Adult Stem Cells are undifferentiatedcells, found throughout the body after embryonic development, that multiply by cell division to replenish dying cells and regenerate damaged tissues. Also known as somaticstem cells (from Greek Σωματικóς, meaning of the body), they can be found in juvenile as well as adult animals and humans.
Scientific interest in adult stem cells has centered on their ability to divide or self-renew indefinitely, and generate all the cell types of the organ from which they originate, potentially regenerating the entire organ from a few cells. Unlike embryonic stem cells, the use of adult stem cells in research and therapy is not considered to be controversial as they are derived from adult tissue samples rather than destroyed human embryos. They have mainly been studied in humans and model organisms such as mice and rats.
Embryonic stem cells.
Embryonic stem cells (ES cells) are stem cells derived from the inner cell mass of an early stage embryo known as a blastocyst. Human embryos reach the blastocyst stage 4–5 days post fertilization, at which time they consist of 50–150 cells.
Embryonic Stem (ES) cells are pluripotent. This means they are able to differentiate into all derivatives of the three primary germ layers: ectoderm, endoderm, and mesoderm. These include each of the more than 220 cell types in the adult body. Pluripotency distinguishes ES cells from multipotentprogenitor cells
found in the adult; these only form a limited number of cell types.
When given no stimuli for differentiation, (i.e. when grown in vitro), ES cells maintain pluripotency through multiple cell divisions. The presence of pluripotent adult stem cells remains a subject of scientific debate; however, research has demonstrated that pluripotent stem cells can be directly generated from adult fibroblast cultures.[1]
Because of their plasticity and potentially unlimited capacity for
self-renewal, ES cell therapies have been proposed for regenerative
medicine and tissue replacement after injury or disease. However
Diseases treated by these non-embryonic stem cells include a number of
blood and immune-system related genetic diseases, cancers, and
disorders; juvenile diabetes; Parkinson's; blindness and spinal cord
injuries. Besides the ethical concerns of stem cell therapy (see stem cell controversy), there is a technical problem of graft-versus-host disease associated with allogeneic stem cell transplantation. However, these problems associated with histocompatibility may be solved using autologous donor adult stem cells or via therapeutic cloning.
Use of human embryonic stem cells as models for human genetic disorders
In recent years there have been several reports regarding the
potential use of human embryonic stem cells as models for human genetic
diseases. This issue is especially important due to the
species-specific nature of many genetic disorders. The relative
inaccessibility of human primary tissue for research is another major
hindrance. Several new studies have started to address this issue. This
has been done either by genetically manipulating the cells, or more
recently by deriving diseased cell lines identified by prenatal genetic
diagnosis (PGD). This approach may very well prove invaluable at
studying disorders such as Fragile-X syndrome, Cystic fibrosis, and other genetic maladies that have no reliable model system.
Yury Verlinsky (Sept, 1, 1943 – July 16, 2009), a Russian-American medical researcher who specialized in embryo and cellular genetics (genetic cytology), developed prenatal diagnosis testing methods to determine genetic and chromosomal disorders a month and an half earlier than standard amniocentesis.
The techniques are now used by many pregnant women and prospective
parents, especially those couples with a history of genetic
abnormalities or where the woman is over the age of 35, when the risk
of genetically-related disorders is higher. In addition, by allowing
parents to select an embryo without genetic disorders, they have the
potential of saving the lives of siblings that already had similar
disorders and diseases using cells from the disease free offspring.
Hematopoietic stem cell transplantation
Hematopoietic stem cell transplantation (HSCT) is the transplantation of blood stem cells derived from the bone marrow (in this case known as bone marrow transplantation) or blood. Stem cell transplantation is a medical procedure in the fields of hematology and oncology, most often performed for people with diseases of the blood, bone marrow, or certain types of cancer.
With the availability of the stem cell growth factors GM-CSF and G-CSF, most hematopoietic stem cell transplantation procedures are now performed using stem cells collected from the peripheral blood, rather than from the bone marrow. Collecting peripheral blood stem cells[1] provides a bigger graft, does not require that the donor be subjected to general anesthesia to collect the graft, results in a shorter time to engraftment, and may provide for a lower long-term relapse rate.
Hematopoietic stem cell transplantation remains a risky procedure
with many possible complications; it has traditionally been reserved
for patients with life-threatening diseases. While occasionally used
experimentally in nonmalignant and nonhematologic indications such as
severe disabling auto-immune disease and cardiovascular disease, the risk of fatal complications appears too high to gain wider acceptance.
Pluripotent
Pluripotency is the ability of the human embryonic stem cell to differentiate or become almost any cell in the body.
Pluripotency in the broad sense refers to "having more than one
potential outcome." In biological systems, this can refer either to cells
or to biological compounds. From the Latin pluri=many, potent=power,
capacity. A pluripotent cell can create all cell types except for extra
embryonic tissue, unlike a totipotent cell, (tot=all), which can
produce every cell type including extra embryonic tissue.
Totipotency
Totipotency is the ability of a single cell to divide and produce all the differentiated cells in an organism, including extraembryonic tissues. [1] Totipotent cells formed during sexual and asexual reproduction include spores and zygotes. Zygotes are the products of the fusion of two gametes (fertilization). In some organisms, cells can dedifferentiate and regain totipotency. For example, a plant cutting or callus can be used to grow an entire plant.
Human development begins when a sperm
fertilizes an egg and creates a single totipotent cell (zygote). In the
first hours after fertilization, this cell divides into identical
totipotent cells. Approximately four days after fertilization and after
several cycles of cell division, these totipotent cells begin to
specialize.
Totipotent cells have total potential. They can specialize into pluripotent
cells that can give rise to most, but not all, of the tissues necessary
for fetal development. Pluripotent cells undergo further specialization
into multipotent
cells that are committed to give rise to cells that have a particular
function. For example, multipotent blood stem cells give rise to the red cells, white cells and platelets in the blood.
Importantly, totipotent cells must be able to differentiate not only
into any cell in the organism, but also into the extraembryonic tissue
associated with that organism. For example, human stem cells are
considered totipotent only if they can develop into any cell in the
body, or into placental cells that do not become part of the developing fetus. This fact is an important aspect of the stem cell controversy
because the human embryonic stem cells used for research purposes are
pluripotent; they are collected from human embryos that have developed
past the totipotent cell stage. All human blastocysts used in stem cell
experimentation are destroyed in the process.
Multipotent Stem Cells
Multipotent progenitor cells
have the potential to give rise to cells from multiple, but a limited
number of lineages. An example of a multipotent stem cell is a hematopoietic cell — a blood stem cell that can develop into several types of blood cells, but cannot develop into brain cells or other types of cells. At the end of the long series of cell divisions that form the embryo are cells that are terminally differentiated, or that are considered to be permanently committed to a specific function. Scientists have long held the opinion that differentiated cells
cannot be altered or caused to behave in any way other than the way in
which they have been naturally committed. New research, however, has
even called that assumption into question. In recent stem cell
experiments, scientists have been able to persuade blood stem cells to
behave like neurons, or brain cells- a process known as transdifferentiation.
Scientists now believe that stem cell research could reveal far more
vital information about our bodies than was previously known. There is
also continuing research to see if it is possible to make multipotent
cells into pluripotent cells.
Blastocyst
The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula, but before implantation. It possesses an inner cell mass (ICM), or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta. The trophoblast surrounds the inner cell mass and a fluid-filled blastocyst cavity known as the blastocoele. The human blastocyst comprises 70-100 cells.
Blastocyst formation begins at day 5 after fertilization in humans,[1] when the blastocoele opens up in the morula.
Differential gene expression in the morula is thought to be the
cause of the lineage divergence of different cell types. For example,
the Oct-3/4
transcription factor is restricted to the ICM, whereas Cdx2 is
expressed at a higher level in the trophoblast than the ICM. This
differential transcription factor expression is likely the result of
positional effect - cells in the middle of the preceding zygote are in
a different environment to those on the outside, thus causing
differential expression.
