stem cells, unspecialized human or animal cells that can produce mature specialized body cells and at the same time replicate themselves. Embryonic stem cells are derived from a blastocyst (the blastula typical of placental mammals; see embryo), which is very young embryo that contains 200 to 250 cells and is shaped like a hollow sphere. The stem cells themselves are the cells in the blastocyst that ultimately would develop into a person or animal. "Adult" stem cells are derived from the umbilical cord and placenta or from blood, bone marrow, skin, and other tissues. The similar embryonic germ line cells come from a fetus that is 5 to 9 weeks old and are derived from tissue that would have developed into the ovaries or testes.
Medical researchers are interested in using stem cells to repair or replace damaged body tissues because stem cells are less likely than other foreign cells to be rejected by the immune system when they are implanted in the body. Embryonic stem cells have the capacity to develop into every type of tissue found in an adult; germ line cells and adult stem cells are less versatile. The processes that control such development, however, are not understood at present. Stem cells have been used experimentally to form the hematopoietic (blood-making) cells of the bone marrow; heart, blood vessel, muscle, tracheal, retinal, and insulin-producing tissue; bone; and sperm cells. Embryonic germ line cells have been used to help paralyzed mice regain some of the ability to move. Since the 1990s umbilical cord blood stem cells have sometimes been used to treat heart and other defects in children who have rare metabolic diseases and to treat children with certain anemias and leukemias. It has been shown that stem cells from this blood can migrate to damaged tissues and repair them.
Human stem cells have typically been extracted from surplus fertilized embryos produced during in vitro fertilization procedures. Some experimenters, however, have used embryos that were fertilized especially to produce stem cells. In so-called therapeutic cloning a nucleus from a patient's body cell, such as a skin cell, would be inserted into an egg that has had its nucleus removed to produce a blastocyst whose stem cells could be used to create tissue that would be compatible with that of the patient. Such a procedure was reported in 2005 to have been successfully undertaken in part by South Korean researchers who produced stem cell lines using genetic material from patients, but the data was subsequently shown to have been fabricated. (It was later determined, however, that the laboratory had produced stem cells using an egg that had developed through parthenogenesis, which does not involve fertilization or result in a viable human embryo.) In 2013, however, scientists at Oregon Health and Science Univ. reported that they had created such stem cells using genetic material from human skin cells and donated eggs.
Because extraction of embryonic stem cells destroys the embryo, the use of embryonic stem cells has been opposed by opponents of abortion. Japanese researchers led by Shinya Yamanaka used retroviruses in 2007 to transfer transcription factors to human skin cells and induce those cells to become stem cells (called induced pluripotent stem cells); Yamanaka's team had previously (2006) achieved similar results with mouse cells. In 2010 an American team announced that they had induced human skin cells to become stem cells using messenger RNA to reprogram the cells. Studies with mice have shown, however, that unlike embryonic stem cells induced stem cells are subject to attack by the recipient's immune system. Treatment with stem cells in humans is experimental and can have unexpected and damaging side-effects; some methods of producing mouse stem cells with retroviruses have led to significant rates of cancer when those cells have been transferred to mice.
The first embryonic stem cells to be isolated were extracted by British researchers from mouse blastocysts; the first human stem cells isolated and cultured were extracted by American scientists in 1998. In 1994 a National Institutes of Health (NIH) panel argued that creating human embryos for use in certain experiments might be justified, but Congress subsequently enacted (1995) a ban on federal financing for research involving human embryos in reaction to that report. The Dept. of Health and Human Services ruled in 1999, however, that that ban did not apply to financing work with stem cells, and guidelines for financing such research were issued by NIH the next year.
President George W. Bush, who had campaigned against financing embryonic stem cell research, announced in Aug., 2001, that he would support federal funding of research with embryonic stem cells, but only with the estimated 60 stem cell lines then existing. Some scientists challenged the assumption that these 60 stem cell lines would be sufficient for experimental and therapeutic needs, while others said the figure included some stem cell lines that had not yet been determined to be viable. In fact, in 2004, there were only 15 approved stem cell lines available to researchers funded by the U.S. government. The restrictions did not prevent other researchers, in the United States and elsewhere, from developing new embryonic stem cell lines and undertaking research with them using private funding, and California voted (2004) to create a $3 billion fund to underwrite embryonic stem cell research. A federal legislation that would have expanded the number of stem cell lines available for federally funded research was vetoed by the President Bush in July, 2006. The executive order issued by Bush was overturned in Mar., 2009, by President Barack Obama.
See also fetal tissue implant.