The complex human body is comprised of somatic (body) cells and reproductive cells (gametes). All cells in the human body originate from a single fertilized egg cell known as the zygote. The zygote then divides into a blastocyst made up of embryonic stem cells that give rise to more than 200 specialized types of cells, according to the International Society for Stem Cell Research. Somatic stem cells – also called adult stem cells – form during fetal development and remain throughout the life span to aid in cell repair.
Stem Cells: Definition
Other names for stem cells that are more precise are embryonic stem cells, adult stem cells or induced pluripotent stem cells, depending on a cell's respective typology. Stem cells have the ability to morph into many other types of cells, which is of great interest to researchers in the field of regenerative medicine. Stem cells share special characteristics that distinguish them from common, ordinary cells such as nerve cells, bone cells and blood cells:
- Stem cells can duplicate themselves many times over, or specialize, as needed in the tissue.
- Stem cells differentiate into specialized cells with specific jobs.
- Stem cells can specialize into many different sizes and shapes of cells.
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Embryonic Stem Cells
Human embryonic stem cells are derived from a developing egg cell in the blastocyst stage, about five days after fertilization. Embryonic stem cells are undifferentiated and can divide indefinitely or differentiate into specialized cells in the lab. Embryonic stem cells have the potential to be genetically or chemically programmed to grow organs and skin for transplant and grafts.
Somatic (Adult) Stem Cells
Embryonic stem cells quickly differentiate into somatic stem cells during fetal development. Small quantities of somatic stem cells remain in the body indefinitely, but they change over the course of a lifetime. Somatic stem cells help the body make internal repairs and regulate homostasis. Progenitor cells are an intermediary step between a dividing stem cell and a more specialized cell.
Unlike versatile embryonic stem cells, somatic stem cells have limited capacity for differentiation. Current studies suggest that adult stem cells only differentiate into cells for the particular type of tissue where they reside. For example, somatic stem cells in muscle tissue can differentiate into various types of muscle cells, but they cannot give rise to nerve cells. However, research is underway that may upend that assumption, according to the University of Nebraska Medical Center.
Function of Somatic Stem Cells
Somatic (adult) stem cells can indefinitely produce more daughter cells, or specialize into certain types of cells, such as red and white blood cells. Adult stem cells are able to renew themselves even after periods of inactivity whenever repair or replacement of cells is necessary. For example, somatic stem cells in the heart and pancreas spring into action under certain conditions when repair work is indicated. However, in the gut and bone marrow, stems cells are continually at work renewing themselves.
Hematopoietic Somatic Stem Cells
Hematopoietic stem cells (HSCs) are blood-forming cells found in bone marrow and in circulating blood. Immature cells can become red blood cells, platelets and white blood cells. Transplanted HSCs cells in bone marrow from matching donors have helped countless patients diagnosed with blood disorders and cancers like leukemia. Autologous transplantation of the patient’s own HSCs is another common therapeutic procedure that has benefited patients by reducing risk of transplant rejection.
Mesenchymal Somatic Stem Cells
Sources of human mesenchymal stem cells (hMSCs) include supportive and connective tissue around body organs. These stem cells differentiate into mesodermal cells like cartilage, bone cells, muscle cells and fat cells. Stem cell research into the use of hMSCs could lead to enhanced treatment of broken bones and cartilage injuries.
Neural Somatic Stem Cells
Neural stem cells (NSCs) generate neurons and glial cells. NSCs are found in the brain and central nervous system. Promising clinical trials are ongoing to investigate NCS stem cell therapy as a treatment for spinal cord injury, stroke and amyotrophic lateral sclerosis (ALS).
Epithelial Somatic Stem Cells
Epithelial stem cells are found in layers of the skin, lungs and in the epithelial layer of the intestine. These stem cells are continually renewing and responding to injury or damage to cells. Medical applications of epithelial stem cell research include creating skin grafts to aid accident and burn victims, for instance.
Induced Pluripotent Stem Cells
In 2007, researchers discovered how to genetically reprogram adult stem cells to act more like embryonic stem cells. Known as induced pluripotent stem cells (iPSCs), these engineered cells can be controlled to act in certain ways in lab cultures. For instance, a somatic cell such as a skin cell may be stimulated to give rise to an entirely different type of cell. The field is still very new, and much is unknown about the mechanisms of the process.
Stem Cell Classification
Stem cells are classified according to their power to give rise to more specialized cell types. Embryonic stem cells are advantageous in research because of their unadulterated condition and high potency for differentiation. The single-cell zygote is called totipotent because it can form a total living organism along with placental cells and tissue. Embryonic stems cells are classified as pluripotent; they form somatic cells, but not placental cells. Cord blood cells and adult stem cells are multipotent; their ability to specialize into different types is more limited than embryonic stem cells.
Early Stem Cell Research
Interest in stem cell research is driven by a desire to find new ways of repairing damaged cells in skin tissue and internal organs critical to survival. In 1981 scientific researchers first isolated embryonic stems cells from mice embryos, according to the National Institutes of Health. By 1998, scientists learned how to derive human stems cells from human eggs created in vitro at fertility clinics, which were no longer needed and were donated for research. Lines of stem cells are grown and shared between scientists.
In 1948, somatic stem cells were first used to produce blood cells. Adult bone marrow cells were used for stem cell transplants in 1968. Since then, stem cell therapies have been used to successfully treat many types of blood disorders. Endless therapeutic possibilities using stem cells are possible, but many are still relatively untested for safety and effectiveness.
Benefits of Stem Cell Research
Scientists use induced pluripotent stem cell lines to study normal and abnormal cell division, including cancer and tumor formation. Deeper understanding of how disease occurs could lead to more effective preventative measures and treatments. Tissues generated in the lab from stem cells can help with testing new drug treatments and reduce testing on animal subjects. Thousands of people suffering from blood-related diseases like leukemia and anemia have been helped through stem cell therapies.
Applications of Stem Cell Research
Stem cell research is a rapidly advancing field with new breakthroughs anticipated soon. Because stem cells are found in so many parts of the body, they may hold the key to figuring out the cause of several diseases. Hematopoietic stem cell therapies like bone marrow transplants are used widely. Some types of skin grafting and stem cell treatments of corneal injuries are also accepted by the medical community.
Risks of Stem Cell Therapy
The public should be wary of overstated claims and misinformation about stem cell therapies, according to the International Society for Stem Cell Research. Patients with serious medical conditions may be especially vulnerable to clinics purporting to offer instant cures. The U.S. Food and Drug Administration’s website warns consumers that they are putting their health at risk by trusting clinics offering treatments not approved by the FDA. To date, only certain products made with the blood-forming stem cells in cord blood are FDA-approved for specific treatments.