About Stem Cells
Stem cells have the ability to replicate and produce specialized cells. Stem cells are not specialized, which allows them to build and repair many types of tissue through differentiation.
What are stem cells?
Stem cells are animal cells that have the ability to self-replicate and to produce specialized cells (Wang, 2002). Stem cells do not have specialized traits, allowing them to build and maintain tissue through differentiation the process by which undifferentiated embryonic material becomes specialized for a specific function (Lee, M.D.). Stem cells have the ability to replicate and produce specialized cells. Stem cells are not specialized, which allows them to build and repair many types of tissue through differentiation. Through differentiation, a stem cell can become any cell in the body, such as those found in blood, bone, and muscle tissue (Anderson, n.d.).
There are three basic types of stem cells: totipotent stem cells, which are able to produce every type of cell in the body and to form an entire organism; pluripotent stem cells, which are also able to produce every type of cell but are unable to form a working organism; and multipotent stem cells, which can only produce a certain number of cell types (Wang, 2002).
Embryonic stem cells are currently the most commonly utilized stem cell (Gerecht-Nir and Itskovitz-Eldor, 2004). When a sperm cell and an egg cell join, the resulting totipotent cell will eventually develop into a hollow sphere of cells called the blastocyst. Within the blastocyst is the inner cell mass, which will go on to form the three primary germ layers of the body: the ectoderm, the mesoderm, and the endoderm (Wang, 2002). The inner cell mass later becomes the embryo and is the source of embryonic stem cells. Because embryonic stem cells can produce all cells of the body but cannot produce a functioning organism itself, they are pluripotent (Wide Potential, 2004). One reason for embryonic stem cells popularity is their ability to inhibit certain immune responses of the body upon transplantation. This unique quality makes it easier for doctors to transplant the stem cells into the body without the body rejecting the foreign material (Immune Properties, 2004).
Though it has the potential to help hundreds of diseases and defects, some people object to embryonic stem cell research due to religious, ethical, and political reasons. In the process of harvesting embryonic stem cells from embryos produced through in vitro procedures, the embryos are destroyed or as some call it, killed (Friedrich, 2004).
What about adult stem cells? Adult stem cells occur in mature tissues. Like all stem cells, they can self-replicate; however, unlike embryonic stem cells, it is usually very difficult to harvest adult stem cells in cultures. Adult stem cells are also only multipotent, as opposed to embryonic stem cells, which are pluripotent (Wang, 2002). However, several types of adult stem cells have proven to be potentially helpful. For example, blood stem cells, called hematopoietic stem cells, have been studied and used for years (Rice and Scolding, 2004). Hematopoietic stem cells are found largely in the bone marrow (Horn, Morris, Neff, & Kiem, 2004). As mature blood cells die, hematopoietic stem cells provide a constant supply of new blood cells through a blood cell generation process called hematopoiesis (Wang, 2002).
Stem cell plasticity refers to the ability of adult stem cells from one tissue to generate the specialized cells of another tissue. It has long been thought that adult stem cells were restricted to only one specialized function. However, recent studies have shown that bone marrow stem cells contribute to skeletal muscle and cardiac cell production. This is extremely significant to further stem cell study because if stem cell differentiation can be directed, then stem cells that are readily available in the body can be used in place of stem cells that are harder to access (Wang, 2002).
How are stem cells used today?
Human embryonic stem cells can differentiate into a variety of specialized cells and can potentially provide differentiated cells for therapeutic uses (Gene Sets, 2004). One area in particular that has been heavily benefited by stem cell research is tissue engineering (Living-Tissue Company,2004). Tissue engineering is the creation and transplantation of new body tissue in order to repair damaged tissue (Elisseeff, 2004). The utilization of adult stem cells in tissue engineering is a hopeful solution to the growing organ and tissue shortage. Mesenchymal stem cells, which are found in adult bone marrow, are capable of producing bone, cartilage, muscle, and fat cells and are currently being researched in order to further the field of tissue engineering (Li et al., 2004).
One possible target of embryonic stem cells is diabetes. In a diabetic person, the pancreatic cells of the body do not produce enough of the hormone insulin. Embryonic stem cells can potentially be used to produce more pancreatic cells, thereby creating more sources of insulin and decreasing the severity of the disease (Tai and Svendsen, 2004).
Stem cells are also being used to treat neurological damage caused by diseases and injury. Adult stem cells called neuronal stem cells can be used to produce neurons and glial cells, the two primary cells of the brain. A possible target of neuronal stem cells is Parkinson's disease, a degenerative neural disease that severely affects motor functions in the elderly. Parkinson's disease develops when brain cells do not produce enough of the chemical dopamine. If neuronal stem cells were introduced into the brain, dopamine levels would hopefully rise and the effects of the disease would lessen (Tai and Svendsen, 2004).
Adult stem cells called retinal stem cells are used to create the retina during the course of development (Klassen, Sakaguchi, & Young, 2004). Researchers have recently found that these retinal stem cells can differentiate into retinal specific cell types and can hinder retinal degeneration (Boulton and Albon, 2004). Another type of adult stem cells, called adult inner-ear stem cells, can be used to aid in inner-ear regeneration after severe hearing loss (Li, Corrales, Edge, & Heller, 2004). Very importantly, adult stem cells can potentially be used to prevent or repair the otherwise irreversible loss of cardiac function that often follows a heart attack by repairing tissue damage and preventing scar tissue from restricting the heart (Fraser, Schreiber, Zuk, & Hedrick, 2004).
There seems to be only one downside to stem cell therapy. Stem cells and cancer cells share one important quality: immortality. This is due to the fact that both use the enzyme telomerase to maintain their telomeres and prevent aging (Campbell and Reece, 2002). Because of this similarity, some scientists believe that stem cells have been the cause of malignant cancer cells in some people in past experiments (Tsai, 2004).
Video
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Dr. Collins speaking about stem cell research and its potential.
References
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