Embryonic stem (ES) cells are derived from undifferentiated cells present at the inner cell mass at the blastula stage of development (1 ,2 ). In the normal course of development, these cells give rise to the primitive ectoderm, which in turn gives rise to the ectoderm, mesoderm, and endoderm through the process of gastrulation. Under the influence of the mesoderm, the midline ectoderm differentiates into the nervous system (3 ). Undifferentiated epiblast cells can be obtained from dissociated blastulas and cultured on mitotically arrested mouse embryonic fibroblast feeder layers and/or in the presence of the cytokine leukemia inhibitory factor (LIF). The epiblast cells expand and give rise to colonies of stem cells that are undifferentiated, nontransformed, have stable diploid karyotype and can proliferate indefinitely. They can integrate within blastocysts or with morulas to generate chimeric animals (4 ). These cell lines, called ES cells, are pluripotent and generate all embryonic derivatives including germ cells (except the trophoblast-placenta). Similar cells can also be generated from primordial germ cells (PGC) present in the fetus. Though the initial culture conditions are slightly different, the resulting stem cells, termed embryonic germ (EG) cells are indistinguishable from ES cells in most of their properties (5 ,6 ).
