Stem cells are special types of cells which have the characteristic ability to renew themselves through mitotic cell division [ A kind of cell division in which two similar cells are produced from one cell] and differentiating into a diverse range of specialized cell types.

Stem cells are present in most multi-cellular organisms.

The two broad types of mammalian stem cells are

• Embryonic stem cells
  These cells are isolated from the inner cell mass of blastocysts in embryo.In a developing embryo, stem cells can differentiate into all of the specialized embryonic tissues.
• Adult stem cells
  These are found in adult tissues and participate in  repair system for the body, replenish specialized cells and maintain the normal turnover of regenerative organs, such as blood, skin or intestinal tissues.

Stem cells can now be grown and transformed into specialized cells with characteristics consistent with cells of various tissues such as muscles or nerves through cell culture.

Highly plastic adult stem cells from a variety of sources, including umbilical cord blood and bone marrow, are routinely used in medical therapies. Embryonic cell lines and autologous embryonic stem cells generated through therapeutic cloning are promising candidates for future therapies.