Environmental lawyer Sanders offers a stem cell primer.

What are Stem Cells?
Stem cells are cells that have the remarkable potential to develop into many different cell types in the body. Serving as a sort of repair system for the body, they can theoretically divide without limit to replenish other cells for as long as the person or animal is still alive. When a stem cell divides, each "daughter" cell has the potential to either remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell.

What classes of stem cells are there?
There are three classes of stem cells: totipotent, multipotent, and pluripotent.
1. A fertilized egg is considered totipotent, meaning that its potential is total; it gives rise to all the different types of cells in the body.
2. Stem cells that can give rise to a small number of different cell types are generally called multipotent.
3. Pluripotent stem cells can give rise to any type of cell in the body except those needed to develop a fetus.

Where do stem cells come from?
Pluripotent stem cells are isolated from human embryos that are a few days old. Cells from these embryos can be used to create pluripotent stem cell "lines" —cell cultures that can be grown indefinitely in the laboratory. Pluripotent stem cell lines have also been developed from fetal tissue obtained from fetal tissue (older than 8 weeks of development).