The idea of a miracle cure, bodies healing themselves, holds a certain fascination. Stem cell research has brought regenerative medicine a step closer, but it has also been fraught with controversy. So what are stem cells, and why are they so important?
Stem cells are a type of cell that can develop into many other different types of cell. Stem cells can also renew themselves by dividing, even after they have been inactive for a long time.
When a stem cell divides, the new cells may either become a stem cell or a specific cell, such as a blood cell, a brain cell, or a muscle.
A stem cell is known as an "undifferentiated cell," because it can still become any kind of cell. In contrast, a blood cell, for example, is a 'differentiated' cell, because it is already a specific kind of cell.
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How can stem cells help in medicine?
Stem cells could help the body to heal itself.
In some tissues, stem cells play an important role in regeneration, as they can easily divide, and they can keep replacing dead cells. Scientists believe this may offer potential treatments for conditions such as diabetes and heart disease.
For instance, if someone has damaged tissue in their heart, it might be possible to stimulate the growth of healthy tissue by transplanting laboratory-grown stem cells into the patient's heart.
A small-scale study published in the Journal of Cardiovascular Translational Research trialed this method. The researchers reported a 40 percent reduction of the size of scarred heart tissue caused by heart attacks.
In the past, this kind of scarring has been seen as permanent and untreatable.
The patients, who all had advanced heart failure, had an average 30 percent improvement in heart function. They also reported a 70 percent improvement in quality of life 24 months after being injected with the stem cells.
However, this study involved only 11 participants. It is difficult to tell whether the improvement in heart function was caused by the transplantation of stem cells, or whether it was caused by something else.
All of the transplants took place while the patients were undergoing heart bypass surgery, for instance, so it is possible that the improvement in heart function could be due to the bypass rather than the stem cell treatment.
To investigate further, the researchers plan to do another study, this time including a control group of patients with heart failure who undergo bypass surgery, but who do not receive the stem cell treatment.
Another study published in Nature Communications suggested that stem cell therapies could form the basis of personalized diabetes treatment.
"In theory, if we could replace the damaged cells in these individuals with new pancreatic beta cells - whose primary function is to store and release insulin to control blood glucose - patients with type 1 diabetes wouldn't need insulin shots anymore."
Jeffrey R. Millman, assistant professor of medicine and of biomedical engineering at Washington University School of Medicine and first author.
Millman hopes that these stem cell-derived beta cells could be ready for research in humans within 3 to 5 years. "What we're envisioning is an outpatient procedure in which some sort of device filled with the cells would be placed just beneath the skin," he said.
Stem cells could have vast potential in developing new therapies.
Stem cells offer new insights into how things work
Stem cell research is also driving forward the scientific understanding of how an organism develops from a single cell, and how defective cells are replaced by healthy cells in people and animals.
Many serious medical conditions that occur in humans, such as cancer and birth defects, are caused by cells dividing abnormally.
By studying stem cells and the process of cell differentiation, we may better understand how these diseases arise and how they may be treated.
How are stem cells already helping?
One way that stem cells are used at the moment is in developing and testing new drugs.
Stem cells can help scientists to understand what causes diseases such as cancer.
The type of stem cells commonly used for this purpose are called induced pluripotent stem cells.
These are cells that have already become differentiated, but then they have been genetically "reprogrammed" using controlled viruses. In this way, they resemble undifferentiated embryonic stem cells.
New differentiated cells can be grown from these pluripotent stem cells to resemble, for instance, cancer cells. Creating these cells means that anti-cancer drugs can be tested using them.
A wide variety of cancer-cell types are already being made using this method. However, because these cells cannot yet be made to mimic cancer cells in a controlled way, the results cannot always be precisely replicated.
In recent years, clinics have opened that provide stem cell treatments. A 2016 study published in Cell Stem Cell counts 570 of these clinics in the United States alone. They offer stem cell-based therapies for disorders ranging from sports injuries to cancer.
However, stem cell therapies are still mostly theoretical rather than evidence-based.
Very few stem cell treatments have even reached the earliest phase of a clinical trial. Most of the current research is being carried out in mice or in the petri dish.
Despite this, clinics are allowed by the U.S. Food and Drug Administration (FDA) to inject patients with their own stem cells, as long as the cells are intended to perform only their normal function.
Where do we get stem cells from?
Stem cells can be harvested in different ways.
Embryonic stem cells come from an embryo that is just a few days old.
Adult stems cells can be extracted from different types of tissue, including the brain, bone marrow, blood vessels, skeletal muscle, skin, teeth, the gut, the liver, among others.
Amniotic fluid contains stem cells. Many women opt for an amniocentesis test that checks for birth defects before the child is born. If the fluid is kept, it could be used to treat certain congenital defects either during gestation or after birth.
Induced pluripotent stem cells (iPS cells) are cells that can be reprogrammed to act as stem cells, for use in regenerative medicine.
What are the ethical issues with stem cells?
The use of stem cells in medical research has been controversial, historically.
New sources of stem cells may make them less controversial.
The reason is that when the therapeutic use of stem cells first came to the public's attention in the late 1990s, it was because scientists had first mastered the technique for deriving human stem cells from embryos.
Many people disagree with using human embryonic cells for medical research, because extracting the stem means destroying the embryo.
This creates complex issues, as there are different beliefs as to what constitutes the start of human life.
For some people, life starts when a baby is born, or when an embryo develops into a fetus. Others believe that human life begins at conception, so an embryo has the same moral status and rights as a human adult or child.
It was because of his strong, pro-life religious views that President George W. Bush banned funding for human stem cell research in 2001.
"At its core, this issue forces us to confront fundamental questions about the beginnings of life and the ends of science," Bush announced at the time. "My position on these issues is shaped by deeply held beliefs. I also believe human life is a sacred gift from our creator."
However, the research restrictions implemented by the Bush administration were partially rolled back during President Obama's time in office.
Pluripotent stem-cell use was introduced in 2006. The stem cells in this method do not come from embryonic stem cells. As a result, this technique is not associated with the same ethical concerns.
With this, and other recent advances in stem cell technology, attitudes toward stem cell research have evolved a little.
What is the next step for stem cells?
Although much more research is necessary before stem cell therapies can become part of regular medical practice, the science around stem cells is developing all the time.
In June 2016, two researchers took second prize in the materials category of the United Kingdom's Royal Society of Chemistry's emerging technology competition for creating a synthetic biomaterial that stimulates stem cells native to a patient's own teeth.
The researchers believe that this material will replace fillings, as the stem cells would simply prompt the damaged teeth to heal themselves.
In almost every therapy area, there are hopes that stem cell technology will revolutionize therapeutic norms and introduce at least a new standard of personalized treatment, and maybe even self-healing bodies.
When that might happen, no one is quite ready to say.