What Are Embryonic Stem Cells?
Any embryonic cell at the stage of 2 or 4 cells is capable in itself to reform an embryo: it is a totipotent cell. But then, this ability disappears.
Stem cells collected at a later stage (morula or blastula) may, in turn, differentiate into any cell in any organ: they are pluripotent. These are the so-called embryonic stem cells.
Adult cells (found in some organs), are, multipotent because they can not differentiate in some cell types. Both embryonic and adult cells represent a great hope for organ repair, but the techniques of differentiation are nevertheless poorly understood.
Both systems have possible advantages and definite shortcomings – for example, the tumorigenic possible of embryonic stem cells, or the difficulty to acquire stem cells in adult organs and their culture in vitro.
Furthermore, the use of differentiated cells obtained from a supernumerary embryo (i.e., an embryo developed during in vitro fertilization in the context of medically assisted procreation) would raise the same risk of rejection as in the traditional transplants.
Therapeutic cloning, in which a cell nucleus (or DNA) of the patient himself would be used to establish the source of embryonic stem cells, would circumvent this problem. It is though banned currently in many countries for ethical reasons.
Barack Obama promised to lift restrictions imposed by the Bush administration to research on embryonic cells. A first step has been done. Two days after his inauguration as President of the United States, the American Drug Agency, the FDA approves the first clinical trial in the world, of therapy derived from embryonic stem cells.