When isolated beating ventricular heart cells from newborn rats were grown in tissue culture on untreated polystyrene surfaces, they showed a striking tendency to grow focally in three dimensions from the single layer cell sheets which were formed early in growth. During this process, they frequently formed miniature spherical heart-like masses, which continued to beat and grow in size. These often were somewhat lobulated in appearance, and grew up to 2 mm in diameter. Histological sections of such structures sometimes revealed evidence of appreciable orientation of the cells to each other, in fiber-like units. Electron microscope sections of such mini-hearts showed structures resembling intercalated discs between myocardial cells. The precise factors which induced the cardiac cells to apparently organize into these heart-like structures are not presently known.

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