Calcium handling precedes cardiac differentiation to initiate the first heartbeat.

The mammalian heartbeat is thought to begin just prior to the linear heart tube stage of development. How the initial contractions are established and the downstream consequences of the earliest contractile function on cardiac differentiation and morphogenesis have not been described. Using high-resolution live imaging of mouse embryos, we observed randomly distributed spontaneous asynchronous Ca-oscillations (SACOs) in the forming cardiac crescent (stage E7.