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BACKGROUND: Embryonic stem (ES) cells are lines of cells that are isolated from blastocysts. The murine ES cells were demonstrated to be true pluripotent cells as they differentiate into all embryonic lineages. Yet, in vitro differentiation of rhesus ES cells was somewhat inconsistent and disorganized. The recent isolation of human ES cells calls for exploring their pluripotential nature. MATERIALS AND METHODS: Human ES cells were grown in suspension to induce their differentiation into embryoid bodies (EBs). The differentiation status of the human ES cells and EBs was analyzed by following the expression pattern of several lineage-specific molecular markers using reverse transcription polymerase chain reaction (RT-PCR) and in situ hybridization. RESULTS: Here we report the induction in vitro of cystic embryoid bodies from human ES cells. Our findings demonstrate induction of expression of cell-specific genes during differentiation of the human ES cells into EBs. In the human EBs, we could show a characteristic regional expression of embryonic markers specific to different cellular lineages, namely, zeta-globin (mesoderm), neurofilament 68Kd (ectoderm), and alpha-fetoprotein (endoderm). Moreover, we present a synchronously pulsing embryoid body that expresses the myocardium marker alpha-cardiac actin. In addition, dissociating the embryoid bodies and plating the cells as monolayers results in multiple morphologies, among them cells with neuronal appearance that express neurofilament 68Kd chain. CONCLUSION: Human ES cells can reproducibly differentiate in vitro into EBs comprising the three embryonic germ layers. The ability to induce formation of human embryoid bodies that contain cells of neuronal, hematopoietic and cardiac origins will be useful in studying early human embryonic development as well as in transplantation medicine.