Mode and determination of the initial contraction stage in the mouse embryo heart

The developing mammalian heart initiates spontaneous contractions shortly after the myocardium differentiates from the splanchnic mesoderm. The precise timing and mode of the onset of heartbeat, however, have not been statistically described in mice. We analyzed the patterns of contractive activity in video-recorded heart regions ranging from the pre-somite stage to day 10.5 (E10.5). The first sign was detected at the 3-somite stage (E8.25), when a few cardiac myocytes constituted small contracting groups on both sides of the heart tube. Fluctuations of the basal [Ca2+]i level were detected in dormant 3-somite-stage hearts, indicating the initiation of electrical activity before visible contractions. After weak and irregular contractions at the 3-somite stage, the contractions were almost coordinated as early as the 4-somite stage. Unidirectional blood flow through the atrioventricular canal was established around the 20-somite stage at E9.25, correlated with the development of the endocardial cushion. We propose that not only the endocardial cushion but also coordinated contractions are essential for unidirectional flow, because induced bradycardia due to short exposure at room temperature caused regurgitation at E10.5 when otherwise highly organized flow was observed. These findings complement and extend earlier observations on functional heart development, providing a reference for fundamental research on mammalian cardiogenesis.