Stem cell is a cell that has the ability to develop into any of the different cell types that make up the tissues and organs of the body. The original cells from which an entire organism develops are stem cells. These cells are also found in adult organs. Stem cells have the ability to divide endlessly, producing more stem cells or other types of cells.
In 1998, scientists succeeded in isolating and growing stem cells from a human embryo in a laboratory. Such stem cells are called embryonic stem cells. Scientists think that stem cells can be used to replace damaged tissues and treat diseases, such as Parkinson disease and diabetes, in people.
Early in development, a human embryo is made up of a hollow ball of cells called a blastocyst. Blastocyst cells divide and eventually develop into all of the tissues and organs of a human being, a process called differentiation.
Embryonic stem cells can be grown in the laboratory from blastocysts and made to differentiate into nerve, liver, muscle, blood, and other cells. Scientists hope to control the differentiation of the cells to replace cells in diseased or damaged organs in human beings. Learning how to control the differentiation of stem cells will help scientists understand how human tissues and organs develop. It may also lead to new treatments for many diseases, such as cancer. Embryonic stem cells can also be used to test the effects of new drugs without harming animals or people.
In adults, stem cells are found in many places in the body, including the skin, liver, bone marrow, and muscles. In these organs, stem cells remain inactive until they are needed. The stem cells supply each organ with the cells needed to replace damaged or dead cells. Some stem cells in the bone marrow may produce new bone and cartilage cells when needed.
Other bone marrow stem cells divide to produce more stem cells, additional cells called precursor cells, and all of the different cells that make up the blood and immune system. Precursor cells have the ability to form many different types of cells, but they cannot produce more stem cells. Scientists can isolate bone marrow stem cells to use as donor cells in transplants. Adult stem cells, however, are rare and more difficult to detect and isolate. Scientists have also found that damaged stem cells may play a role in the development of certain cancers.
The discovery and isolation of embryonic stem cells has led to debate over whether it is right to use cells taken from human embryos for research. People have expressed concern about using human embryos and collecting some of their cells. Some people consider embryos already to be human beings. The embryos are destroyed in the process of isolating the stem cells. Many people consider it wrong to destroy human embryos, but other people believe that the potential medical benefits of stem cells justify their use.
By 2004, scientists in the United Kingdom and South Korea began producing cloned human embryos to produce stem cells. This process first involves destroying the nucleus of a human egg cell. A nucleus is then removed from another human cell and injected into the egg cell. The egg, with its new nucleus, develops into an embryo with the same genetic makeup as the donor. Some scientists are working to clone human embryos to produce a collection of stem cells that are genetically identical to sick and injured patients. This kind of cloning is known as therapeutic cloning because doctors might be able to use the stem cells to replace damaged human tissues and treat diseases. In 2008, physicians in Spain successfully replaced a woman’s trachea (windpipe) with one that was grown using her own stem cells.
In the United States, the National Institutes of Health (NIH) sets the standards for medical research that can receive federal funding. For several years, the NIH forbade laboratories that received federal funding from isolating human embryonic stem cells. In 2001, President George W. Bush allowed federal funds to support scientific research on existing supplies of stem cells that had been isolated previously in privately funded laboratories. However, in 2006 and 2007, Bush vetoed legislation that would have further eased restrictions on the use of federal funds for embryonic stem cell research.
A law passed by the California Legislature in 2002 allows laboratories that receive funding from that state to isolate human embryonic stem cells. In 2004, California voters approved a plan to provide state funds for stem cell research. Other states, including Massachusetts and New Jersey, have passed similar laws. Studies of new embryonic stem cells remained ineligible for federal funds until 2009, when President Barack Obama lifted all restrictions on federal funding for embryonic stem cell research.
In 2007, Japanese and American scientists announced that they had developed a technique to transform human skin cells into stem cells. The technique is experimental, but scientists believe it will eventually mean they would no longer need to use embryonic stem cells for research.
