Stem cells are undifferentiated cells that have the potential to become one of 200 types of cells in the body, including blood, nerve, muscle, heart, glandular, and skin cells.

Some stem cells can be triggered to become any kind of cell in the body. Others are already partially differentiated and can only become, for example, some kinds of nerve cell.

Stem cells divide, producing more stem cells, until they are triggered to specialize. Then as they continue to divide, they become more and more specialized until they lose the ability to be anything but one kind of cell.

Researchers hope to use stem cells to repair or replace cells or tissues damaged or destroyed by such disorders as Parkinson disease, diabetes, and spinal injuries. By triggering certain genes, researchers may be able to cause the stem cells to specialize and become the cells that need to be replaced.

Researchers are so far able to obtain stem cells from the following sources:

• Embryos

• Fetuses

• Umbilical cord blood

• Bone marrow of children or adults

• Induced pluripotent stem cells (certain cells from adults that can be altered to act like stem cells)

Embryos: During in vitro fertilization, sperm from the man and several eggs from the woman are placed in a culture dish. The sperm fertilizes the egg and the resulting cell divides, forming an embryo. Several of the healthiest-looking embryos are placed in the woman’s uterus. The rest are discarded or frozen to be used later if needed.

Stem cells can be obtained from the embryos that are not used. Because the embryos then lose the ability to grow into a complete human being, the use of stem cells from embryos is controversial. But researchers think that these stem cells have the most potential for producing different kinds of cells and for surviving after transplantation.

Fetuses: After 8 weeks of development, an embryo is called a fetus. Stem cells can be obtained from fetuses that have been miscarried or aborted.

Umbilical cord: Stem cells can be obtained from the blood in the umbilical cord or placenta after a baby is born. These stem cells can produce different types of blood cells.

Children and adults: The bone marrow and blood of children and adults contain stem cells. These stem cells can produce only blood cells. These stem cells are most often used for transplantation.

Induced pluripotent stem cells: Scientists are developing ways of enabling (inducing) other cells (such as a blood or skin cell) to act as stem cells. These cells are taken from adults. One way to induce these cells is to inject them with material that affects their genes, a process called reprogramming.

Development and use of induced stem cells is still considered experimental.