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Stem cells are cells that can differentiate into other cell types that can perform specific functions (this is termed potency). This ability allows cells of complex multicellular organisms which have a finite lifespan to be renewed and replaced throughout the life of the organism. Stem cells can divide to create new stem cells of the same type, and they may do this indefinitely (i.e. they are capable of self-renewal).
Because of these traits of self-renewal and potency, they have great potential for both research to increase the understanding of cell development and function within an organism, as well as to investigate therapeutic applications. Stem cell therapy has been used to varying degrees of success for many disorders and much is expected to come from this in the future.
There are several different types of stem cell, these can be classified according to their source:
1. Embryonic Stem Cells
2. Adult Stem Cells
3. Induced Pluripotent Stem Cells
4. Cord Blood Stem Cells
As the name suggests, are responsible in the generation of new cells during the early stages of development of an embryo into the mature organism. These stem cells are further defined by how they may differentiate into cell types (potency) as follows:
Adult stem cells are those that are present in the fully developed organism and act to replace the cells in the body as they are required e.g. immune cells or blood cells. Adult stem cells are said to be multipotent as each of the categories of adult stem cells can only change into a limited number of specific cell sub-types. For example, haematopoietic (blood generating) cells differentiate into the range of blood cell sub-types, but not other classes of cells such as those of the immune system. Adult stem cells become less able to maintain the homeostasis of tissues over time, leading to the ageing process. Just how these metabolic and epigenetic changes, and accumulation of molecular changes are induced and regulated in these cells are therefore of great interest.
Induced pluripotent stem cells are normal cells that have been “engineered” to act as stem cells. By analysing the expression of stem cells and the normal differentiated cells it has become possible to compare the differences between what genes are “on” or “off” in each case. To simplify this for this article, what this means is that the key stem cell genes can effectively be switched on, and the genes that are expressed in normal cells can be switched off. In doing this we now have cells that can be induced to act as stem cells.
A fourth source of stem cells is umbilical cord blood which contains stem cells. These stem cells have gained interest in recent years as they are easy to obtain at childbirth and pose less ethical questions than for example some types of embryonic stem cells. These cells carry the same MHC (Major Histocompatibility Complex) proteins and other cell self-identifiers and thus mitigate problems around the need for immuno-suppression. The idea that is promoted by some commercial companies is that cells can be stored and potentially used for treatment of medical conditions of a sibling or family member. Public cord cell blood bank programs are also present in some areas meaning that these sources are available as potential sources for stem cell treatment of unrelated recipients.
Stem cell therapy is widely heralded as being a cure for a great many conditions. There are a growing number of applications wherein stem cells may be applied towards therapeutic applications i.e. to repair or return damaged, dysfunctional or diseased tissues within an organism back to their normal state. This is because many cells in the body do not have the ability to replenish themselves. However, with a few exceptions stem cell therapies have not reached the stage where they are approved for use by the FDA or other equivalent regulatory bodies.
Bone marrow transplants are one of the better-known applications for stem cell therapy that has been successfully used to treat various forms of leukaemia. Stem cell therapy is also being actively investigated for many autoimmune and inflammatory conditions including Crohn’s disease, Lupus and for neurodegenerative diseases such as Parkinson’s and even for regeneration of skin following burn damage.
A recent example that shows how stem cell treatments are developing to help otherwise fatal disorders is that of patients with MLD (Metachromatic Leukodystrophy). MLD is a neurodegenerative lysosomal storage disorder due to a faulty ARSA enzyme. By removing the patients own stem cells prior to progression of the disease, gene therapy can be performed to add in the functional ARSA gene. The existing faulty stem cells within the bone marrow are killed off by chemotherapy. When the gene-corrected stem cells are later added back into the patient it will allow normal metabolism and halt the onset of MLD, potentially permanently and without the need for further treatments.
There is also promise for stem cells as an alternative to organ transplants. Organ donors are in short supply and even with a close match there is a need for continual immune-suppression and other drugs to prevent rejection of “non-self” tissue and to prevent opportunistic infections. In this role the aim of stem cell therapy would be to inject stem cells of the correct type into damaged tissue, at the precise location, and potentially aid the return of function of the tissues in the liver, heart, kidney or other organs. Similarly, there are studies underway for spinal cord and nerve injury to return function. Finally, one alternative use for stem cells is in drug discovery as specific cell types can be produced. Toxicity and other studies can be conducted on these cells as part of the screening processes to aid the development of more effective drug treatments with less side effects.
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