Stem cells are primal cells found in all multicellular organisms that retain the ability to renew themselves through mitotic cell division and can differentiate into a wide range of specialized cell types (Becker et al., 1963 and Siminiovitch et al., 1963).

The rigorouis definition of a stem cell requires that it possesses two properties:
• Self-renewal: the ability to go through numerous cycles of cell division while maintaining the undifferentiated state.
• Unlimited potency: the capacity to differentiate into any mature cell type. In a strict sense, this makes stem cells either totipotent or pluripotent, although some multipotent and/or unipotent progenitor cells are sometimes referred to as stem cells.

Potency specifies the differentiation potential (the potential to differentiate into different cell types) of the stem cell.
• Totipotent stem cells are produced from the fusion of an egg and sperm cell. Cells produced by the first few divisions of the fertilized egg are also totipotent. These cells can differentiate into embryonic and extraembryonic cell types.
• Pluripotent stem cells are the descendants of totipotent cells and can differentiate into cells derived from the three germ layers.
• Multipotent stem cells can produce only cells of a closely related family of cells (e.g. hematopoietic stem cells differentiate into red blood cells, white blood cells, platelets, etc.).