What are stem cells?<br /><br />Stem cells are a special type of cell with two important characteristics. First, they are capable of producing more copies of themselves, meaning they are self-renewing. Second, they can transform into other types of cells that perform different functions, through a process called differentiation. Stem cells are found in almost all tissues in the body. The body needs them to maintain and repair tissues after injury. Stem cells can develop into different tissues, depending on their location. For example, hematopoietic stem cells are found in the bone marrow and can produce all the active cells in the blood. Stem cells can also develop into brain cells, heart muscle cells, bone cells, or other cell types.<br /><br />Stem cells come in many different types. The type most capable of transforming into cells with diverse functions is embryonic stem cells, which can develop into all cell types that occur in the fetus. Most stem cells in the body have a limited ability to form new cells, and their only benefit is maintaining and repairing the tissues and organs in which they reside.<br /><br />No other cells in the body have this natural ability to form new cell types.