What are stem cells?
Just as the stem of a plant gives rise to other structures, stem cells give rise to all the other cells in the body. Stem cells are undifferentiated, or unspecialized, cells that are able to renew and regenerate themselves through the process of cell division. In addition, if conditions are right, they can differentiate and mature into the specialized cells that make up all of the body’s tissues and organs.
There are two main types of stem cells. Embryonic stem cells have the greatest potential to differentiate into specialized cells. Indeed, just a few stem cells present at the very beginning of embryonic life eventually differentiate into cells as different as beating heart muscle cells, nerve cells that generate and transmit vital signals, and pancreas cells that produce insulin. Embryonic stem cells give rise to each and every one of the body’s 10 trillion cells.
Adult stem cells play a different role. It now appears that all adult organs contain a small number of stem cells that reside in a special protected region, or niche. Because most cells in a tissue or organ have a limited life span, stem cells function as a reserve of cells that can be moved out of their niche, begin dividing, and differentiate into specialized organ cells to replace dying or injured cells. But unlike embryonic stem cells, which have the potential to morph into any cell type, adult stem cells ordinarily develop only into the same cells as their parent organs. For example, adult stem cells from the liver become normal, functioning liver cells but not brain cells.