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The Stem Cell Controversy
Stem cells constitute one of the most fascinating -- and controversial -- areas of biology today. Researchers, who are still learning how the body uses these cells to restore or regenerate tissue, hope to harness the power of stem cells and make them a human "repair kit." But there is also a deep division over how safe -- or how ethical -- using these cells will be. While stem cells could revolutionize medicine, they also raise profound ethical questions about what steps should be taken to restore health or save lives.
What Are Stem Cells?
Stem cells, the building blocks of the body, have two essential characteristics that make them unique. They are unspecialized cells that can renew themselves for long periods through cell division. In addition, under the right conditions, they can develop, or "differentiate" to become cells with more specialized functions. Although there are three major types of stem cells -- embryonic stem cells, embryonic "fetal" germ cells, and adult stem cells -- scientists mainly work with adult and embryonic stem cells from animals and humans. (Embryonic germ cells are found in a specific part of the embryo/fetus called the gonadal ridge; they normally develop into mature gametes -- eggs and sperm.) The labels "adult" and "embryonic" -- relate to the stem cells' place of origin.
Adult Stem Cells
Adult stem cells are unspecialized, undifferentiated cells that exist in very small numbers among specialized cells in an adult organ or tissue. Their main function is to maintain and periodically repair the tissues in which they are found. Adult stem cells are rare and hard to detect, but so far researchers have found them in a number of places, including the brain, the bone marrow, peripheral blood, blood vessels, skeletal muscle, skin, and liver. One adult stem cell, the hematopoietic stem cell, has been used for decades to treat diseases, including leukemia, lymphoma, and inherited blood disorders, and to replace cells destroyed by cancer chemotherapy. Researchers are investigating whether it is possible to expand adult stem cells, increasing their numbers in either a cell culture or within the body so that more diseases might be treated.
Researchers long assumed that adult stem cells could yield only those specialized cells of the tissue or organ where they were located -- for example a skin stem cell could only give rise to a skin cell. But the last few years has brought provocative evidence that some adult stem cells may have "plasticity" and may be able to transform into different cell types, a process known as transdifferentiation.
Embryonic Stem Cells
Embryonic stem cells, as their name implies, are taken from embryos. These cells are "pluripotent," or capable of differentiating into any cell type derived from the three embryonic germ layers (the three initial tissue layers arising in an embryo) -- mesoderm, endoderm, and ectoderm. Under the right conditions, human embryonic stem cells will proliferate indefinitely without specializing or differentiating into specific cell types, to form an embryonic stem cell line. Embryonic stem cells were first isolated in mice more than 20 years ago, but it was not until 1998 that James Thomson of the University of Wisconsin managed to derive and grow the first stable line of human cells.
The embryos from which these stem cells are derived form under two different conditions. Until very recently, these cells were always taken from surplus eggs left over from in vitro fertilization procedures and contributed with the informed consent of the donors. In vitro fertilization is a technique used to assist women who are experiencing difficulty in conceiving a child. A woman's eggs are removed from her ovary and then fertilized in a laboratory culture dish. The fertilized eggs are maintained in a laboratory dish and allowed to develop into pre-implantation embryos, or pre-embryos. A little over two days later, they are delivered to a healthy uterus to continue development.
Many times, surplus embryos remain after the in vitro procedure. These can be donated to other couples or discarded, but they can also be frozen and stored for future IVF attempts or donated for research. The best estimate, by the RAND Institute in May 2003, is that nearly 400,000 embryos are frozen and stored in the U.S. alone, with about 11,000 such embryos designated for research.
In February 2004, scientists in South Korea became the first to derive human stem cells from embryos using a second method called somatic cell nuclear transfer (SCNT) -- a procedure often referred to as therapeutic cloning. [See the Stem Cell Challenge for a detailed description of both the SCNT and IVF methods of obtaining stem cells.]
Whether the embryonic stem cells are isolated from surplus IVF embryos or through SCNT, the embryo is destroyed in the process. This fact is the root of the controversy over using these cells in research.
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Both adult and embryonic stem cells offer the potential for therapeutic treatments of various medical conditions, from Parkinson's disease to spinal chord injuries. Much will depend on future advances in stem cell research. Yet, over the last fifty years, the scientific community has already learned a great deal. Read on to see how the promise of stem cells has unfolded.
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