Adult or Somatic Stem Cells: Facts and Potential Medical Uses
What Are Adult Stem Cells?
Adult stem cells are unspecialized but very important cells that are found at various sites in our body. Most of our cells are specialized and have specific characteristics to help them do their jobs. They are generally unable to divide. Stem cells divide repeatedly to produce the specialized cells that our body needs in order to repair minor injuries.
Adult stem cells have been found in many places in the body. For example, they've been discovered in the bone marrow, heart, brain, muscles, teeth, and gastrointestinal tract. The list of locations is growing as scientists do more research. Despite their name, the cells are present in both adults and children. Some researchers prefer to call them somatic stem cells.
Scientists are very excited by the potential benefits of adult or somatic stem cells. It may be possible to stimulate them to become more numerous and more active. It may also be possible to trigger them to produce specific target cells. In the future, the stem cells may be useful in the treatment of major problems in the body.
Although most forms of stem cell therapy are not used as medical treatments yet, bone marrow transplants are. Bone marrow contains hematopoietic stem cells. These produce all types of blood cells.
"Potency" is the ability of an unspecialized stem cell to produce specialized cells. There are four types of potency.
- Totipotent cells can produce all the cell types in the body as well as extraembryonic tissues, such as those needed to form the placenta. Extraembryonic tissues are needed for the embryo's development but aren't part of its body. The zygote is an example of a totipotent cell. A zygote is produced when an egg joins with a sperm. It's the first cell of a new individual.
- Pluripotent cells can form any cell type in the body. However, unlike totipotent cells, pluripotent cells can't make extraembyonic tissue.
- Multipotent cells can produce several types of specialized cells. For example, stem cells in the bone marrow can make red blood cells, white blood cells, and platelets.
- Unipotent cells can produce only one type of specialized cell.
Progenitor and Target Cells
When an adult stem cell divides, it produces either two stem cells or one stem cell and one progenitor cell. The progenitor cell is slightly more specialized than the stem cell and undergoes additional divisions. The cells become more and more differentiated (or specialized) with each division. Eventually, the target cells are produced. The target cells are generally ones needed by the tissue in which the stem cells are located.
Functions of Adult Stem Cells in the Body
The job of adult stem cells is to replace cells that are damaged or have died. These cells include our red blood cells or erythrocytes, which live for only about 120 days, and our white blood cells, which are sometimes killed when fighting bacteria or other microbes. Stem cells in the lining of the small intestine produced the specialized cells needed for digestion and absorption. In the brain, stem cells can produce neurons (nerve cells) as well as the astrocytes and oligodendrocytes that support the neurons. Satellite cells are a type of stem cell in muscles.
The end products of stem cell division are controlled by factors inside the cell as well as by factors outside the cell. Gene expression (the turning on of specific genes), the prevention of gene expression, chemicals produced by other cells, additional chemicals in the cell's surroundings, and physical contact with neighbouring cells can all have an effect on cell differentiation as cells become more specialized.
Mesenchymal stem cells are found in various tissues. They are multipotent and form bone, cartilage, and fat cells. They may form other types of cells, too, although this is uncertain.
Satellite Cells in Muscles
Stimulating Stem Cell Activity
Under natural conditions, stem cells have a limited ability to repair damage in our bodies. They can't produce an entire organ or body part after we're injured, for example. In addition, they are more numerous or more active in some areas of the body than in others. Still, their potential to replace injured tissue is exciting. It may be possible to stimulate their activity.
Researchers hope to discover the signals that trigger a stem cell to produce specific target cells. Scientists may then be able to produce large numbers of target cells in the laboratory or large numbers of stem cells directed to produce the target cells. The hope is that either the stimulated cells or unstimulated ones that have multiplied will be able to repair serious damage in a person's body. There have been some clinical successes using stem cell technology, but more research is needed before the technology is widely used.
An exciting discovery is that under certain laboratory conditions some multipotent stem cells can be stimulated to produce any other cell type instead of just a few types. These pluripotent cells may have very useful applications. However, the manipulation of stem cells to make them pluripotent or to direct their multipotency involves controlling genes in the cells, which raises safety concerns.
Embryonic Stem Cells
In the early stages of stem cell research, stem cells from human embryos were used instead of cells from adults. Embryonic stem cells are still used in some research labs today.
The embryos that supply the stem cells are usually ones that remain after in-vitro fertilization. This is a procedure that helps some couples who have been unable to have children. Donated eggs and sperm are joined in laboratory equipment and then one or more of the resulting embryos are implanted in the woman's uterus. Multiple embryos are produced because the first embryo or embryos that are used may not implant in the lining of the uterus successfully. The embryos that are not needed by the couple are frozen and may eventually be used in embryonic stem cell research, with the couple's consent.
Embryos are used for research when they have completed about five days of development. At this stage an embryo is known as a blastocyst. The inner cells of the blastocyst are pluripotent stem cells. Once the embryonic cells are obtained they are allowed to replicate in the laboratory, producing a huge number of stem cells for research as they continue to divide. The fact that the cells are pluripotent makes them especially useful. Since stem cells make more stem cells as they divide, it's unnecessary to continually use new embryos in research.
Controversy and a Problem
The use of embryonic stem cells is controversial, since when the original cells are obtained the intact embryo is destroyed and is unable to develop any further. Some people believe that a new human life begins the moment that a sperm joins with an egg. Another problem is that if cells derived from an embryo are placed inside someone's body, the recipient's immune system may treat the transplanted cells as invaders and destroy them.
A "somatic" cell is a body cell, or one that isn't involved in reproduction, so the word is appropriate in an alternate name for adult stem cells.
Reducing the Controversy
The use of adult stem cells is much less controversial than the use of embryonic cells because the adult cells can be obtained from a donor without destroying human life. If the donor is also the person who needs a transplant of specialized cells or stem cells, the problem of rejection is eliminated, since the person's body won't attack its own cells.
At first it was thought that adult stem cells weren't as valuable as embryonic ones, since the adult cells are multipotent but not pluripotent. Now scientists have found a way to trigger adult stem cells to become pluripotent.
Pluripotent Stem Cells
Adult stem cells may be transformed from multipotent cells into pluripotent ones by controlling which genes are expressed and by altering the chemical environment of the cells. The transformed cells are known as induced pluripotent stem cells, iPSCs, or iPS cells. They have a state much like that of an embryonic stem cell.
iPS cells are not ready to use in humans yet. One problem with the creation of induced pluripotent cells is that viruses are sometimes used to change the genetic programming of the original cells. Viruses are useful in biotechnology because some can add or remove genes from host cell DNA and because some can change the gene expression of the host cell. In tests with animals, the viruses added to cells to reprogram them have sometimes caused cancer in their host. Researchers are trying to find ways to force adult stem cells to become pluripotent without increasing the risk of cancer development.
Some researchers have found that certain chemicals can trigger pluripotency in stem cells, which may be a viable alternative to virus use. Transforming adult stem cells into pluripotent ones safely could have enormous benefits in the field of medicine.
Additional Benefits of Adult Stem Cells
Even if stem cells and their products aren't used for transplants, they may still be useful. For example, they may be helpful in testing drug action in cells and in studying human development and birth defects. They may also be useful in the study of cancer. In addition, learning how stem cells work may help researchers develop new therapies for illnesses.
Early tests indicate that adult stem cell technology may have important benefits, but many questions about multipotent and induced pluripotent stem cells remain. For example, is it possible to stimulate multipotent cells to produce cells found in different tissues from the one in which the stem cells are located? What are the most efficient methods for making adult stem cells pluripotent? Are genetically altered stem cells safe?
In 2012, the Nobel Prize for physiology or medicine was awarded to Sir John Gurdon and Shinya Yamanaka for their discovery that adult cells can be reprogrammed to become pluripotent cells.
Potential Uses of Stem Cell Technology
Adult stem cell technology appears to be very promising, since in tests it has helped to improve certain symptoms and conditions. However, there is a great deal that researchers don't yet know or understand about stem cells and their actions in our bodies.
Some individuals, organizations, and companies are making wonderful claims about the benefits of stem cell therapy. We need to be cautious when assessing these claims. Once scientists have a better understanding of how stem cells work and of how to control them, we may discover that some of the claims are true. We're not at this stage yet, however.
Researchers hope to be able to help a range of disorders with stem cell technology, including heart disease. A few ailing hearts have been helped by the technology in test procedures. It's hoped that in the future insulin-secreting cells will be produced for implanting into the pancreas of people with Type 1 diabetes. Other diseases which may respond to stem cell therapy include Alzheimer's disease, osteoarthritis, rheumatoid arthritis, strokes, spinal cord injuries, and burns.
A Doctor's Warning About Unproven Therapy
Requirements for Successful Therapy
There are many requirements in order for stem cell technology to be successful as a therapeutic treatment.
- The transplanted cells must be able to survive in the patient's body after the transplant.
- They cells must be incorporated into the correct tissue or organ.
- Unspecialized stem cells must divide repeatedly to produce a sufficient number of replacement cells for the patient.
- The new cells must differentiate into the correct target cells.
- The cells must remain active in the person's body for a long time.
- The stem cells mustn't harm the recipient.
If all these requirements are met, some wonderful medical treatments may be available. A lot of researchers are investigating stem cells. Hopefully, new and effective ways to treat certain health problems will soon exist.
© 2012 Linda Crampton