Embryonic stem (ES) cells are continuously growing stem cell lines of embryonic origin first isolated from the inner cell mass of developing mouse blastocysts. More recently, it has been shown that embryonic germ (EG) cell lines, established from primitive reproductive cells of the fetus, are functionally equivalent to ES cells. The distinguishing features of ES cells are their capacity to be maintained in an undifferentiated state indefinitely in culture and their potential to develop into every cell of the body. The most rigorous test of the developmental potential of mouse ES cells is their ability to contribute to all cell lineages-including the germ-line-of chimeric animals. In addition, under appropriate culture conditions, ES cells differentiate into a broad spectrum of cell types and when injected into immunocompromised animals, they form teratomas composed of multiple lineages. It is this ability to develop into a wide range of cell types that has drawn so much attention to ES cells as a basic research tool and as a novel source of cell populations for new clinical therapies. 
To appreciate the potential importance of human ES/EG cell lines, it is worth examining recent advances made with mouse ES cells. Although mouse ES cells are most noted for their ability to produce new strains of animals with specific genetic alterations, it is their capacity to differentiate and generate multiple lineages in culture that is most relevant to human cell lines. When removed from conditions that maintain them in an undifferentiated state, ES cells will spontaneously differentiate to form multicellular structures known as embryoid bodies (EBs) that contain elements of all three embryonic germ layers: ectoderm, mesoderm and endoderm. As differentiation continues, a wide range of cell types, including hematopoietic, endothelial, muscle, and neuronal develop within the EBs in a defined and reproducible temporal order. When analyzed in detail, the developmental programs associated with lineage commitment in EBs show remarkable similarities to those found in the normal embryo. Thus, EBs provide a rich source of normal developing cell populations with both embryonic and adult phenotypes. Given this potential, what opportunities do human ES and EG cell lines offer?
The most obvious application of human ES/EG cells and the one that receives the most attention is in cell¨Creplacement therapies: to replace diseased or degenerating tissues, or cell populations (such as those of the hematopoietic system) that have been destroyed by chemotherapy. In theory, EBs could provide an unlimited supply of specific cell types for transplantation. To date, ES cell-derived cardiomyocytes, neural precursors and hematopoietic precursors have been transplanted into recipient animals. Although the analyses of the long-term outcome of such experiments are limited, the findings suggest that the transplanted cells are able to function in the host animal.

