Histology

Question 1

Why do Axons not Regenerate
in the CNS?:

Answer 1

1. Oligodendrocytes and CNS myelin express proteinsthat actively inhibit axonal growth.
2. Astrocytes form a scarlike tissue at the siteof injury in the CNS
   * “Glial scar” a mechanical barrier to growing axons
   * Express various growth-inhibitory proteoglycans
3. Microglia (resident CNS macrophages) phagocytose cellular
   debris and degrade growth inhibitory ECM components, but
   may also kill injured CNS neurones.

Question 2

How Schwann Cells Support
Axon Regeneration

Answer 2

1) Phagocytose and recycle cellular debris
2) Provide a growth-promoting substrate for
axons (expression of cell adhesion molecules,
e.g. N-CAM, L1)
3) Support neurone survival and axon re-growth
through production of neurotrophic factors
(e.g. NGF)

Question 3

Events after Nerve Injury:
The neuron contains lots of Nissl bodies
* Schwann cells ensheathe the entire length of the axon
* The axon forms a synaptic terminal on the muscle fibre

Answer 3

*
Schwann cells form a substrate for axon growth

• Proximal: Nissl bodies re-forming, cut axon forms sprouts
• Distal: Schwann cells proliferate and fill endoneurium
• The axon re-grows along the Schwann cell

Question 4

What is the The Growth Cone

Answer 4

The moving and sensing tip of a growing axon

Question 5

Requirements for Functional
Axon Regeneration

Answer 5

1) The injured nerve cells must be able to survive after the lesion, and re-express genes required for axon outgrowth
2) The surrounding tissue (microenvironment) must be conducive to axon re-growth
3) The re-growing axons must be able to find their proper target areas (presence of guidance cues), and establish synaptic contact with target neurones