Pluripotential stem cells, present in the early stages of embryo development, can generate all of the cell types in a fetus and in the adult and are capable of self-renewal. A renewable, tissue culture source of human cells capable of differentiating into a wide variety of cell types would have broad applications in basic research and transplantation therapies. A major step in realizing this goal has now been taken with the demonstration that human embryonic stem cells can be grown in culture. These stem cells have been derived in culture from two embryonic tissues: inner cell masses of blastocysts (those cells within the conceptus that form the embryo proper) and primordial germ cells. Embryonic stem (ES) cells were first derived from the inner cell masses of mouse blastocysts in the early 1980s (1, 2). More recently, primordial germ cell cultures were found to give rise to cells with characteristics of ES cells and were designated EG (embryonic germ) to distinguish their tissue of origin (3, 4). ES and EG cells have now been derived from embryos of other mammals, including primates (5–10). Now on page 1145 of this issue, Thomson et al.(11) report the derivation of ES cell lines from human blastocysts.

Pluripotential stem cells, primarily ES cells, have been used extensively in studies of embryogenesis, gene function, and development in the mouse (11). ES and EG cells transferred to a mouse blastocyst can contribute substantially to all differentiated cell types in the fetus, including the germ line. Consequently, gene targeting within ES cells has enabled both whole-animal studies of gene function and the production of mouse models of human genetic diseases and abnormalities. ES and EG cells have also been used to study the differentiation of various cell types and tissues in vitro, such as neural cells (12–16), hematopoietic lineages (17–19), and cardiomyocytes (20). ES-derived cells have been successfully transplanted into fetal and adult mice, where they have demonstrated morphological and functional integration (19–23).