Biomedical Engineering Reference
In-Depth Information
Stem cells represent a viable treatment option following spinal cord injury.
Undifferentiated stem cells excrete a variety of neurotrophic factors that encourage
axon growth, promote the replacement of damaged non-neural structures such as
blood vessels, promote the breakdown of the glial scar, and temper inflammatory
responses. Embryonic stem cells in particular have a penchant for adopting the glial
phenotype, that is they will readily transform into the support cells required by
neurons (e.g., astrocytes, oligodendrocytes) once they are transfused into the site of
injury. They may also be used to overcome glial repulsion of axons; myelinating
cells produce inhibitory factors that can prevent an axon from regenerating. With
that in mind, there are a couple exciting papers coming out. The first [
80
] points to
uses embryonic stem cells in the rat. These cells, when added to the site of damage
along with a PDE-4 inhibitor to block the axon-repulsive effects of glia, were
experimentally differentiated into a
neural
phenotype to form bridge connections
between the degenerating axons and the muscle. The interesting manipulation in
this paper was the infusion of cells that produce the trophic factor GDNF into the
target muscle; GDNF provides a signal that attracts growth of axons from the
embryonic stem cells.
A second paper [
81
] expands upon the promise of animal stem cell models of
spinal cord injury by using human neural stem cells in a rodent model. They
demonstrate differentiation of the human cells into neuronal and glial tissue, axon
remyelination, synapse formation, and locomotor recovery. It seems, then, that stem
cell therapies hold promise for treatment of traumatic spinal cord injury. While
much work remains to develop a stable, consistent model in animals we are definitely
making progress and a variety of very creative approaches are being used. Some of
these approaches point directly to potential of human adult and/or embryonic stem
cells [
81-
83
]. A truly pro-life culture would embrace the exploration and use of
these technologies for the benefit of all its citizens. One strategy to repair a damaged
spinal cord involves stimulation of axon regrowth in order to reestablish the broken
connection. When the dendrites receive information the cell body generates a nerve
impulse, which travels along the axon to another “target” cell. At the target cell—a
muscle cell, another nerve cell, or gland cell—the axon divides into a multitude of
nerve endings. The tip of each of these endings is called the axon terminal and
located very close to the target cell [
80,
81
]. Here the axon forms a synapse allowing
neurotransmitters to travel across a small gap (25 nm wide) and fuse with the recep-
tors of the next cell—this is how electric signals are sent from the brain to other
parts of the body. When neurons die, connection between axon terminals and
receptors is broken and the CNS can no longer function.
There are over 100 billion neurons in the CNS and as many as 10,000 different
subtypes of these neurons [
82
]. The incredible power of the brain to process infor-
mation exists in the massive amount of neurons and synapses. Neurons are not the
only cell in the CNS—glial cells also exist in even greater numbers than neurons.
These cells come in different forms with a variety of different functions, all helping
the CNS to operate. Two glial cells related to the spinal cord are oligodendrocytes
and astrocytes. Oligodendrocytes are responsible for producing myelin, a fatty
substance that provides electrical insulation on the axons. Myelin allows electric
