Stem cells differ from other kinds of cells in the body. All stem cells - regardless of their source - have three general properties:

1)  They are capable of dividing and renewing themselves for long periods.

2)  They are unspecialized.

3)  They can give rise to specialized cell types. 

Stem cells are unspecialized.  One of the fundamental properties of a stem cell is that it does not have any tissue-specific structures that allow it to perform specialized functions.  A stem cell cannot work with its neighbors to pump blood through the body (like a heart muscle cell); it cannot carry molecules of oxygen through the bloodstream (like a red blood cell); and it cannot fire electrochemical signals to other cells that allow the body to move or speak (like a nerve cell).  However, unspecialized stem cells can give rise to specialized cells, including heart muscle cells, blood cells, or nerve cells.

Stem cells are capable of dividing and renewing themselves for long periods. Unlike muscle cells, blood cells, or nerve cells - which do not normally replicate themselves - stem cells may replicate many times.  When cells replicate themselves many times over it is called proliferation.  A starting population of stem cells that proliferates for many months in the laboratory can yield millions of cells.  If the resulting cells continue to be unspecialized, like the parent stem cells, the cells are said to be capable of long-term self-renewal.   The specific factors and conditions that allow stem cells to remain unspecialized are of great interest to scientists. It has taken scientists many years of trial and error to learn to grow stem cells in the laboratory without them spontaneously differentiating into specific cell types. For example, it took 20 years to learn how to grow human embryonic stem cells in the laboratory following the development of conditions for growing mouse stem cells.  Therefore, an important area of research is understanding the signals in a mature organism that cause a stem cell population to proliferate and remain unspecialized until the cells are needed for repair of a specific tissue.  Such information is critical for scientists to be able to grow large numbers of unspecialized stem cells in the laboratory for further experimentation. 

Stem cells can give rise to specialized cells. When unspecialized stem cells give rise to specialized cells, the process is called differentiation.  Scientists are just beginning to understand the signals inside and outside cells that trigger stem cell differentiation.  The internal signals are controlled by a cell's genes, which are interspersed across long strands of DNA, and carry coded instructions for all the structures and functions of a cell.  The external signals for cell differentiation include chemicals secreted by other cells, physical contact with neighboring cells, and certain molecules in the microenvironment. 

Stem cells can give rise to specialized cells.  When unspecialized stem cells give rise to specialized cells, the process is called differentiation.  Scientists are just beginning to understand the signals inside and outside cells that trigger stem cell differentiation.  The internal signals are controlled by a cell's genes, which are interspersed across long strands of DNA, and carry coded instructions for all the structures and functions of a cell.  The external signals for cell differentiation include chemicals secreted by other cells, physical contact with neighboring cells, and certain molecules in the microenvironment.