ABSTRACT
Stem cells are defined as cells that have extensive, some would say indefinite, proliferation potential, differentiate into several cell lineages, and repopulate tissues upon transplantation. Embryonic stem (ES) cells, which are derived from the inner cell mass of a blastocyst, are by all accepted criteria true stem cells capable of generating mature progeny of all cell types. ES cell isolation was initially reported in mouse embryos (1) and more recently in nonhuman primates and humans (2,3). When introduced into mouse blastocysts, ES cells can contribute to all tissues of the mouse (4). Following transplantation in postnatal animals, ES cells generate teratomas (3,5), which again demonstrates their pluripotency, but at the same time raises serious concerns when clinical applications of ES cells are considered. Additional legal, ethical, and moral problems accompany the derivation of ES cells from human blastocysts.
