1) Cells from which other types of cells can develop.

2) Relatively undifferentiated cells of the same lineage that retain the ability to divide and cycle throughout postnatal life to provide cells that can become specialized and take the place of those that die or are lost.

3) A stem cell is a cell with the potential to form many of the different cell types found in the body. When stem cells divide, they can form more stem cells or other cells that perform specialized functions. Embryonic stem cells have the potential to form a complete individual, whereas adult stem cells can only form certain types of specialized cells. Stem cells continue to divide as long as the individual remains alive.