The First Heartbeat—Origin of Cardiac Contractile Activity

The amniote embryonic heart starts as a crescent of mesoderm that transitions through a midline linear heart tube in the course of developing into the four chambered heart. It is unusual in having to contract rhythmically while still undergoing extensive morphogenetic remodeling. Advances in imaging have allowed us to determine when during development this contractile activity starts. In the mouse, focal regions of contractions can be detected as early as the cardiac crescent stage. Calcium transients, required to trigger contraction, can be detected even earlier, prior to contraction. In this review, we outline what is currently known about how this early contractile function is initiated and the impact early contractile function has on cardiac development.