Stem cells are the cell which are capable to divided and renew and also produce progeny. These can differentiate into a variety of different cells types, e.g., tissues continuously renew themselves throughout the life. The stem cells have two properties which increases their importance. 

(i) They have potential to form more differentiated cells and 

(ii) these are self-renewing because each division of a stem cells creates at least one stem cell. 

Various types of stem cells : On the basis of development potential, these are divided into following levels: 

• Totipotent Cells : which give rise to entire organism. 

• Pluripotent Cells : w hich form totipotent cells and also give rise to most but not all of cell types that are necessary for foetal development. 

• Multipotent Cells : cells which are formed after further differentiation of pluripotent cells. These can give rise to a limited number of cells types. 

• Unipotent Cells : are also formed from further differentiation of multipotent cells. These give rise to a single cell type.