Rivka Zimm
Scientific and Ethical Considerations in the Advancement of Stem Cell Research
A SC injury (SCI) is defined as an insult to the spinal cord (SC) resulting in a change,
either temporary or permanent, in the cord’s normal motor, sensory or autonomic function. Every
year, over a million Americans suffer from SCIs resulting in significant impairment to their
ability to work and participate in their activities of daily life.1 The use of stem cells in the
treatment of SCI’s for both functional recovery of cells and regeneration of damaged nerves has
seen great advancement in the past several years and continues to advance as the scientific
community of the world begins realizing the potential of the stem cell. This paper will review the
current advancements and address the ethical issues concerning the use of stem cells in human
subjects.
To understand SCI and its treatment, one must first understand the building blocks of the
SC itself. Within the SC, there are two broad categories of cells: neurons, which process
information and are responsible for transmitting the electrical signals throughout the body, and
glia, which support neurons metabolically and mechanically.2 Each neuron is surrounded by a cell
membrane, which is critical for the neuron to be able to send electrical signals. All neurons
consist of a cell body, also called a soma, dendrites, and an axon. The cell body contains the
nucleus and intracellular organelles. The dendrites are extensions of the cell body that receive
chemical signals from the other neurons and pass those signals to the soma of the neuron. The
axon transmits information from the soma of the neuron to the dendrites of the next neuron. The
connection point of the axon of one neuron and the dendrites of another neuron is called the
1
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for Clinical
Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
2
"Cells of the Nervous System." Cells of the Nervous System. N.P., n.d. Web. 2014.
1
synapse.3 Both the dendrites and the axon are extensions of the cell body and are also known as
processes.
Within the broad class of neurons, there are three categories of cells: receptors,
interneurons, and effectors/motor neurons, that vary based on their functions. Receptors act to
receive and encode sensory information. Through this action, receptors begin the process of
sensation and perception. Interneurons process information, by sending and receiving signals.
Because of this tie to signals, interneurons constitute the bulk of the nervous system. The final
kind of neurons are effectors/motor neurons which send signals to the all of the muscles and
glands of an organism, thus making them responsible for the behavior of the organism.4
The second broad class of neural cells, glial cells, is defined as the glue of the nervous
system. These are the accessory cells that are required for neurons to function. Many glial cells
come together to fill the space between neurons. The reason that so many of these cells are able to
fit between just two neurons is because glia are considerably smaller than neurons; in fact, they
outnumber neurons by a factor between 10 and 50. Even so, the miniscule size of glial cells is
responsible for the fact that the entire population of glial cells barely accounts for half of the
brain’s mass.5
There are two main categories of glial cells within the SC and the rest of the central
nervous system (CNS): macroglia and microglia. The macroglia of the nervous system, are the
larger type of glial cells, and are split into two classes: astrocytes, and oligodendrocytes.
Astrocytes are unusual in that they lack organelles. Because of this, astrocytes cannot do the
functions of other cells, such as the synthesis of proteins; rather, they provide structural support
for their neighboring neurons, and help repair the brain in the case of damage. They also regulate
3
Ibid.
Ibid.
5
Ibid.
4
2
the flow of ions and larger molecules through the synaptic region (the space between two
neurons). The cells of the second type of macroglia, oligodendrocytes, have organelles and
microtubules. These organelles allow the oligodendrocytes to produce the myelin that makes up
the myelin sheaths, which are the insulating coats of axons. Microglia are the macrophages of the
CNS and do most of its “housekeeping”.6 They are the main source of active immune defense for
the CNS, and clear out and digest all of the dead cells (mostly neurons) from within the brain and
the rest of the CNS.
The macroglia and microglia work mainly in the CNS. There is another type of glial cell
specifically found in the peripheral nervous system (PNS), called Schwann cells. Schwann cells
act as the oligodendrocytes of the developing PNS. In place of producing myelin, a Schwann cell
will wrap itself around the axon of the neuron, and build a myelin sheath. As this happens, the
cytoplasm of the SC is pushed forward, leaving only the membrane of the Schwann cell wrapped
around the once-naked axon. This process, called myelination, greatly increases the speed with
which action potentials are carried along an axon.7
In the case of SCI, neurons and glial cells may become damaged, causing loss of
functionality within the organism with the injury. This loss is dependent on where along the
spinal cord the injury takes place. SCI is categorized as cervical injury, thoracic injury, lumbar
injury, and sacral injury. The first, cervical injury, usually results in paralysis or weakness of
limbs (arms and legs), and effects all bodily functions below the level of injury, in the neck.
Additionally, many with cervical injury find that they have loss of physical sensation, respiratory
6
7
Ibid.
Ibid.
3
issues, bowel, bladder, and sexual dysfunction. This area of the SC controls signals to the back of
the head, neck and shoulders, arms and hands, and diaphragm.8 (See Figure 1)
The second type of SCI, thoracic injury, is less common than cervical SCI. This is because
the area of injury is found directly beneath the rib cage, which is protective. Thoracic SCI results
in paralysis or weakness of legs, and will affect bodily functions below the level of injury. Just as
in cervical injury, patients with thoracic injury can experience loss of physical sensation,
respiratory issues, and sexual dysfunction. In addition, patients may experience bowel and/or
bladder dysfunction. This area of the SC controls signals to some of the muscles of the back and
part of the abdomen.9 (See Figure 1)
The third type of SCI, lumbar SCI, results in the paralysis or weakness of the legs similar
to thoracic injury. This area of the SC controls signals to the lower parts of the abdomen and the
back, the buttocks, some parts of the external genital organs, and parts of the leg.10 (See Figure 1)
The fourth and final type of SCI, sacral SCI, results in the paralysis or weakness of hips
and legs. Just like thoracic injury, bowel, bladder, and sexual dysfunction are known to occur.
This area of the SC controls signals to the thighs and lower parts of the legs, the feet, and genital
organs.11 (See Figure 1)
8
"Spinal Cord Injury Types." Spinal Cord Injury and Paralysis Research Center. Christopher and Dana Reeve
Foundation, n.d. Web. 2014.
9
Ibid.
10
Ibid.
11
Ibid.
4
Figure 2
All types of SCI can be classified as complete or incomplete injury. In the case of
complete injury, the patient experiences a loss of functionality throughout all of their axons and/or
nerves below the level of injury. The axons and nerves may be still intact, but they are not
functioning properly. In contrast, the axons and nerves of an organism with incomplete SCI can
still interact with the brain, and convey messages, whether sending them or receiving them.
5
Because of this, sensation and movement below the line of injury is possible for a patient with
incomplete SCI.12
SCI can be divided into two phases: primary and secondary. The first, the primary phase,
takes place at the time of the actual injury, and can be caused by contusions and compressions.
When contusion occurs, the vertebral bones shatter, and when compression occurs, there is
extreme pressure placed upon the spinal cord. Primary injury occurs most often with lumbar and
cervical SCI. It affects both lower and upper motoneurons which play a part in the skeletal system
and SC. The primary injury usually results in hyperreflexia (overactive or over responsive
reflexes), hypertonia (increased rigidity, tension, and spasticity of the muscles) and muscle
atrophy or weakness (atrophy in lower motoneurons, and weakness in upper neurons).13 Contrary
to popular belief, the primary injury phase does not cause the most damage to the nervous system.
Rather, the secondary phase of SCI is mostly responsible for long term damage.14
The secondary damage phase of SCI results in complex damage at the cellular level.
During this phase, massive cell death can occur, due to the inflammatory response of the host
immune system. This massive cell death includes both necrosis (the death of most or all of the
cells in an organ or tissue due to disease, injury, or failure of the blood supply) and apoptosis (the
death of cells that occurs as a normal and controlled part of an organism's growth or
development.)15 Additionally, the hemorrhaging and production of chemokines that occurs
during this phase breaks the blood brain barrier (BBB), which usually protects the CNS from a
variety of substances that are in circulation throughout the bloodstream. These chemokines
12
Ibid.
Ibid.
14
Ronaghi, M., Erceg, S., Moreno-Manzano, V. and Stojkovic, M. (2010), Challenges of Stem Cell Therapy for
Spinal Cord Injury: Human Embryonic Stem Cells, Endogenous Neural Stem Cells, or Induced Pluripotent Stem
Cells?. STEM CELLS, 28: 93–99. doi: 10.1002/stem.253
15
Ibid.
13
6
include IL 1, which activates and recruits inflammatory cells, thus causing a local inflammatory
response.16
There are three major phases of the secondary phase of damage of SCI, which follow: the
acute phase, the subacute phase, and the chronic phase. The acute phase lasts between two hours
and two days, the subacute phase can last for weeks or even months, and the chronic phase may
last for months or even years.17
Figure 2 (below) summarizes the damage phases, both primary and secondary.18
16
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
17
Ibid.
18
Ronaghi, M., Erceg, S., Moreno-Manzano, V. and Stojkovic, M. (2010), Challenges of Stem Cell Therapy for
Spinal Cord Injury: Human Embryonic Stem Cells, Endogenous Neural Stem Cells, or Induced Pluripotent Stem
Cells?. STEM CELLS, 28: 93–99. doi: 10.1002/stem.253
7
Figure 2
8
Due to the damage that occurs in both of the phases, (the inflammatory component of SCI,
and subsequent demyelination of surviving axons), the treatment of SCI relies largely on
inhibiting the inflammatory response of the host, reducing and/or eliminating cell death (necrosis
and apoptosis), and enhancing neural regeneration and remyelination.19 To accomplish these, and
promote recovery in the host, many scientists advocate the use of stem cells. To be successful,
cell transplantation should accomplish the following: 1) secrete neurotropic molecules (that will
nourish nervous tissues and cause cell recovery), 2) act as a scaffold for axon regeneration, and 3)
actually replace lost neurons.20
There are many different types of cells that are being transplanted into SCI patients in an
attempt to achieve the best results regarding SCI treatment. These cells can be broken up into two
major categories: embryonic stem cells (ESCs) and adult stem cells (ASCs). The cells of the first
category, ESCs, are derived from the inner mass of the cell embryo. They are quite useful, as they
are pluripotent, meaning they can differentiate into all cell types and they can replicate
indefinitely.21 They also have anti-inflammatory properties. However, there are many
disadvantages to these cells as well. First, there are moral and practical considerations in harvesting
the cells, as the harvesting of ESCs requires the death of the embryo that the cells are from. The
cells also have karyotypic instability because of repeated freeze thaw cycles (when the cells divide
they lose pieces of chromosomes or entire chromosomes). Finally, these cells have been known to
19
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
20
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
21
Stem Cell Basics: Introduction. In Stem Cell Information [World Wide Web site]. Bethesda, MD: National
Institutes of Health, U.S. Department of Health and Human Services, 2002 [cited 2014] Available at
<http://stemcells.nih.gov/info/basics/pages/basics1.aspx>
9
be teratogenic in the host, meaning they may cause birth defects in future children of the recipient.22
However, they are still marginally better than ASCs.23
Within the category of ESC there are additional distinctions within the subtypes. These
subtypes are induced pluripotent stem cells (iPSCs) and mesenchymal progenitor cells (MPCs).
iPSCs can be generated from ASCs and reprogrammed to perform like ESCs. MPCs are
embryonic progenitor cells (they can differentiate to form one or more kinds of cells but they
cannot divide and reproduce indefinitely) that are isolated from first trimester fetal blood.24
The cells of the first type of ESCs, iPSCs, are really ASCs that have been reprogrammed
to perform the functions of ESCs. Scientists have discovered that one of the most beneficial parts
of iPSCs is the cells that can be derived from them. These include: motoneuron grafts,
GAbAergic neurons, and neural supporting cells such as oligodendrocyte progenitors (OPCs).25
This eliminates the classic ethical considerations with ESCs.
iPSCs appear as if they may become a valuable resource in the future, with the help of
more advanced technology. As of yet, the only real sensory and/or motor recoveries that have
occurred with the help of iPSCs have been when they were co transplanted with other beneficial
cells. These include: adding collagen as a scaffold to add initial support (which actually increased
differentiation and motor and/or sensory recovery), the addition of Schwann Cells, which would
probably be beneficial on their own, the addition of cells that overexpress NGN-2, and the
22
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
23
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
24
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web. It is important to keep in mind that there are
further subtypes of these subtypes (iPSCs and MPCs) within the category of ESCs.
25
Ibid.
10
transplantation of iPSCs, along with sonic hedgehog, and retinoic acid as cofactors.26 These
efforts have actually paid off, as studies have shown that remyelination and axonal regeneration
have occurred, functional motor recovery has been obtained, as well as neuronal differentiation.27
The motoneuron grafts of iPSCs were extraordinarily useful. They can be used to replace
damaged motor neurons, extend axons through muscles to reinnervate them, and recover sensory
and motor functions. These cells express active growth factors such as neurotrophins three and
four, nerve growth factors, and vascular endothelial derived growth factor.28 Even the other grafts
derived cells were useful. The GAbAergic neurons increased sensory function, and the neural
supporting cells caused axon remyelination and motor function improvement.29
The simplicity in the method of differentiation of iPSCs is also a concrete advantage.
Neurogenins (which are transcription factors involved in neuron differentiation) especially NGN2, which is essential for the development of the CNS, are added to the blank (reprogrammed)
stem cells, creating cells essentially similar to the common ESC cell (the NPC).30 However, this
causes a lack of consistency in the cells that are implanted, in regards to their differentiation and
proliferation. Once the cells have become NPCs, they will behave as such, and will no longer fit a
scientific mold.31
26
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
27
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
28
Ibid.
29
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
30
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
31
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
11
Additionally, the iPSCs have been tumorigenic in the past.32 They are impractical, as
genetic modification and reprogramming is a painstaking process, and they require extensive
retrospective classification to ensure that they truly behave like ESCs. Unfortunately, the attempts
at genetic modification have not been very effective as of late.33
The other type of ESCs, mesenchymal progenitor cells (MPCs), are isolated from fetal
blood from the first trimester of pregnancy. This presents a tremendous ethical dilemma.
However, they are highly proliferative, and they can undergo repeated freeze thaw cycles
without losing their viability, without losing their mesodermal differentiation potential, and
without accumulating karyotypic abnormalities. The cells are non-immunogenic, and can even
suppress regular immune response, which is good for allogeneic transplantation. Additionally,
they are highly pathotropic, as they can secrete a wide range of trophic factors. This promotes
neural cell survival, which is especially useful when used for cell rescue and as support cells.
MPCs are not only nontumorigenic, but they even have anti-tumor properties, and
immunosuppression is not necessary for implantation.34
The use of adult stem cells (ASCs) also has advantages and disadvantages. They are
morally/ethically, and practically more efficient than ESCs. Additionally, their proliferative
and differentiation potential can be raised to compete with ESCs when coadministered with
chondroitinase treatment (especially in a subtype of ASCs known as olfactory ensheathing
cells, or OECs).35 However, they are naturally less effective than ESCs, in regards to this
proliferative and differentiation potential. They also fall short in regards to post implantation
32
Ibid.
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
34
Ibid.
35
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
33
12
survival, migration and integration within the host/recipient CNS, and in their increased
neuropathic pain.36
ASCs have three distinct categories, each with two types of stem cells within them. The
first category is made up of neural progenitor cells (NPCs). NPCs are like stem cells in that they
can differentiate to form one or more kinds of cells. However, they cannot divide and reproduce
indefinitely. The second category is made up of cells that originate in bone marrow. These highly
important cells include bone marrow mesenchymal/stromal cells (BMSCs), and mesenchymal
stem cells (MSCs). The third and final category includes all ASCs that originate outside of the
bone marrow. This includes olfactory ensheathing cells (OECs) and Schwann cells (SCs- not to
be confused with stem cells or spinal cord).
The first of the ASC types, NPCs, react the best when implanted seven to ten days postinjury, and have been known to last for 2.5 years after implantation. To implant NPCs, the
technique known as derived cell lines is used. This means that the cells are grown and
differentiated in an in vitro culture, and implanted to be in vivo after this differentiation. They are
also beneficial, as NPCs can undergo in vitro manipulation (including immortalization), and they
have a lack of tumorigenicity.37
NPCs are taken from the inner cell mass of an early embryo, as well as from fetal, adult
and postnatal CNS, including the sub ventricular zone of the brain, the central canal of the SC, the
hippocampus, and the cortex.38 The NPCs extracted from the cortex prove especially beneficial,
as they can be expanded in culture as nonadherent neurospheres. (A neurosphere is composed of
36
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
37
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
38
Ibid.
13
two different cell types, and can be isolated from fetal, postnatal, and adults CNS.)39 This variety
of hosts (fetal, postnatal, and adult CNS) gives NPCs an extraordinary advantage. Neural
progenitor neurospheres are also beneficial as they can remain multipotent (they have the ability
to differentiate into all cells within a limited family of cells) for long periods of time before
implantation, and can be easily maintained and expanded during this time.40
NPCs can generate all 3 neural cell types (neurons, astrocytes, and oligodendrocytes).
Unfortunately, this is more of a curse than a blessing, since the cells waste their complete
differentiation potential, by tending towards astrocyte development, as well as a small portion of
oligodendrocyte development. This is a problem as was evident in a trial with shiverer mice.41 It
was apparent that the best form of differentiation for the body is differentiation into neurons,
rather than glial cells. Neurons are essential, as they promote remyelination and synaptic contact.
This trial also shows that the creation of oligodendrocytes is vital to a SCI patient’s survival, as
without them the patient will become myelin deficient.42 Additionally, NPCs must be cotransplanted with fibroblast growth factor (FGF) and epidermal growth factor (EGF) to achieve
their full potential.43
To solve the classic problems found with NPCs, a revolutionary type of cell has been
found, known as human adult immortalized NPCs. These cells are effective in that they are
morally better than NPCs, since they are harvested from adult bone marrow, which is accessible.
Additionally, these cells are advantageous in that they overcome the limited supply of cells, since
39
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
40
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
41
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
42
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
43
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
14
the donor’s age, is irrelevant. These cells also have less variability than regular NPCs, so the
clinical trials (which foreshadow future use) will be more even and predictable. These cells also
enable research and classification of cells prior to implantation.44
Although these cells (adult immortalized NPCs) are more predictable than regular
NPCs, there are still many differences between studies, dependent on the age of the donor, the
immortalization strategy, and the conditions in the culture.
This is a clear distinction between immortalization and oncogenic transformation (which
leads to cancer). In immortalization, cells stop dividing when they run out of space for replication.
However, in oncogenic transformation, the cells will not stop replicating, which is tumor
inducing. Therefore, oncogenic transformation (and its tumor-inducing characteristics) had best
be avoided as much as possible.45
Because of all of the potential surrounding the use of NPCs in treating SCI, two subtypes
of NPCs are being investigated. The first are OPCs (oligodendrocyte progenitors), which were so
efficient in one of the early trials that they were reconsidered as a viable option when treating
thoracic SCI. In the follow-up trial, known as the Geron trial (2010), OPCs were proved to be
relatively effective in the treatment of thoracic SCI, yet they began to cause abnormal cyst
formation on the rats involved in the trial.46
In an attempt to solve this problem, the second type of NPCs was used. Known as GRPs
(glial restricted progenitors), these secondary cells were coupled with the first, and together they
44
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
45
Ibid.
46
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
15
were proven to be neuroprotective, and to inhibit neurotropic pain.47 Currently, more trials are
being planned to determine the safety of the procedures.
The second category of bone marrow derived ASCs, mesenchymal stem cells (MSCs),
are divided into two types. The first type of cells is called Human Wharton’s Jelly Cells
(WJCs)/Umbilical Cord Matrix Cells (named such because of their natural location). In fact,
WJCs are generally found in non-embryonic tissues, such as bone marrow, peripheral and
umbilical cord blood, and the umbilical cord matrix. This is a colossal advantage for these
cells, as they are plentiful, easily accessible, and their collection process is ethical. 48 Saved
umbilical cords are the source of WJCs. Each umbilical cord has an outer layer of amniotic
epithelial cells, enclosing a gelatinous matrix. These epithelial cells are the WJCs.49
There are many advantages to WJCs. Not only are they plentiful, ethical, and easily
accessible, but they are highly proliferative, and they can undergo repeated freeze thaw cycles
without losing their viability, without losing their mesodermal differentiation potential, and
without accumulating karyotypic abnormalities. The cells are non-immunogenic, and can even
suppress the immune response, which is good for allogeneic transplantation (transplant
between two immunologically incompatible individuals of one species).50 Additionally, WJCs
are highly pathotropic (they can secrete a wide range of trophic factors). This promotes neural
cell survival, which is especially useful when WJCs are being used for cell rescue and as
support cells. In addition, WJCs are not only nontumorigenic, but they even have anti-tumor
properties. Unfortunately, these cells do not survive and proliferate as well as their fetal
47
Ibid.
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
49
Ibid.
50
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
48
16
counterparts when transplanted. Rather, there is only a five to ten percent survival rate of
grafted cells. Additionally, there is a lack of neuronal differentiation amongst WJCs, and the
treatment causes scars.
The cells of the second type of MSCs, BMSCs, are from the stromal compartment of adult
bone marrow. This section of the bone marrow also comprises hematopoietic cells (creates blood
cells). Since BMSCs also fall into the MSC category, in addition to the advantages and
disadvantages as WJCs, they have added benefits. These self-renewing cells (BMSCs) have
crucial anti-inflammatory and immunomodulatory effects.51 Although these cells may not be
pluripotent, they are multipotent, as they have mesodermal differentiation potential.52 This
differentiation potential does not end there, as BMSCs can differentiate in vitro into osteoblasts,
adipocytes, chondroblasts, neural cells, and myoblasts. Additionally, BMSCs secrete neurotropic
factors, and help halt relapses and formation of MS lesions. As far as advantages go, these are
probably the best ones a stem cell can have. In fact, BMSCs are currently being used as a
treatment for leukemia.53
BMSCs have many potential uses. They are effective at CNS cell rescue, and can also be
used during autologous transplants as they will provide trophic support to endogenous and co
implanted cells by promoting cellular and axonal growth, differentiation, and survival.54 BMSCs
are favored over other stem cells as they are not rejected by the patient's immune system. They
51
Ibid.
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
53
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
54
Ibid.
52
17
can be frozen and stored, can duplicate quickly in vivo, and have been shown to prevent or reduce
demyelination and neuronal inhibitory molecules.55
A study was conducted in rat models, using BMSCs inserted directly into the lesion site.
On a positive note, the cells down regulated the inflammatory response at the injection site,
restored motor function in the patient, and lessened the pain of the SCI. However, the BMSCs
were unsuccessful at restoring bladder function in the patient.56
Despite these positive results, there is no conclusive evidence to support the
differentiation potential of BMSCs, and they have been shown to have limited migration beyond
injection site. While these cells may be effective at the injection site, it appears that they have no
more potential than that. Rather than healing the rest of the body, BMSCs will stay close to the
injection site working their magic there. Also, BMSCs have inter-donor variability, in regards to
efficacy and immunomodulatory potency (they do not fit a scientific mold; rather the results may
differ throughout multiple trials).57
Nevertheless, BMSCs may present a practical solution to SCI. An autologous BMSC
transplantation method is used to get the cells into the recipient’s body. According to this method,
the patient receives his or her own bone marrow or stem cells that were collected and frozen
before admission for high-dose chemotherapy or radiation. While this has been found to be safe,
it is unfortunately not beneficial for chronic SCI patients.58
Due to the propensity of BMSCs to remain near the injection site, there are only two
methods of injection: intra-spinal (within close proximity to the lesion site), and intrathecal
55
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
56
Ibid.
57
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
58
Ibid.
18
(between the SC and the surrounding sheath).59 To prove the effectiveness of BMSCs, a trial was
conducted on rats with thoracic SCI. The injection of BMSCs promoted axonal regrowth and
sprouting, which is most definitely a step in the right direction. It has been hypothesized that the
amount of success in the patient depends on whether cells are injected by IV or at the site of the
lesion. The amount of cells implanted may also be a contributing factor to the variety of
successes.60 Additionally, BMSCs must be pre-differentiated in vitro. To do so, they are placed in a
culture with basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), and heparin.
Otherwise, bFGF must be injected directly into bMSCs, which will make the bMSCs overexpress
bFGF.61 This is a viable option, as it has been found to promote axon regeneration and functional
recovery.
An alternative method to make bMSCs more successful treatment-wise is to co-transplant
them with a collagen scaffold. There are two types of scaffold: those derived from blood plasma,
called platelet rich plasma, and those with a gelatin sponge. To use the second type of scaffold,
MSCs must be inoculated onto the scaffold, and then these scaffolds (with the MSCs on them)
must be transplanted. This technique has been successful in dogs, by showing functional
recovery and a reduced secondary injury phase. These dogs were also helpful in that they proved
the ideal time of transplantation of bMSCs. Originally, there was a debate between transplantation
at twelve hours after injury, one week after, and two weeks after, but the dogs showed that the
most improvement occurred when implanted after one week.62 Even with these remarkable trials
59
Ibid.
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
61
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
60
62
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
19
and discoveries, it is clear that there is still more work to be done on bMSCs, just as there is with
every other stem cell.
In the final category of ASCs, the non-bone marrow category, olfactory ensheathing cells
(OECs) are important. They are isolated autologously from the nerve fiber layer of olfactory bulb,
and/or the lamina propria of the olfactory epithelium (the nasal cavity).63 OECs are safe and
feasible, and have shown promising results in some transection models. They are also potentially
capable of creating a microenvironment permissive for axogenesis, the growth and development
of axonal processes by neurons.64 Unfortunately, there is a variation amongst results, which
depends on the source of the OECs, the difference in culture conditions, and the changing
phenotype in prolonged culture. Even so, there was a successful trial that was successfully
repeated. In this trial, human olfactory bulb OECs were implanted 1-week post-injury for
thoracic contusion and hemisection SCI. Because of this treatment, cavitation and gliotic scarring
were reduced, and functional recovery was attained.65 Nevertheless, the best results are always
attained when the OECs are co transplanted with Schwann cells.
Schwann cells (SCs), the second kind of non-bone marrow ASCs, have been shown to be
effective only when they are co transplanted with either OECs or BMSCs, as these cells help the
SCs invade the lesion site, in a process called schwannosis. Usually, SCs are effective in cases of
cervical crush SCI or thoracic models of contusion compression and transection SCI.66 The use
of SCs as stem cells was only effective after the implantation of 50,000 cells into young rats.
However, in a trial where purified autologous sural nerve-derived (a sensory nerve in the leg) SCs
63
Ibid.
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
65
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
66
Li, Jun, and Guilherme Lepski. "Cell Transplantation for Spinal Cord Injury: A Systematic Review." BioMed
Research International 2013 (2013): 1-32. Web.
64
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were implanted, no such success occurred. This was also the case for MS patients. Nevertheless,
these trials proved that the transplant of SCs is safe and feasible, and that remyelination will
occur, as it did in one of the failed trials.67
The use of SCs as stem cells has its benefits. Schwann cells (SCs) can be obtained
autologously, and are not tumorigenic. They can also myelinate and remyelinate axons of both the
CNS and PNS. However, it is possible that the repair process of the SCs will hinder the
endogenous repair process. In fact, should an axon want to re-enter the CNS (and its
parenchymal microenvironment), the transplanted SCs would not allow it unless another type of
cells was cotransplanted (with the SCs).68 SCs cannot stimulate corticospinal tract regeneration
(schwannossis) when transplanted alone. The results with SCs have been varied. In fact, only 1/4
of the adult derived nerve cells passed BBB assessment for functional outcome (blood-brain
barrier permeability). To take it even further, only 2/5 of these (of the ¼) had improved motor
function.69
Despite the numerous setbacks in trials and treatments, the use of stem cells for SCI is an
incredible advancement of science. There may be moral dilemmas regarding the use of ESCs, and
unforeseen effects of the treatments, but such challenges may be viewed as surmountable in the
effort to rehabilitate and sustain life. Stem cells have an exorbitant amount of unrealized
potential. It is the task of the scientific community of the world today to release that potential into
the world in order to save lives.70
67
Fehlings, Michael G., and Reaz Vawda. "Cellular Treatments for Spinal Cord Injury: The Time Is Right for
Clinical Trials."Neurotherapeutics 8.4 (2011): 704-20. PMC. Web.
68
Ibid.
69
Ibid.
70
For a complete summary of all of the cells reference Table 1 (2) at
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3210356/table/Tab1/
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