July 17, 2006

Key to Stem Cells Lies in Their Power

WASHINGTON (Reuters) - The U.S. Senate will start a debate on Monday on a bill aimed at encouraging stem cell research, and experts agree that stem cells hold the power to transform medicine, allowing doctors to one day regenerate damaged and diseased tissue.

Disagreement lies in which types of stem cells hold the most promise, and whether it is unethical to experiment on the types taken from human embryos.

Terminology is key to the debate about stem cells.

-- Totipotent stem cells are produced in the first few days after sperm meets egg. Each of these very early embryonic cells can give rise to an entire organism, including the placenta.

-- Embryonic stem cells come from a ball of cells known as a blastocyst, which develops five to seven days after conception. These embryonic stem cells are pluripotent -- meaning they can differentiate into all the types of cells that make up an animal, including a human being, but do not form placenta and cannot become a fetus.

-- Fetal stem cells come from early fetuses and appear to be slightly more malleable, than adult stem cells.

-- Adult stem cells are found, with difficulty, throughout the body -- in bone marrow, the brain, the liver, the blood and elsewhere. They are the body's master cells but unless they are incubated with the right compounds, only give rise to certain tissue types.

For instance, bone marrow cells usually only become blood cells such as white blood cells and red blood cells. Mesenchymal stem cells are the source of muscle tissue, and oval cells form liver.

Adult stem cells are difficult to find and to separate from other cells. They are also more difficult to keep alive in the lab, while true embryonic stem cells are virtually immortal and in theory can be directed to form any tissue.

But adult stem cells taken from a patient would be a precise genetic match. Any tissue or blood product made from such cells could be used without having to suppress the immune system, as is done now with organ transplants.

To make precisely matched embryonic stem cells, scientists would have to make a clone from a patient, which is not only controversial but would require the use of human eggs, which are difficult and expensive to obtain.

Eventually, scientists want to learn just what is unique about stem cells, so they could direct any cell to take on such properties.