In general, a cell can be defined as a stem cell if two basic criteria are met. First, the stem cell is capable of self-renewal for an indefinite period throughout life while maintaining an undifferentiated state, that is, it can divide and produce two identical daughter cells and thus maintaining the pool of stem cells. Second, stem cells possess the ability to differentiate into specialized, functional progeny under the right conditions or by receiving the right signals. It can divide asymmetrically to yield an identical cell and a daughter cell that acquires the properties of a particular cell type, such as morphology, phenotype and functional physiology that classify it as a particular tissue (Burns and Zon 2002 ; Preston et al. Stem cells can also be classified into a hierarchy based on their differentiation potential (Figure 1). At the very top of the hierarchy is the totipotent stem cell. This cell is capable of giving rise to all kinds of cells which constitute the embryonic and extra-embryonic tissues and then contribute to the whole organism (for example the fertilized egg or the zygote).)...