Peripheral nerve injuries

Question 1

Why are nerve injuries one of the most challenging in regenerative medicine?

Answer 1

CNS and PNS = different cell types! Composition of cells therefore very different

Question 2

Epidemiology of nerve injuries ?

Answer 2

9000 per year in UK
Mainly younger population (car accidents etc)
Therefore large financial burden on society

Question 3

ANATOMY OF NERVE

Answer 3

Axons are surrounded by myelinated schwann cells
They are enclosed by the endoneurium (surrounds axons)
These are bundled together into fascicles
Fascicles are enclosed by the perineurium. Mutiple fascicles = nerves!
Whole nerves are surrounded by the epineurium

Question 4

INJURY TYPES?

Answer 4

Elongation –> only 10/20% extension of nerve is allowed before structural damage occurs
Laceration –> make up 30% of nerve injuries
Compression –> external mechanical pressure on conductive membrane of nerve

Question 5

Wallerian degeneration - how long does the process take ? why does it take longer in the CNS?

Answer 5

1-3mm each day

In CNS, macrophages infiltrate much more slowly, inhibiting the removal of dead myelin

Question 6

Role of reactive astrocytes in wallerian degeneration?

Answer 6

Reactive astrocytes produce glial scars –> this inhibits regeneration in the CNS and promotes it in the PNS!!!

Question 7

Approaches to repair in the PNS?

Answer 7

• Surgery
• Grafts
• Nerve conduits

Question 8

Surgical reconstruction of nerves – what makes it possible ? Overall process?

Answer 8

• Suture individual fascicles of the nerve back together
• Only possible if the nerve severs are close together. If the gap is too large, there is a decrease in blood flow and overstretching of the nerve is seen.

Question 9

Stretch-repair relationship?

Answer 9

8% nerve stretch = 50% decrease in blood flow.

Complete ischaemia at 15% of nerve stretching

Question 10

GRAFTS - autologous - advs and disadvs

Answer 10

Autologous grafts are taken from other nerves within the body of the same person.
ADVS - low risk of immune rejection
DISADVS - loss of function at donor site, 2 surgeries required, limited nerve size and type available

Question 11

GRAFTS - allogeneic - advs and disadvs

Answer 11

Graft taken from different person.
ADVS - no second surgery required. No loss of function at donor site due to taking from same person
DISADVS - limited availability and higher risk of immune rejection

Question 12

NERVE CONDUITS - how do they work?

Answer 12

Guide regenerating axons and prevent infiltration of scar tissue.
Increase the concentration of intraluminal proteins and important in guiding regeneration.

Question 13

Properties of an ideal nerve conduit?

Answer 13

• Permeable to proteins and plasma
• Porous
• Mechanically malleable (if too stiff the surrounding tissue may become injured)
• Degradable in the long term but lasts long enough for regeneration to take place
• Biocompatible
• Suturable
• Sterile / sterylisable

Question 14

Classes of biomaterials that may be appropriate for designing a nerve conduit?

Answer 14

Collagen
Fibronectin
Gelatin

Question 15

Top-down approach to designing nerve conduits - DECELLULARISED NERVE CONDUIT ?

Answer 15

Retain ECM architecture and remove antigens
Nerve chemically and biologically decellularised –> creates a scaffold for nerve regeneration, clears pathways to allow cell migration/axonal regeneration
The axon will distribute evenly throughout the nerve thickness allowing functional incorporation of the neve conduit

Question 16

Bottom - up approach of nerve conduit –> what does this involve?

Answer 16

Design own nerve conduit from scratch!
Make own design for what is needed / going to work best for that particular situation
Several materials and a mixture of natural and synthetic could be used. Can also be biodegradable!

Question 17

Synthetic materials used to produce nerve conduits?

Answer 17

• PLA (polylactic acid)
• Poly (lacto-go-glycolic) acid
• Poly caprolactone
• Polyethylene glycol

Question 18

Commerically available nerve conduits?

Answer 18

• Neurotube - PGA
• Neuragen - Type 1 collagen
• Neurolac
  A range of fabrication methods are employed for these

Question 19

Synthetic, non-biodegradable materials used for nerve conduits?

Answer 19

Silicone

Gore-Tex

Question 20

Length limitations - what is the relevance?

Answer 20

Chance of survival of nerve regeneration decreases once the gap reaches a certain size
A short gap means a fibrin cable is robust enough to provide a regeneration platform
At longer lengths thinning of the nerve restricts regeneration –> no fibrin cables are seen at long length

Question 21

Critical gap length - what is meant by this?

Answer 21

Length at which regeneration of the nerve occurs 50% of the time
Current strategies are aiming to increase the critical gap length

Question 22

What approaches are involved in increasing the critical gap length

Answer 22

• Use of ECM componants
• Cell grafts
• Intralumenal support
• Neurotrophic factors
  OR A COMBINATION OF THE ABOVE

Question 23

Use of ECM in increasing critical gap length

Answer 23

• Hydrogels (high concs h20 prevent axonal penetration)
• Laminin, fibronectin and collagen – > problem is that batches of each greatly vary, meaning that they are not massively robust therefore always suitable for clinic

Question 24

Intralumenal proteins in increasing critical gap length

Answer 24

Engineered constructs to contain different proteins inside to recreate an accurate nerve scaffold - CASE STUDY EXAMPLE

Question 25

Neurotrophic factors in nerve regeneration –> how used / why important?

Answer 25

• Support axonal growth, migration and proliferaton of schwann cells and modulation of intrinsic signalling pathways
• Improves functionalisation
• Need controlled release (like BMPS) into scaffold - encapsulation methods used often

Question 26

Schwann cells in nerve regeneration - why important? How are they recapitulated?

Answer 26

• Schwann cells secrete factors critical for regeneration. Stem cells used where schwann cells unavailable
  CELL GRAFTS –> genetically modified cells –> can produce appropriate stimuli for regeneration. Grafts must contain NT factors, schwann cells, stem cells and GM cells