|Stem cells are those cells that can develop into many different types of cells in the body during early life and growth. In some cases, in many tissues they act as internal repair system, as they divide essentially and continuously without limit to replace other cells as long as the person or animal is still alive. Whenever a stem cell divides, the new cell formed can remain a stem cell or become another type of cell having some specialized function, like a brain cell, a muscle cell or a red blood cell.
Stem cells can be distinguished from other types of by two important characteristics. First, all stem cells are unspecialized cells which are capable of renewing themselves by regular cell division, sometimes after long periods of inactivity. Second, under certain conditions like physiologic or experimental, they can be made to grow as tissue- or organ-specific cells having special functions. In some organs, like the gut and bone marrow, stem cells continuously divide in an orderly manner to repair and replace worn out or damaged tissues. But in other organs, like in the pancreas and the heart, stem cells divide only under special conditions. Once stem cells divide and propagate in an orderly and controlled culture, that collection of healthy, dividing, and undifferentiated cells is called a stem cell line. These stem cell lines formed can be subsequently managed and shared among researchers. And when under the control, the stem cells can be made to grow as specialized cell as needed by the researcher and this process is called directed differentiation.