Most cells in the human body are highly specialized---meaning they perform one particular function---but cannot divide to replace themselves when they die due to normal aging or injury. Instead, a specific population of cells, stem cells, is the source of these replacement cells. Their specialized function is to generate new cells that will become nerves, skin, muscles or other types of tissues. Stem cells also produce more stem cells, maintaining a constant pool of this unique cell population.
Embryonic Stem Cells
Embryonic stem cells (ES cells) are derived from the stages of development immediately following egg fertilization and are the most versatile type of stem cells. At this stage, the embryo consists of relatively few cells (about 100), but these cells will produce the trillions of specialized cells that will make up the adult organism. ES cells first divide to increase the total number of cells present and, as fetal development continues, they start to produce specialized cells that will make up the different tissues of the body. ES cells are called pluripotent and have the potential to generate virtually any cell type. The broad range of possible fates for ES cells makes them a particularly attractive source for future therapeutic applications.
Adult Stem Cells
Adult stem cells are actually present in the body at birth and are found in many types of tissues. Unlike ES cells, adult stem cells are limited to producing cells of a particular class. For instance, the adult stem cells found in bone marrow (hematopoietic stem cells) produce the various blood cell types but cannot make nerve or muscle cells. Adult stem are responsible for growth in size throughout childhood, sexual maturity and the replacement of damaged or worn-out cells. Blood cells are renewed every few months. The lining of the digestive tract and the surface of the skin are continually shedding their outermost layers. Some organs, like the liver, can repair themselves if damaged by injury or disease. During wound healing, specialized skin cells migrate to the site of injury and begin diving to replace the lost layers of skin. All of these processes are carried out by naturally occurring stem cells.
Laboratory-Derived Stem Cells
The other common types of stem cells are produced in the laboratory and are used for research purposes.
Mature, specialized cells can be harvested and, through special treatments, are "reprogrammed" to behave more like ES cells. These are typically called "induced stem cells" because they do not naturally have any of the characteristics that define a stem cell.
Nuclear transfer, or cloning, has also been used to make nonhuman, laboratory-derived stem cells. The nucleus (the part of the cell that contains the DNA) of a mature cell is transferred into an egg that lacks its own nucleus. The egg is then allowed to develop as a normal, fertilized egg and, at the appropriate stage, stem cells displaying the same traits as conventional ES cells are collected.
