Stem cells can divide into more than one form, such as those present in embryos or those in bone marrow. These cells play important roles naturally, but scientists have found applications for them in treatment of some medical conditions. The potential usefulness of stem cells is based on their ability to replace cells elsewhere in the body that have been damaged beyond repair. There is controversy in many countries over the use of embryonic stem cells as therapy for other people, and adult therapeutic stem cells tend not to be as useful as the embryonic cells.
When a fetus grows in the womb, it must build a body from an initial egg fertilized by a sperm. This splits over and over to make more cells, which each develop into a specific cell of the body. For example, a newborn baby has brain cells, skin cells and nerve cells, which were all produced by the initial fertilized egg cell. As development goes on, the baby's cells tend to become specialized in these roles, and lose their original ability to produce daughter cells of a variety of types.
In adults, though, some stem cells survive, in places where new cell production is a necessary part of life. This includes areas like the bone marrow, which make a variety of blood cells and so needs to keep its versatility. Working with adult stem cells, as of 2011, can be less useful than working with embryonic stem cells, as the ability of the cells to divide and specialize is not as productive.
As a relatively new field of science, therapeutic stem cells have potential for many applications. Typically, the therapeutic stem cells are tested in medical conditions that arise because of irreversible damage to the cells that should be working properly in a particular part of the body. An example are the corneal cells of the eye, which when replenished with therapeutic stem cells from an adult eye can regrow the necessary tissue, thus improving an eye condition. With complex techniques, scientists may even be able to produce necessary organs in the laboratory from an initial group of unspecialized stem cells.
Sometimes, treatment for a medical condition may cause certain groups of cells to become weakened. Drugs that are designed to kill cancer cells can also kill useful cells as a side effect, which may increase the chance of further illness. Studies suggest, as of 2011, that therapeutic stem cells may be able to replace the depleted population and prevent complications such as infections from arising.