The use of stem cells in the clinical arena has gathered
tremendous momentum over the last decade, advanced by varying
levels of success in clinical trials and by the advancement in
our understanding of the mechanisms by which stem cells exert
their seemingly favorable effects. Broadly speaking, stem cells
can be characterized as either embryonic or adult stem cells. In
theory, embryonic stem cells (ESs) appear to be the most
versatile stem cell type for application in regenerative
medicine. In the hierarchy of ESs, cells taken from the
fertilized oocyte are called totipotent ]. These totipotent
cells are then able to specialize, forming the blastocyst from
which the embryo will develop. ESs from within this blastocyst
are called pluripotent as these cells go on to specialize to
form all three of the germ layers (Fig. ). Fully developed adult
tissues and organs contain niches of multipotent adult stem
cells. Originally these multipotent adult stem cells were
described as being able to differentiate into varyi