What is Stem Cell ?

                  A Stem cell is a  special kind of cell hat has a unique capacity to renew itself and to give rise to specialized cell types.  Most of the body cells are committed to perform a  specialized function , and a stem cell is uncommitted to a specialized function and remain uncommitted, until it receives a signal to develop into a specialized cell. The proliferative capacity and their ability to make a specialized cells make them unique. Stem cells with this unique property comes from embryo and fetal tissues.

                 There are three types of stem cells- Totipotent, Pluripotent and Multipotent, each representing a different stage in development. Totipotent stem cells form after the division of a fertilized egg first and can  develop into a complete individual. These totipotent cells forms a blastocyst, a mass of cells. The inner layer of blastocyst contains pluripotent stem cells. Pluripotent cells have the potential to develop  more than 200 different known cell types.  Pluripotent stem cells are also called embryonic stem cells. Multipotent stem cells are found in mature tissue and are formed by the body to replace worn out cells in tissues and organs. Blood cells are the example of multipotent cells. These  are also called  somatic or adult stem cells.

                   In 1998 James Thomson at the University of Wisconsin Madison University isolated cells from the inner cell mass of the blastocyst  and developed the first human embryonic stem cells lines. John Gearhart of Johns Hopkins University reported the first derivation of human embryonic germ cells from the fetal tissues Since its discovery evidence has emerged that these stem cells are capable of becoming almost all of the specialized cells of the body and thus have the capacity to replace cells for various type of cells.

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