

Question 2

State the action of notochord

Answer 2

• Notochord is a solid rod of cells running in the midline with important signalling role
  
  • Diffusion limited - only cells that respond to signals are ectoderm cells
• Notochord directs conversion of overlying ectoderm to neurectoderm

Question 3

State how the neural plate forms the neural tube

Answer 3

• Elevation of lateral edges of neural plate creates a depressed mid-region known as the neural groove
• Neural folds gradually approach each other in the midline and fuse, producing the neural tube

Question 4

State potential neural tube defects

Answer 4

• Failure of the neural tube to close
• Cranial (anterior neuropore) defect results in anencephaly
  
  • Without a brain - not compatible with life
• Caudal (posterior neuropore) defect results in spina bifida
  
  • Can occur anywhere along length of spinal cord, most common lumbosacral region
  • Neurological deficits occur, though not associated with cognitive delay
  • Hydrocephalus nearly always occurs - excessive accumulation of CSF in brain

Question 5

Describe the types of spina bifida

Answer 5

• Occult - one or more vertebrae do not form properly, creating a gap in the spine
  
  • Usually does not create problems
• Meningocele - meninges push out through the spine to form a cyst-like swelling containing CSF
• Myelomeningocele - meninges and spinal cord push out through the spine creating a swelling containing CSF and neural tissue

Question 6

Describe rachischisis

Answer 6

• Failure of the neural tube to fuse, thereby resulting in the vertebrae overlying the open portion of the spinal cord to not fuse
• Exposes the spinal cord to the outside

Question 7

How can neural tube defects be diagnosed

Answer 7

• Raised maternal serum α-fetoprotein
• Ultrasound scan
• Folic acid intake in the first trimester reduces incidence by 70%

Question 8

Explain how the cauda equina forms

Answer 8

• At the 3rd month, the spinal cord is the same length as the vertebral column
  
  • After, the vertebral column grows faster
• Spinal roots must elongate because they still exit at their intervertebral foramen - forms the cauda equina

Question 9

Describe how the brain derivatives form

Answer 9

• During neural fold formation, 3 primary brain regions can be distinguished
  
  • Forebrain, midbrain, hindbrain
• After neural tube closure in the 4th week, these dilations at the cranial end become the three primary brain vesicles
• At 5 weeks of development, 5 secondary brain vesicles are formed
• Forebrain forms telencephalon (cerebral hemispheres) and diencephalon (thalamus)
• Midbrain forms mesencephalon (midbrain)
• Hindbrain forms metencephalon (pons, cerebellum), myelencephalon (medulla)

Question 10

How does the ventricular system form

Answer 10

• Tubular hollow structure of the neural tube persists as development proceeds
• Becomes the ventricles filled with CSF
• Acts to cushion brain and spinal cord within their bony cases

Question 11

Describe the organisation of neural tube in terms of sensory and motor portion

Answer 11

• Sensory portion known as alar plate and in dorsal end
• Motor portion known as basal plate and in ventral end
• Sulcus limitans is the junction between dorsal and ventral plates

Question 12

Describe what neural crest cells are

Answer 12

• Cells of the lateral border of the neuroectoderm tube
• Become displaced and enter the mesoderm and undergo epithelial to mesenchymal transition
• Become structures including cranial nerve ganglia, dorsal root ganglia, sympathetic/parasympathetic ganglia, Schwaan cells, glial cells etc

Question 13

State the name for defects in neural crest cells

Answer 13

• A defect in one structure of neural crest cell migration or morphogenesis - Hirschspring’s disease
• Defects affecting multiple structures - DiGeorge syndrome