Embryoid bodies are aggregates of cells derived from embryonic stem cells, and have been studied for years with mouse embryonic stem cells. Cell aggregation is imposed by hanging drop, plating upon non-tissue culture treated plates or spinner flasks; either method prevents cells from adhering to a surface to form the typical colony growth. Upon aggregation, differentiation is initiated and the cells begin to a limited extent to recapitulate embryonic development. Embryoid bodies are composed of pluripotent cells and therefore cannot form trophectodermal tissue or embryonic tissue (which includes the placenta), cells of virtually every other type present in the organism can develop.

The aggregate, while first simply appearing as a ball of cells, takes on an increasingly more complex appearance, becoming after a few days a hollow ball (cystic embryoid body), and next forms internal structures such as a yolk sac, and cardiomyocytes, heart muscle cells which beat in a rhythmic pattern to circulate nutrients within the increasingly larger embryoid body.

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