What are stem cells?
Stem cells are unspecialized cells that have the ability to develop into cells with specific functions—for example, muscle cells or red blood cells.
The two main types
Stem cells are found both in embryos and in certain tissues. Embryonic stem cells develop into all the different types of cells the body needs. Stem cells found in tissues serve to repair damage and replace old cells.
Most human embryonic stem cells come from embryos that are four or five days old and still microscopic.
Similarities and differences
• All stem cells can divide many times without turning into specialized cells. In a laboratory, a small batch of stem cells can multiply over many months and yield millions of cells.
• Embryonic stem cells can develop into any of the more than 200 types of cells found in the human body.
• Embryonic stem cells can be produced fairly easily in the laboratory, by cell division.
• Adult stem cells (also called somatic stem cells) are found in specific tissues of the body. Usually, they develop only into the type of cells needed in those tissues. Adult stem cells have been found in the brain, bone marrow, blood, skeletal muscle, skin, teeth, heart, and liver.
• Adult stem cells are difficult to isolate from the surrounding specialized cells, and scientists haven’t yet learned how to grow them in the laboratory. This makes them less promising for therapies that would involve replacing damaged tissues with tissues grown from stem cells. However, certain adult stem cells are already being used in medical therapy: for example, in bone marrow transplants for leukemia patients. (Bone marrow contains stem cells that produce blood cells.)
Why are researchers so excited about stem cells?
Stem cells offer the hope that doctors will one day be able to replace damaged cells with new, healthy ones derived from the same person—thereby avoiding the problem of tissue-rejection. (Patients who receive transplants must take anti-rejection drugs, which have unwanted side-effects. And the need for organs and tissues to transplant far exceeds the supply.) Millions of Americans suffer from diseases that may some day be treated with stem cell therapy.
The first medical therapy using embryonic stem cells was approved by the U.S. Food and Drug Administration in March, 2010. The treatment is for a type of blindness known as Stargardt’s Macular Dystrophy.
Scientists also hope to use stem cells to study the effectiveness of new drugs: experimental medications could be tested on tissue grown from stem cells, rather than on human subjects. And stem cell research helps advance our understanding of how adult organisms develop from a single fertilized cell. Finally, scientists hope to learn more about cancer and birth defects by studying stem cells.
What diseases and injuries may some day be treated with stem cells?
Among others, Type 1 diabetes, heart disease, spinal cord injuries, vision loss, hearing loss, Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, A.L.S. (Lou Gehrig’s disease), stroke, burns, and arthritis.
The ethical controversy over stem cell research
In order to extract human embryonic stem cells, the embryo—which has the potential to develop into a human being—must be destroyed. This is the crux of the ethical debate.
Most of these embryos come from fertility clinics. Many couples having difficulty conceiving children opt for in vitro fertilization, in which sperm and egg are fused in a laboratory; a resulting embryo is then implanted in the mother’s uterus. Several eggs are usually fertilized, but only one is implanted in the mother. With the consent of the parents, the extra embryos are donated for research.
Proponents of research using embryonic stem cells say that the potential medical advances—including treatments for diseases that doctors can’t cure at present—far outweigh the ethical concerns. They also point out that the excess embryos would be destroyed anyway if they weren’t used for research.
Opponents say that this research disrespects human life. (Most of these opponents also oppose abortion.) The Catholic Church is the most prominent opponent of stem cell research using human embryos.
There is less controversy over research involving adult stem cells, because no embryo is destroyed in extracting them. The Catholic Church supports this type of research.
Milestones in stem cell research
1960s While studying rats, scientists discover cells in the brain that divide and eventually become nerve cells.
1981 Researchers learn how to derive stem cells from mouse embryos.
1998 Researchers at the University of Wisconsin-Madison extract the first stem cells from human embryos that can be kept alive in the laboratory.
2006 Researchers learn how to induce some specialized adult cells to return to an unspecialized, stem-cell state.
2010 U.S. doctors begin the first official trial using human embryonic stem cells in patients. The trial, at an Atlanta hospital, involves patients with spinal injuries. (See this BBC story for more information.)
What research is currently allowed by law?
A 1996 law known as the Dickey-Wicker amendment bans the use of tax dollars to create human embryos, or to conduct research in which human embryos are destroyed, discarded, or injured. That law prevented federal funding of embryonic stem cell research—until August, 2001, when President George W. Bush found a compromise by ordering that tax dollars could be used for research on the 60 stem cell lines that already existed. (A stem cell line is a population of healthy stem cells, derived from a single parent group of stem cells, that have divided for at least six months without differentiating into specialized cells.)
In March, 2009, President Obama removed the limits set by President Bush. His executive order will allow research on all embryonic stem cells—but federally-funded researchers still won’t be allowed to create their own stem cell lines.
Some countries, including Austria, Denmark, France, Germany and Ireland, have banned the production of embryonic stem cell lines. Countries that permit it include Finland, Greece, the Netherlands, Sweden, and the United Kingdom.
8/24/10: In a surprise to researchers, the New York Times reports, Federal District Court Judge Royce Lambert issued a preliminary injunction blocking the president’s 2009 executive order that expanded embryonic stem cell research. Agreeing with two scientists who sued the government, Judge Lambert said the president’s order violated the ban on federal funding for research that destroys embryos.
8/26/10: British scientists have created liver cells from stem cells taken from human skin, by “reprogramming” the skin cells. This seems to support hopes that healthy cells can someday be used to repair damaged organs—and that adult stem cells can be used in place of controversial embryonic stem cells. See Yahoo News for more.
5/3/11: A majority of the U.S. Court of Appeals for the District of Columbia has ruled that embryonic stem cell research can go on while the legality of the Obama executive order is decided.
11/14/11: In two separate studies, patients treated for severe heart disease with infusions of stem cells showed significantly improved heart function within a year. This is the first time anyone has grown living heart tissue, and it disproves the conventional wisdom that heart cells, once destroyed, can never be replaced. For details, see CNN.
Terms to know
blastocyst: an embryo that’s 3 to 5 days old. A hollow, microscopic ball, the blastocyst consists of 50-150 cells. Its inner cells eventually develop into the entire adult body. Blastocysts are the source of human embryonic stem cells.
cell-based therapies: medical treatments using stem cells to generate replacements for injured or diseased tissue.
cell culture: growing cells in a laboratory.
differentiation: the process by which an unspecialized cell develops into a specialized one.
directed differentiation: inducing embryonic stem cells to turn into specific cell types. Scientists have learned how to create a few different cell types this way.
embryo: in humans, a fertilized egg up to the age of eight weeks. (After eight weeks, it’s called a fetus.)
pluripotent: able to develop into any type of cell in the body.
induced pluripotent stem cells: cells that have been reprogrammed to behave like embryonic stem cells, by implanting embryonic genes into them.
regenerative medicine: using cell-based therapies to treat disease.
transdifferentiation: the differentiation of adult stem cells found in one type of tissue into cells of a different type. This has been observed in some species, but scientists disagree on whether it happens in humans. (Note: Scientists have recently “reprogrammed” certain types of adult cells into other types of cells through genetic modification.)
Why are they called stem cells?
Think of the stem of a plant: from one stalk, many smaller limbs branch off. Similarly, from one stem cell, many different types of specialized cells develop.
For an overview of ethical concerns on the use of embryonic stem cells, see ReligiousTolerance.org.
Stem Cell Opinion Roundup, Biopolitical Times (website), 3/11/09: opinion and commentary on the change in federal human embryonic stem cell research policy as ordered by President Obama
Two letters to the editor in the New York Times, published 8/26/10, sum up the opposing positions on stem cell research:
Arthur L. Yeager, Edison, N.J.: “By what distorted sort of moral values are thousands of frozen embryos, no longer needed by couples for in vitro fertilization and about to be thrown into the medical waste trash bin, prohibited from being used for vital research to cure diseases like diabetes, cancer, heart disease and Parkinson’s, to find remedies for debilitating injuries and to promote vital organ regeneration?”
Paul Kokoski, Hamilton, Ontario: “The American people should not be forced to pay for experiments that destroy human life. Human life begins at conception with the formation of a genetically complete, self-directing human entity, the embryo. Human beings are not raw materials that can be exploited or commodities that can be bought and sold. We must help those who are suffering, but we may not use a good end to justify an evil means. The respect for every human life is an essential condition of our society. Any method of genetic manipulation that involves the alteration or destruction of human embryos is nothing more than Frankenstein science.”
To learn more about stem cells
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Last updated 11/16/11