Embryonic stem cells. Embryonic stem cells, which come from the inner cell mass of a human embryo, have the potential to develop into all or nearly all of the tissues in the body. The scientific term for this characteristic is "pluripotentiality."
Adult stem cells. Adult stem cells are unspecialized, can renew themselves, and can become specialized to yield all of the cell types of the tissue from which they originate. Although scientists believe that some adult stem cells from one tissue can develop into cells of another tissue, no adult stem cell has been shown in culture to be pluripotent.
The potential of embryonic stem cell research. Many scientists believe that embryonic stem cell research may eventually lead to therapies that could be used to treat diseases that afflict approximately 128 million Americans. Treatments may include replacing destroyed dopamine-secreting neurons in a Parkinson's patient's brain; transplanting insulin-producing pancreatic beta cells in diabetic patients; and infusing cardiac muscle cells in a heart damaged by myocardial infarction. Embryonic stem cells may also be used to understand basic biology and to evaluate the safety and efficacy of new medicines.
The creation of embryonic stem cells. To create embryonic stem cells for research, a "stem cell line" must be created from the inner cell mass of a week-old embryo. If they are cultured properly, embryonic stem cells can grow and divide indefinitely. A stem cell line is a mass of cells descended from the original, sharing its genetic characteristics. Batches of cells can then be separated from the cell line and distributed to researchers.
The origin of embryonic stem cells. Embryonic stem cells are derived from excess embryos created in the course of infertility treatment. As a result of standard in vitro fertilization practices, many excess human embryos are created. Participants in IVF treatment must ultimately decide the disposition of these excess embryos, and many individuals have donated their excess embryos for research purposes.
Fact Sheet: Embryonic Stem Cell Research http://www.whitehouse.gov/news/releases/2001/08/20010809-1.html
NIH Human Embryonic Stem Cell Registry
Stem Cells at the National Academies