Stem cells are unspecialized cells that can perpetually replicate themselves. They can also undergo cell differentiation to become specialized cells of one or more types. Therefore, stem cells are able to both replenish their own population and to generate cells that travel down specific differentiation pathways.
In other words, stem cells can be induced to become tissue- and/or organ-specific cells with special functions. In many tissues, human or animal, stem cells work as a sort of internal repair system, dividing without limit to replenish other cells which in turn can repair damaged or "worn out" tissues. Stem cells can be either embryonic — originating in an embryo — or somatic — originating in an adult.
Embryonic stem cells are pluripotent, meaning they have the ability to differentiate into derivatives of the three primary germ layers: ectoderm, endoderm and mesoderm. These cells can generate all cell types in the body and are also capable of propagating themselves indefinitely. They are isolated from early embryos at the blastula stage — just before they develop into specialized cells. Thus far, the only way to obtain embryonic stem cells has been to harvest them from human embryos, which has raised ethical and political concerns in some cases.
The adult body has somatic stem cells, which serve to replace non-reproducing specialized cells when needed. Unlike their embryonic counterparts, adult stem cells are not able to give rise to all cell types in the organism. They can generate multiple cell types, however.
One of the several types of stem cells in adult bone marrow, for example, can generate all of the different kinds of blood cells. Another type can differentiate into bone, cartilage, fat, muscle, and the linings of blood vessels. The adult brain has also been found to contain stem cells that continue to produce certain kinds of nerve cells. Given the limitations of adult stem cells, embryonic stem cells seem to hold more promise for medical application.
Research with both types of stem cells, embryonic and adult, is a source of valuable data about cell differentiation and has enormous potential for overcoming obstacles in modern science. The main goal of this research is to supply cells for the repair of damaged or diseased organs. For example, stem cells might eventually be used to make insulin-producing pancreatic cells for people with diabetes, or certain types of brain cells for people with Parkinson’s disease or Huntington’s disease.