FAQ Science

Commonly asked questions about stem cells

What are stem cells?

The human body contains a trillion cells and 200 cell types. Scientists know that all of the body’s different cell types STEM from a master cell. Stem cells are unspecialized cells that have two important characteristics that distinguish them from other cells in the body. First, they are immortal, and in most tissues they are a very rapidly expandable population. Second, they have the remarkable potential to differentiate, or transform into specialized cells with specific functions, such as a heart cell or nerve cell. Stem cells can be classified by the extent to which they can differentiate into different cell types: totipotent, pluripotent, multipotent and progenitor cells.

What are the sources of stem cells?

Embryonic stem cells (ESCs).All humans start their lives from being a single cell, called the zygote, which is formed by the union of sperm and egg. The zygote divides and at 4- to 5-days-old, the embryo is known as a blastocyst. Each blastocyst consists of 200 cells. The inner cell mass contains about 50 pluripotent cells which go on to form all the cells of the body. Embryonic stem cells come from excess fertilized eggs stored in freezers at in-vitro fertility (IVF) clinics. Currently, there are over 400,000 unused frozen embryos in U.S. fertility clinics, which could be donated for research purposes with the informed consent of the donors.

Adult stem cells are undifferentiated cells that are found in small numbers in most adult tissues. The primary roles of adult stem cells in the body are to maintain and repair the tissues in which they are found. They are usually thought of as multipotent cells, giving rise to a closely related family of cells within the tissue. An example is hematopoietic stem cells, which form all the various cells in the blood. Blood from the placenta and umbilical cord that are left over after birth is a rich source of hematopoietic stem cells. Disadvantages of using adult stem cells are that they are rare in mature tissues and it is more difficult to expand their numbers in cell culture, compared with ESCs.

Reprogrammed Cells: In November 2007 scientists announced the development of iPS (induced pluripotent stem) cells. A retrovirus is used to inject genes into a normal cell’s nucleus to turn back the clock on an adult cell to behave embryonic stem cell-like. Unfortunately these cells cannot be used therapeutically for transplantation because of the introduction of a virus and the use of genes that may cause cancer. Scientists are looking for ways to modify the technique so that the cells do not carry this risk and scientists still need to confirm the potential and behavior of these cells.

What are the potential uses of human stem cells?

Because stem cells are the source of all tissues of the body, scientists can learn how stem cells grow into different cell types in the body and help them better understand normal and abnormal development. Scientists need to understand cell renewal and differentiation in order to offer the possibility of a renewable replacement source to treat diseases.

Many scientists see stem cell research as the beginning of the new era that will allow us to utilize everything that we have learned from the Human Genome Project. Stem cell research can lead to new insights about disease from studying the proteins that surround and organize the DNA in our cells. Examples of some genetic defects scientists are studying include: some cancers, ALS, infertility, Type 1 diabetes.

The first potential application of human embryonic stem cell technology may be in the area of drug discovery. Stem cell technology could permit the rapid screening of hundreds of thousands of chemicals that must now be tested through much more time-consuming processes.

How might embryonic stem cells be used to treat disease?

The ability to grow human tissue for transplantation of all kinds opens the door to treating a range of cell-based diseases. Stem cells can be used to generate healthy and functioning specialized cells, which can replace diseased or dysfunctional cells through transplantation.

• Heart Damage: Beating-heart cells could be grown to make a patch and fix damaged heart tissue.
• Parkinson’s: Dopamine-producing brain cells could replenish those destroyed.
• Spinal Cord Injury: Oligodendrocyte cells, the building blocks of myelin, could re-insulate nerve fibers. This is critical for electrical conduction in the central nervous system to restore sensory and motor function.

What are some of the challenges?

One of the first obstacles is the difficulty in identifying stem cells in tissue cultures, which contain numerous types of cells. Second, once stem cells are identified and isolated, scientists need to figure out how to control differentiation. The cells must then be integrated into the patient’s own tissues and organs and “learn” to function in concert with the body’s natural cells. Cardiac cells that beat in a cell culture, for example, may not beat in rhythm with a patient’s own heart cells. And neurons injected into a damaged brain must become “wired into” the brain’s intricate network of cells and their connections in order to work properly. Like organ transplantation there is also the issue of tissue rejection. Taking drugs to suppress an immune system is dangerous. Researchers are trying to find the balance between growth of the new cells and making sure the cells don’t overgrow and form cancerous tumors.

Do adult stem cells have the same capability as embryonic stem cells?

The general consensus is that adult stem cells seem to be less versatile. Scientists think that embryonic stem cells have a much greater utility and potential than the adult stem cells, because embryonic stem cells may develop into virtually every type of cell in the human body. Adult stem cells, on the other hand, may only be able to develop into a limited number of cell types. Embryonic stem cells also continue to divide indefinitely when placed in a culture, while this may not be the case for adult stem cells. Adult stem cells also have a reduced their capacity to form new cell types. Both adult and embryonic stem cell research should continue simultaneously as they are both critical to our understanding of the etiology, progression and treatment of disease.

What is therapeutic cloning?
Therapeutic cloning is not the same as reproductive cloning, which is intended to genetically duplicate a person.

Therapeutic cloning is based on a technology called somatic cell nuclear transfer which involves transferring a nucleus from a donor cell, such as a skin cell, into an unfertilized egg. Scientists first remove the nucleus, the part of the cell that contains the genetic material, from a normal unfertilized egg cell of a woman. They then extract a nucleus from a somatic cell—that is, any body cell other than an egg or sperm—from a patient who needs an infusion of stem cells to treat a disease or injury and insert the nucleus into the egg. The injected egg, which now contains the patient’s genetic material, is then induced to divide, and when it reaches a few hundred cells, the so-called blastocyst stage, it can be used to derive embryonic stem cells that are genetically identical to the original donor. No sperm is involved, and therefore no fertilization occurs, in this procedure. The advantage of therapeutic cloning is that the resulting ESCs have the patient’s cell surface proteins and are unlikely to be rejected by the patient’s immune system when transplanted into the body. This technique can also be used to make stem cells that carry disease genes. These cells could provide a powerful new tool for studying the basis of human disease and for discovering new drugs.

What is the future of cell therapy?

Despite the many challenges, most scientists believe that cell therapy will revolutionize medicine. With the use of cell therapies, we may have dramatic cures for cancer, Parkinson’s, diabetes, ALS, multiple sclerosis, macular degeneration and a host of other diseases. Cell therapies have also shown great promise in helping to repair catastrophic spinal injuries, and helping victims of paralysis regain movement. We may even have the ability one day to grow our own organs for transplantation from our own stem cells, eliminating the danger of organ rejection. While we will undoubtedly encounter the limits of cell therapy one day, there is every reason to hope that this revolutionary new approach will result in radically improved ways to treat disease.