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Heart development produces the pumping organ, its chambers, lining and supporting structures through coordinated differentiation and morphogenesis. The early heart tube changes shape through cardiac looping and receives further cellular contributions; changes in shape, cell number and cell identity are separable processes.

In zebrafish, fluorescent developmental-timing assays and a photoconvertible fluorescent protein distinguished early differentiation from later addition at the arterial pole of the heart. These experiments showed why growth of the heart cannot be explained solely by division of already differentiated cardiac cells.^[ref]