In order to better acquaint users of the Fox News Web site with the scientific questions surrounding the controversy over stem-cell research, the editors and research department of the Fox News Channel have put together the following primer.

What Are Stem Cells?

Stem cells are the fundamental building blocks for all the body's tissues.  They have the ability to develop not only into brain, bone and muscle cells, but into the cells of all other organs as well. 

About 30 hours after a woman's egg, or ovum, is fertilized by a man's sperm, the newly formed zygote begins to divide. During these initial cell divisions, all of the developing embryo's cells are structurally similar. These are called totipotent stem cells, because of their ability to form all the cells necessary to become a human being.

After one week, the embryo contains more than one hundred cells clustered in a hollow, ball-like structure called a blastocyst. By this time, the blastocyst's cells begin to differentiate and take on different roles. The outer cells, called the trophoblast, will develop into the placenta, the embryo's life-support system.

The other cells, called the inner cell mass, are the embryonic stem cells used in research. Scientists call these cells pluripotent, because they can give rise to many, but not all, types of cells — they cannot form the placenta.

If the blastocyst successfully implants into a woman's uterus, those pluripotent stem cells will form the embryo and eventually differentiate into all of the body's specialized tissues, such as blood, hair and skin. But, when isolated in the laboratory, pluripotent stem cells divide indefinitely — making them an almost inexhaustible resource.

In the lab, scientists add and remove certain proteins, which may help direct the stem cells to develop into new heart, bone or muscle cells.

Harvesting Stem Cells

The blastocyst is opened using precision tools and the stem cells are removed.  The cells multiply after being mixed with proteins and enzymes.  From here, scientists are learning how to direct the cells' development into specialized cells.

Scientists obtain the stem cells in three ways. 

Discarded Embryos From Fertility Clinics

Scientists can purchase unused embryos from fertility clinics. These embryos were not created for the sole purpose of research.  Not all embryos created for fertility treatments will be used, so they remain frozen indefinitely or are discarded.

Contrived Conception 

The process of contrived conception, where sperm and eggs are purchased from consenting donors and joined to create embryos, is another potential source of stem cells.  These embryos are created for the sole purpose of harvesting their stem cells.

Coupling and Copying

Under a third way of stem cell generation, called coupling and copying, donor eggs and cells could be combined using experimental cloning techniques.  The egg is then stripped of its nucleus, so it can be replaced.  Cells from a donor — perhaps from the person who will receive the stem cells — are fused with the empty egg using an electrical current.  Reprogrammed with the donor's genetic material, the egg is prompted into developing, yielding stem cells.

Embryonic Versus Adult Stem Cells

Embryos are not the only source of stem cells — they can be harvested from adults, as well.  However, there is much scientific debate over which should be used in research.  Some researchers feel embryonic stem cells are best because they are most versatile and they grow vigorously, forming colonies that would expand forever. They say adult stem cells are less promising because they are committed to generating specific tissue types.

But research has shown that adult stem cells are also able to form many different types of cells.  Some experiments on mice have shown that brain tissue can be coaxed into growing muscle. Still, adult stem cells do not survive in the lab as long as embryonic cells.

However, the embryo is killed when the stem cells are removed, and this is not the case with the removal of adult stem cells.