Stem Cells Demystified
Indeed,
as discoveries seems to outmaneuver controversy, Stem Cells and
their enormous potential for use in clinical therapy, are the
focus of much interest and debate. However, the medical jargon
and the dissemination of scientific information to the general
public can be quite confusing and rather overwhelming for the
uninitiated.
If you have had the need to seek out stem cell therapy options,
then the chances are that you may already have familiarised yourself
with certain terminologies in the field. What follows is a synopsis
of pertinent concepts, procedures and terminologies that should
simplify matters, inform you further, and empower your effective
decision-making.
What is a Stem Cell?
Stem cells are undifferentiated cells. Pluri–
or totipotent stem cells have the potential,
given the required microenvironment, to develop from a progenitor
or parent cell into each cell type of the human body. When stem
cells are referred to as being multipotent then
their developmental capacity is a bit more limited and they have
an ability to differentiate into many, but not all, cell types
(thus multi lineages). Stem cells are capable of dividing and
renewing themselves infinitely, acting as a regeneration and maintenance
system.
The potency of stem cells makes them key to developing new regenerative
and transplant cures.
Embryonic Versus Adult Stem Cells
The term adult stem cell is used for any stem
cell derived from the postnatal animal (especially the human)
to differentiate it from embryonic stem cells.
Use of, and research on human embryonic stem cells is controversial,
with debate focusing on moral and ethical concerns. Adult stem
cells are typically sourced from bone marrow, fat, umbilical cord
and placenta.
Source of cells
1. Embryo
Embryonic stem cells are a population of self-renewing, pluripotent
cells that are derived from the inner cell mass of mammalian embryos
at the blastocyst stage (3 - 4 days after conception). These cells
are the most versatile of the stem cell group, having the ability
to transform or differentiate into any cell in the human body.
Differentiation is dependent on micro-environmental cues. Development,
broadly speaking, follows a haematopoeitic (blood) or a non-haematopoeitic
lineage.
The moral and ethical issues surrounding the use of human embryonic
stem cells have frustrated efforts for medical advancement. Scientists
have proposed a new and less controversial source of embryonic
stem cells; human eggs discarded in fertility treatments - up
to 1/3 of eggs used during in vitro fertilisation are found to
be unusable and consequently discarded.
2. Umbilical Cord and Placenta
Stromal cells isolated from the human umbilical chord mesenchymal
tissue, namely Wharton's jelly, have been shown to exhibit stem
cell potency. Since umbilical cord is a postnatal organ discarded
after birth, the collection of cells does not require an invasive
procedure with ethical concerns. Placentas also, are typically
thrown away by hospitals and are a source of mesenchymal stems
cells.
Mesenchymal stromal cells of foetus-derived tissue possess multipotent
properties between embryonic stem cells and adult stem cells.
They can be successfully differentiated into mature adipocytes,
osteoblasts, chondrocytes, skeletal myocytes, cardiomyocytes,
neurons and endothelial cells. More on mesenchymal cells later!
3. Bone marrow
Bone marrow, particularly of the long bones, is the source of
haematopoetic stem cells that have been used for the past several
years in transplants. There are no ethical issues regarding their
use, but the method for accessing them is invasive.
Cell Types
1. Mesenchymal stromal cells (MSC)
Mesenchymal cells are considered to be multipotent adult
progenitor cells that possess stem cell features. They have a
considerable capacity for self-renewal while maintaining their
multipotency.
These stem cells can differentiate into a range of different cell
types (related tissue types are in brackets) associated with the
musculoskeletal system such as osteoblasts (bone), chondrocytes
(cartilage), adipocytes (fat), myocytes (muscle), myofibroblasts
(tendon and ligament). Their ability to give rise to cells of
non-musculoskeletal tissue such as brain or liver or pancreatic
islets cells has been reported. Given this potential, mesenchymal
stem cells make excellent candidates for use in treating a range
of tissues affected by disease or damage; tissue remodeling depends
on mesenchymal cells. As previously mentioned, these cells can
be derived from Wharton's jelly present in the umbilical cord.
Allogeneic transplantation of mesenchymal stem cells between different
individuals may also be possible as these cells appear to be immune
privileged in that they are not necessarily rejected when implanted
into unmatched recipients.
2. CD34 + cells
These are haematopoietic stem cells, found in umbilical
cord and bone marrow, with cell surface markers specific for CD34.
The CD 34 molecule functions in cell-to-cell adhesion (CD is an
abbreviation for: cluster of differentiation). CD34+ cells may
be useful in several areas of clinical stem cell transplantation,
including stem cell expansion and haematopoietic recovery. As
a matter of clinical distinction, mesenchymal cells are CD34 negative.
What is Stem Cell Therapy?
This novel kind of therapy has as its basic tenet that stem cells
recruited to sites of injury are involved in tissue repair, regeneration,
and remodelling. Stem cells are isolated from cord blood or other,
cultured in a laboratory and then therapeutically applied to ameliorate
degenerative diseases, and serious injury in patients. The hope
is that human stem cells can be used to heal patients with Alzheimer's,
Parkinson's disease, diabetes, liver disease, multiple sclerosis
and other chronic diseases.
What Does Cell Culture Entail?
An essential prerequisite to the successful development of stem
cell-based therapies is the development of techniques to propagate
pure populations of stem cells on a large scale.
Mesenchymal stem cells are very rare, existing at an estimated
frequency of about 1 in 100,000 bone marrow cells(1).
To obtain sufficient numbers, cells are encouraged to multiply
indefinitely in the laboratory. Briefly, the process entails isolating
cells from a source such as umbilical cord and growing them up
in a cell culture facility whereby the multipotency of the cells
is expanded over several population doublings. Mesenchymal cells
are traditionally grown on a fibroblast feeder layer that provides
them with the necessary chemical signals to remain undifferentiated
and to continue dividing over and over. After expansion these
cells retain their ability to differentiate into a variety of
mature cell types.
It had been thought previously that stem cells are directly influenced
by cells in the local environment or ‘niche’, but
Canadian researchers have recently (July 2007) published groundbreaking
findings in the prestigious journal Nature that human embryonic
stem cells can actually produce distinctive niche cells in vitro,
which then release stem cell nourishing proteins to help keep
their ‘parents’ ticking over.
Researchers understandably are interested in the relationship
between stem cells and their niche, because the niche represents
a route for modifying stem cell behaviour. If human stem cells
can be reliably guided down a particular pathway, then they can
be more readily used for future clinical therapy to regenerate
damaged tissue such as neurons for Parkinson’s disease,
or insulin producing cells for diabetes.
Expansion
Expansion refers to the proliferation capacity and the
cell population doublings during a particular culture period.
Undifferentiated Embryonic Stem cells have the potential for unlimited
expansion in culture with retention of an ability to generate
cells of all three germ layers - endoderm, mesoderm, and ectoderm.
The early germ layer cells can then further differentiate into
many specific cell lineages.
Umbilical cord blood (and adipose tissue) are attractive alternatives
to bone marrow in isolating mesenchymal cells. Cord blood MSCs
particularly seems to be expandable to higher numbers, meaning
they can be cultured longest with the highest proliferation capacity(2).
Optimised culture conditions resulted in more than 50 population
doublings of umbilical cord mesenchymal cells after 15 weeks.
Importantly, a clinical quantity of 100 million mesenchymal stromal
cells with retained differentiation potential could be obtained
from umbilical cord MSCs within approximately 7 weeks (3).
Cryopreservation
After the clinically desired numbers of cells are cultured,
standard cryogenic protocols are used for stem cell storage. During
storage, it is critically important that stem cells maintain their
undifferentiated status.
Regenerative Medicine
The essential aim of regenerative medicine is to obtain
adequate specific cell types or tissue to restore the normal physiology
of a part of the body damaged by injury or disease.
Transplantation
Matched tissue
For bone marrow transplantation as treatment for certain
haematological diseases, matched tissue is required. In the general
population the odds of finding someone with matching stem cells
is thought to be one in 100 000, while 25% of siblings have matching
tissue types. Globally there is a shortfall of matched bone marrow,
and this is propelling the research towards improving the engraftment
of umbilical cord blood.
Autograft
For transplantation, the ideal in ensuring graft compatibility
is to be able to take cells from the individual requiring treatment.
This may occur either directly or following a period of growth
and differentiation in vitro.
Allograft
There is a considerable risk of graft incompatibility
and a very real chance of immunorejection when cells or tissue
are transplanted from an allogeneic source –
i.e. source of independent origin / other than the patient.
Rejection
Reports that transplanted umbilical cord mesenchymal
cells do not precipitate an immunorejection reaction (4)
(and may in fact suppress ongoing immune cell induced reactions
in humans) offer much promise to those folk who are exploring
stem cell therapy of this nature.
If you would like any further information, please email Dr.
Jeff Peimer -Medical Consultant to Regenecell. Alternatively,
you can follow him on facebook.
References
1. www.stemcell.com
2. Kern et al. Stem Cells 2006, 24(5): 1294-301
3. Reinisch et al. Regenerative Medicine 2007, 2(4):371-382
4. Can & Karahuseyinoglu Stem Cells Aug, 2007 epub
