1. Development of the CNS

Question 1

What are the different layers in the embryonic disk?

Answer 1

There are various layers in the embryo – endoderm is inner layer, ectoderm is the outer layer and middle layer is the mesoderm.

Question 2

What is the notochord and what will it develop into?

Answer 2

Notochord – cartilaginous strip through the embryo which will be the vertebral column

Question 3

What happens in the early development of the CNS?

Answer 3

• The first thing that happens is that you get a proliferation of the ectoderm in the dorsal midline of the embryonic disc
  
  • This proliferation is called the neural plate
  • As this thickens, the neural plate starts to fold up on the sides
  • Eventually the two neural folds fuse dorsally to form a tube
  • The space in the middle of the tube is called the neural canal
  • You get a little bunch of cells at the tip of the neural fold called neural crests
  • The neural crest is separate from the neural tube and doesn’t take part in the fusion but lies alongside it
  • At the end of this process you have a neural tube lying in the midline dorsally in the embryo and on either side of this you have two strips of neural crest tissue

Question 4

What is the difference between the neural tube and the neural crest?

Answer 4

The neural tube and the neural crest are two independent sources of nervous tissue
• Neural Tube = all CNS cells
• Neural Crest = all PNS cells

Question 5

What is the wall of the neural tube called?

Answer 5

The neuroepithelium

Question 6

What are the different types of cells that come from the neuroepithelium? What do they become?

Answer 6

THREE types of cell comes from neuroepithelium
• Neuroblasts
○ Precursors of all neurones that have their cell bodies within the CNS (this includes motor neurones)
• Glioblasts
○ These become neuroglia - astrocytes and oligodendrocytes
• Ependymal Cells
○ Lining ventricles and central canal of the spinal cord
○ These remain close to the inner membrane of the neural tube and they spread out and form a lining around the developing ventricular system

Question 7

What do neural crest cells differentiate into?

Answer 7

• Sensory neurones of dorsal root ganglia and cranial ganglia. Cell bodies close to the spinal cord in the ganglia and they project into the CNS
  
  • Postganglionic autonomic neurones – ganglia that are in the periphery. PNS ganglia will be close to the organ being innervated whereas for sympathetic ganglia they are down the side of the spinal cord.
  • Schwann cells. Non-neuronal derivatives e.g. melanocytes

Question 8

Outline the differentiation of the neuroepithelium and the different cells involved

Answer 8

• Overall movement of the cells from the inner layer to the outer layer.
  
  • Firstly, you get the cells withdrawing from the outer membrane towards the inner membrane and then it goes through mitosis
  • One of the daughter cells will stay attached to the inner cell membrane, it gets bigger and goes into the cell cycle again
  • The other daughter cell migrates away from the inner membrane and then develops into neuroblasts
  • They develop processes (one will become the axon) and these axons are directed away from the inner membrane again
  • As this occurs over and over you end up with THREE layers
  • There is one layer by the inner membrane where you get mitosis occurring (ependymal), another layer where you mainly have cell bodies (grey matter) and another that has mainly axons (white matter)
  • This is the beginning of the difference between grey and white matter
  • Glioblasts also show a similar pattern of differentiation and migration to neuroblasts but glioblasts can migrate into WHITE matter as well
  • REMEMBER: glioblasts DO NOT develop axons (but do develop processes)
  • Ependymal cells just remain in the ependymal layer

Question 9

What are the layers of the neural tube in cross section?

Answer 9

• The ependymal layer surrounds the neural canal
  * The names in the brackets are the correct embryological terms
  * There is an inner grey matter and an outer white matter which is what the spinal cord looks like unlike the brain where there is an outer grey matter and inner white matter.
  * The roof plate and the floor plate are landmarks that show the most dorsal and the most ventral parts of the canal

Question 10

How is differentiation and migration controlled in the neural tube?

Answer 10

• This whole process of differentiation and migration is controlled by signalling molecules that are secreted either from the tissues surrounding the developing neural tube or sometimes secreted by cells within the neural tube
  * These signalling molecules interact with receptors on the developing neuroblasts
  * These signalling molecules also guide the developing axons as well as proliferation

Question 11

What are the factors that determines the effect of the signalling molecules on the developing neural tube?

Answer 11

Concentration and timing
• The signalling molecules can either cause attraction or repulsion and the direction is determined by the concentration gradient - close to the source of the signalling molecules there will be a higher concentration
• The other important factor is TIMING - there is no point producing signalling molecules if the developing neurones don’t have the right receptors
• Some of the signalling molecules have different effects at different stages of development

Question 12

What are the features of the neural tube later in development?

Answer 12

The neural canal is even smaller compared to the thickness of the wall
The grey matter has split into TWO different types:
• Alar Plate - Dorsal
• Basal Plate - Ventral