STEM CELL, in developmental biology, an unspecialized cell that is capable of reproducing itself for long periods and of differentiating, through the activation or inactivation of various genes, into specialized cells making up the organs and tissues of the body. At the beginning of the 21st century, many scientists believed that research on human stem cells offered extraordinary potential for advances in biological knowledge and in the treatment of a variety of diseases.

Pluripotent and Multipotent.
Stem cells are generally “pluripotent” or “multipotent.” Pluripotent cells can develop into almost every type of cell in the body but cannot produce a functioning organism. Multipotent cells can produce only a limited number of cell types. By contrast, a cell that can give rise to every cell type and to a functioning organism is called totipotent; examples include the fertilized egg and the two identical cells into which it divides in the initial hours after fertilization.

Cells that can renew themselves but give rise to just a single mature cell type are also sometimes referred to as stem cells; they are said to be “unipotent.”

     A distinction is made between embryonic and adult stem cells. Embryonic stem cells are derived from the so-called inner cell mass found in embryos a few days old and are cultured in the laboratory. So-called embryonic germ cells, which are derived from primordial reproductive cells (progenitors of egg and sperm cells) in a five- to ten-week-old fetus, are similar in some regards to embryonic stem cells and are also under intensive study. Both embryonic stem cells and embryonic germ cells are pluripotent.

Adult stem cells, obtained from developed tissue (regardless of the age of the organism), appear by and large to be multipotent or unipotent. They give rise to specialized cells needed by the body for tissue regeneration after the organism develops beyond the embryonic stage, and they generally produce cell types found in the tissue in which they reside.
Among the better studied examples in humans are hematopoietic (“blood-forming”) stem cells and mesenchymal stem cells, both of which can be found in bone marrow. “Peripheral,” or circulating, blood and blood remaining in the umbilical cord and placenta after birth are also notable sources of hematopoietic stem cells. Hematopoietic stem cells can give rise to blood and immune cells. Mesenchymal stem cells can produce such tissue types as bone, cartilage, muscle, and fat.

Evidence suggests that at least some adult stem cells may possess a degree of plasticity; that is, it may be possible under certain conditions to genetically reprogram them in the laboratory to produce specialized cells other than those found in the tissue in which the stem cells originated. Adult stem cells, however, occur relatively rarely in the body and are less readily grown in quantity in laboratory cultures than are embryonic stem cells.

Stem cells have been extensively studied in mice. Scientists learned in the 1960s that certain mouse cells could develop into multiple tissue types, and stem cells were identified in mice in 1971. Ten years later, researchers reported success in growing mouse embryonic stem cells in the laboratory.
Human adult stem cells also came under study by researchers, and were introduced into therapy: transplants of hematopoietic stem cells are performed for several disorders—for example, they may be used to help cancer patients recover from high-dose chemotherapy or radiation therapy.
 In 1998, scientists reported that pluripotent human cells—embryonic stem cells and embryonic germ cells—had been isolated and grown in laboratory cultures. A number of self-renewing colonies, or “lines,” of such pluripotent cells have been developed in the laboratory. The possibility of an additional source of pluripotent cells seemed to be opened up when researchers reported apparent success in “reprogramming” ordinary adult body cells, such as skin cells, to have pluripotent characteristics. Such “induced pluripotent cells” were produced using mice cells in 2006 and human cells the following year.

Preliminary study of the developmental flexibility of stem cells, especially embryonic stem cells, raised hopes that they would find numerous applications in scientific research and medical treatment. It seemed possible that they could be used to study chromosome abnormalities in early stages of development; to provide tissues for testing experimental drugs; to carry genes to specific body tissues; or to create replacements for faulty organs and tissues in such conditions as spinal cord injury, multiple sclerosis, PARKINSON"S disease, ALZHEIMER"S disease, type 1 diabetes, chronic heart disease, end-stage kidney disease, liver failure, and cancer.

Such practical medical applications of stem cells, may still be years away. Considerable basic research on the process of cell differentiation and specialization remains to be done, and serious concerns must be addressed, including the possibility that a cell transplant might carry a virus or provoke an immunological reaction in the receiving body. Nonetheless, researchers have reported intriguing, and promising, findings. In late 2007, for instance, it was announced that induced pluripotent stem cells had been used in devising a therapy for a form of sickle-cell disease in mice.

Basic research on human stem cells and exploration of their possible applications require access to an ample supply of the cells from a variety of genetically diverse viable cell lines. Many researchers believe pluripotent stem cells to be potentially the most promising for medical applications, but traditionally such cells have been obtained (at least originally) from an embryo or fetus in a way that entails the embryo or fetus’s destruction. This fact touches on ethical issues concerning abortion. For people who believe human personhood begins at conception, the destruction of an embryo or fetus involves the taking of a human life and is therefore presumptively wrong. But many argue that the embryo should not be regarded as a human life or, even so, that its destruction is justified by a greater good—potentially life-saving medical advances—that may result. It may be possible to sidestep this disputed issue if researchers’ efforts to find alternative sources of fully functional pluripotent cells yield practical fruits. One possibility might be the use of induced pluripotent stem cells. Another might be the development of nondestructive techniques for extraction of stem cells from embryos; a report claiming experimental success in this approach appeared in early 2008.

Additional issues also figure in the stem cell debate. For example, many people are opposed to the cloning of humans, a technology that conceivably could be used to produce embryos to supply stem cells. Another question is whether at least the extra embryos often created in the course of in vitro fertilization (IVF) procedures may ethically be used as a source of stem cells; such embryos are generally either discarded or placed in frozen storage. A further issue is the morality of using IVF specifically for the purpose of producing embryos for stem cell research rather than for creating a new human life.

Seeking to allay ethical concerns while still allowing some federal funding of research potentially valuable for human life and health, U.S. President GEORGE W. Bush in 2001 decided to support federal funding of embryonic stem cell research only for projects using stem cell lines that already existed at that time. For these cell lines, said Bush, “the life and death decision has already been made.” Bush determined that for such cell lines to be used, they had to meet certain other criteria as well: they were derived from embryos that had been created for reproductive purposes and were no longer needed; informed consent was obtained for the donation of the embryo; and the donation did not involve financial inducements. The Bush administration policy did not, however, bar or restrict private organizations or nonfederal government bodies from using their own funds to use stem cells for research. In November 2004, California voters approved Proposition 71, providing $300 million in state funds annually over a 10-year period to fund embryonic stem cell research in that state. Several other states have also authorized funding for stem cell research. New York, for example, in 2007 established a special revenue fund, called the Empire State Stem Cell Trust, to distribute grants, earmarking $100 million for the first fiscal year and $50 million for each year of the following decade.

At the time Bush made his determination, most countries had few or no restrictions on scientific study of human embryos. In January 2001, Britain explicitly legalized stem cell research on human embryos and also authorized the use of cloning to create embryos for such research.

In July 2006, Congress passed, but President Bush vetoed, a measure that would have permitted federally funded research on stem cell lines from surplus embryos that fertility clinics planned to discard. The veto, the first of his presidency, was sustained in the House of Representatives.