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Stem Cell Research
The Science Behind the Controversy

Three main types of stem cells form at different times during human development:

Totipotent stem cells are formed at conception and exist until the embryo is four days old. These cells can develop into all of the cells of the body.

Pluripotent stem cells (also known as embryonic stem cells) begin forming when the embryo is four days old and exist until the embryo is eight months old. These stem cells can develop into most of the cells in the body. It is these cells that have caused the most debate.

Multipotent stem cells begin forming when the embryo is eight months old and exist throughout a person's lifetime. While these cells can develop into many of the cells in the body, they have significant limitations that pluripotent stem cells do not.

diagram of stem cell development
Stem cells can be derived from either an embryo or a fetus.
Graphic courtesy of the National Institutes of Health

Scientists have successfully isolated pluripotent stem cells, or human embryonic stem cells, in two ways:

1. Scientists remove the inner layer of cells from a blastocyst, an early stage embryo. These cells are pluripotent and can be grown in a lab to become most of the cells in the body.

2. Specific cells destined to become egg and sperm cells are removed from the developing fetus. These cells are also pluripotent and can be grown in a lab to become most of the cells in the body.

For a complete primer on stem cell research, visit the National Institutes of Health stem cell Web site.