Stem cells obtained from human embryos seem to offer the best chance of new therapies, because unlike other stem cells they have the ability to morph into almost any type of tissue. But researchers complain that political roadblocks are keeping them from determining the full potential of these cells. Six years ago, President Bush limited federally funded research to about 20 viable lines of cells that had been extracted from embryos prior to August 9, The stem cell community has repeatedly called for the restrictions to be lifted, charging that the designated cell colonies have been compromised or corrupted and that failure to ease the ban is hobbling U. But opponents say such research is immoral, because embryos must be destroyed to obtain their cells. Last week, Congress sent legislation to Bush that would allow federally funded scientists to study cells from frozen embryos that fertility clinic patients no longer need and have chosen to donate instead of discard.

Jump to navigation. For many years, researchers have been seeking to understand the body's ability to repair and replace the cells and tissues of some organs, but not others. After years of work pursuing the how and why of seemingly indiscriminant cell repair mechanisms, scientists have now focused their attention on adult stem cells. It has long been known that stem cells are capable of renewing themselves and that they can generate multiple cell types.

Adult stem cells, also called somatic stem cells, are undifferentiated cells that are found in many different tissues throughout the body of nearly all organisms, including humans. Unlike embryonic stem cells, which can become any cell in the body called pluripotent , adult stem cells, which have been found in a wide range of tissues including skin, heart, brain, liver, and bone marrow are usually restricted to become any type of cell in the tissue or organ that they reside called multipotent. These adult stem cells, which exist in the tissue for decades, serve to replace cells that are lost in the tissue as needed, such as the growth of new skin every day in humans. Scientists discovered adult stem cells in bone marrow more than 50 years ago.