The types of cells that a stem can become during differentiation determine its classification. One classification of stem cells is pluripotent; these cells are capable of becoming virtually any type of tissue, but cannot develop into a living organism (NIH, Backgrounder). With just a basic understanding of stem cells, it is easy to see the potential benefit of a cell that regenerates itself and has the possibility of becoming any of the cells in the human body. .
One source of stem cells is a human embryo; these stem cells provide the greatest promise for medical advancement. An ES cell is derived from a group of cells called the inner cell mass, which is part of a four to five day old embryo called the blastocyst (NIH, Basics III). The embryos are derived from eggs that have been fertilized in-vitro (NIH, Basics III). ES and some fetal cells are currently the only stem cells that are pluripotent, which means they are the only stem cells that can become any of the cells in a living organism (NIH, Progress ES-2). ES cells also have the highest vitality for reproducing themselves (NIH, Progress ES-4). Given the self-renewing ability and unlimited pluripotency of ES cells, without the difficulty of receiving federal funding, they would be an obvious choice for stem cell research.
The opponents of ES cell research site inferior stem cells derived from adult sources as an alternative source for stem cell research (Do No Harm). Adult stem cells are unspecialized cells that reside in tissue made up of specialized cells and through differentiation are capable of becoming all of the cells of the tissue from which they were removed (NIH, Progress, ES-2). An example of an adult stem cell would be the "hematopoietic (blood-forming) stem cells isolated from bone marrow and blood" (NIH, Progress ES-3). While recent research has found some adult stem cells that have limited ability to form cells other than the differentiated cells that they were removed from, none have been isolated that are pluripotent like ES cells (NIH, Backgrounder).
