Stem cells have two important characteristics that distinguish them from other types of cells.  First, they are unspecialized cells that renew themselves for long periods through cell division.  The second is that under certain physiologic or experimental conditions, they can be induced to become cells with special functions such as the beating cells of the heart muscle or the insulin producing cells of the pancreas.

Stem cells are important for living organisms for many reasons.  In the 3- to 5-day-old embryo, stem cells in developing tissues give rise to the multiple specialized cell types that make up the heart, lung, skin, and other tissues.  In some adult tissues, such as bone marrow, muscle, and brain, discrete populations of adult stem cells generate replacements for cells that are lost through normal wear and tear, injury, or disease.

Scientists want to study stem cells in the laboratory so they can learn about their essential properties and what makes them different from specialized cell types.  As scientists learn more about stem cells, it may become possible to use the cells not just in cell-based therapies, but also for screening new drugs and toxins and understanding birth defects.  It has been hypothesized by scientists that stem cells may, at some point in the near future, become the basis for treating diseases such as Parkinson's disease, diabetes, and even heart disease.