Stem cells are defined as cells that have the ability  to perpetuate themselves through self renewal and to generate mature cells of a particular tissue through differentiation. Stem cells from a variety of organs might have the potential to be used for therapy in the future, but hematopoietic stem cells the vital element in bone marrow transplantation have already been used widely in therapeutics.

Studies in human and rat mammary gland suggest that stem cells are located in specific locations or stem cell niche.