Overview on Tissue Engineering of Nervous System. The application of Nerve guide.

1. TISSUE ENGINEERING OF
NERVOUS SYSTEM
Presented by
S.Shashank Chetty
2nd year M.Tech, NAST

2. Objectives:

Introduction to
Nervous System.

Anatomy and Origin
of Peripheral nerve
System.

Autologous nerve
grafts (autograft)

Nerve guides

critical gap length in
peripheral nerves.

Matrices and scaffolds

selection of
approaches

Summary

4. Introduction

The loss of touch, sight, hearing or movement often results
from diseases or injuries to the nervous system.

The retina, cochlear, spinal cord, brain and peripheral nerve
have very different molecular and cellular environments, and
tissue engineering within each requires different strategies.

The limited regeneration in the central nervous system (CNS) is
primarily due to the different cellular and molecular
environment, and less to the internal properties of the
corresponding neurons.

Scaffolds and matrices in the nervous
system, therefore, provide a substrate for axonal growth that
has efficacy even in the inhibitory CNS environment.

The most popular approach in peripheral nerve tissue
engineering involves in vivo implantation of artificial scaffolds
and substrates that will guide naturally regenerating axons to
the distal segment.

5. Peripheral nerve

Peripheral nerve injuries (PNI) can lead to lifetime loss of function and
disfigurement.

Figure: Anatomy of the peripheral nerve. The axons from the selected
fascicle are indicated in yellow. The Schwann cell myelin sheath is drawn to
demonstrate the wrapping of the cell body around the axon; however, the
myelin is compact and only a single dark ring is typically seen with histology
(right).

6. Ependymal cells
• line ventricles of brain &
central canal of spinal cord
• produce, monitor & help
circulate CSF (cerebrospinal
fluid)
Astrocytes
• create supportive
framework for neurons
• create “blood-brain
barrier”
• monitor & regulate
interstitial fluid
surrounding neurons
• secrete chemicals for
embryological neuron
formation
• stimulate the formation
of scar tissue secondary
to CNS injury
Microglia
• “brain macrophages”
• phagocytize cellular wastes
& pathogens
Oligodendrocytes
• create myelin sheath
around axons of neurons in
the CNS. Myelinated axons
transmit impulses faster than
unmyelinated axons
Schwann cells
• surround all axons of
neurons in the PNS creating
a neurilemma around them.
Neurilemma allows for
potential regeneration of
damaged axons
• creates myelin sheath
around most axons of PNS

7. Autologous nerve grafts (autograft)

Figure: End-to-end suturing of
peripheral nerves.

Alignment of the fascicles is
critical in successful
regeneration and microsurgical
techniques have been
developed to optimize this
surgery.

8. Use of tubes (nerve guides) in the PNS

Figure: Sequence of events in empty silicone nerve guides.
Initially, fluid and cytokines fill up the nerve guide

(a), followed by the formation of a fibrin matrix inside the
lumen which initially supports invading fibroblasts (blue)

(b), followed by Schwann cells (green) and axons (red)

(c), which eventually penetrate to the distal stump. The
regenerated nerve usually is thinner in the middle of the
bridge (yellow).
Figure: The chance of
successful reinnervation
with nerve guides is
dramatically reduced once
an injury gap reaches a
certain value. This length,
termed the critical gap
length, L C, is where
successful regeneration
occurs 50% of the time.

9. Basic Nerve
Guide
Requirements
• Sterilizable
• Porous
• Low antigenicity
• Persists during
regeneration
• Biodegradable over
long-term
• Resists compression or
collapse
• Pliable (not too rigid)
• Contains regeneration
stimuli

10. In PNS, the proximal segment may be able to regenerate
and reestablish nerve function. Proliferating Schwann
cells, macrophages, and monocytes work together to
remove myelin debris, release neruotrophins, and lead
axons toward their synaptic targets, resulting in restored
neuronal function.
In CNS, an impermeable glial scar
composed
of
myelin,
cellular
debris, astrocytes, oligodendrocytes,
and microglia formed. Regeneration
neurons are blocked from reaching
their synaptic target.

11. Matrices and scaffolds

Peripheral nerve tissue engineering matrices have either oriented
or random structures, while scaffolds predominantly have an
oriented morphology.

Figure: Selected examples of the critical gap length
gap, LC, adapted from Yannas and Hill (2004) for the rat sciatic
nerve. Empty silicone nerve guides

(a) have a L C of 9.7 mm and the introduction of a collagen–GAG
matrix

(b) increases the L C to at least 14.8 mm with a L of 5.1 mm. When
oriented fibrin matrices are used

(c), this results in a L of at least 4.7 mm when compared to the
same, nonoriented matrices. Degradable collagen nerve guides

(d) have a L of at least 5.4 mm compared to silicone tubes. When
polyamide or collagen fibers are introduced

(e), the L is at least 7.4 mm compared to empty nerve
guides, while Schwann cells

(f) result in a similar L value of 7.4 mm, compared to nerve guides
filled with phosphate-buffered saline.

Note: the shift length ( L ) is the result of the therapy – not
necessarily compared to empty nerve guides .

12. 
First, there is the scaffold, this being a microporous, usually three
dimensional, structure, within which a cell suspension can be placed
and where the cells themselves have the opportunity to attach to the
free surfaces of the material, and the media, with nutrients and other
agents, can circulate.

Fibrin is commonly used as a matrix and can be modified with peptide
sequences from laminin or used to deliver neurotrophic factors with a
heparin-binding site

Secondly there are the matrices, which are gels, and more
specifically hydrogels, involving a network of structural, usually crosslinked, molecules, within a water-based viscous matrix.

Guidance scaffolds/polymers based upon poly(-hydroxy acids)
demonstrate the ability to stimulate axon growth

The principal advantages with the porous scaffold approach are the
potential control over the morphology and mechanical characteristics,
while the matrix has the very considerable advantage of resembling
the extra cellularmatrix within which most cells normally reside.

13. 
Figure: A selection of approaches for
delivering growth factors into peripheral
nerve guides.

Growth factors can be incorporated into
the nerve guide (a) or

introduced closer to the lumen with a
rod(b) or

as growth factor containing microspheres
(c).

The growth factors may be incorporated
into the matrix as non bound (d) or as a

Non-covalent controlled release
system(e).

14. Sequence of events
after implantation
of a fibronectin
scaffold in the
hemisection lesion
of the spinal cord

15. Summary

Neurotrophic factors play an important, but complicated, role in regenerative
therapies.

The limited regeneration in the central nervous system (CNS) is primarily due to
the different cellular and molecular environment and less to the internal
properties of the corresponding neurons.

A range of cell and scaffolds implanted into the injured spinal cord reduces
astrocytic scarring, promotes tissue sparing, and facilitates axonal regeneration
and myelination.

Age and gender differences may also need to be taken into account.

Tissue engineering therapies for treating traumatic injury (acute/chronic) versus
degenerative diseases differ greatly (i.e. what to do and when to do it?).

Genetic modification of cells prior to transplantation into the injured brain and
spinal cord can increase neuronal survival and enhance the regeneration response.

Neural tissue engineering strategies commonly adopt multifactorial strategies
both in terms of diversity of therapies (i.e. scaffold and cells and drugs) and timing
of interventions (parallel orsequential).

16. Thank you for Kind attention