After scientists remove an inner cell mass (ICM) from a blastocyst (usually one left over from an attempt at in vitro fertilization), the cells are placed on a plate with feeder cells. Scientists have found that cells from mouse skin work particularly well for this. A totipotent stem cell comes from a fertilized egg and can develop into any type of cell in the human body. A pluripotent stem cell is slightly more limited. The ability to divide and produce more cells is key. If all goes well in the process, then in a few days new cells grow and form colonies. These cells are determined to be embryonic stem cells only if they display certain molecular markers agreed upon by scientists and go through several generations of cell divisions.

Stem cells in an adult are present in lesser numbers than they are in embryos, though bone marrow in all people is filled with stem cells. The role of adult stem cells (also called somatic stem cells) is generally believed to be in repair of damaged and injured tissue. As early as the 1960s, Till and McCulloch developed a cell-cloning technique that permitted bone marrow transplants for diseases like leukemia and other blood disorders.

Many scientists feel that embryonic stem cells are a better source for further medical research and application, as these cells have not yet become specialized in any way. (Parents today are now encouraged to have stem cells from their newborn's umbilical cord frozen for possible use later for to-be-discovered medical cures.) There is particular hope that stem cells will be helpful in managing degenerative illnesses such as Parkinson's disease. Neurological diseases like this are so complex that drugs, or even gene therapy, often prove to be inadequate. Stem cells offer hope.

Only 22 stem cell lines have been available for federally funded research in the United States, and during George W. Bush's administration it was mandated that the National Institutes of Health (NIH) should not support work on lines created after August 2001. (Once created a stem cell line can be kept going perpetually through freezing and storage.) One of President Barack Obama's early actions in 2009 has been to loosen these restrictions, though scientists had already begun to create new ways to make stem cell lines.

Because of the Bush administration rulings, scientists had been looking for alternative ways to gain stem cells. In 2005, scientists at Harvard succeeded in turning ordinary skin cells into what appear to be embryonic stem cells--without having to use human eggs or make new human embryos. This was a big step forward in gaining access to stem cells without having to work with embryos, which stir up such controversy. A possible benefit of the technique is that the cells come directly from the patient, so the DNA in the new stem cells is an exact match. This could enhance the chances of a person's body more readily accepting whatever it is the stem cells need to be used to replace.

Embryonic stem cells must be directed to specialize before they are used. In experiments with mice, scientists saw that direct introduction of embryonic stem cells can cause cancer. If stem cells were to be used to produce insulin for diabetics, for example, it would be vital that the stem cells had been fully converted and that there were absolutely no stem cells remaining before they were introduced into a patient. Scientists are just learning what variables and nutrients are necessary to accurately guide stem cells so that they become stable nerve or muscle cells. . .

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