Direct intracellular measurement of electrical events in the early embryonic heart is impossible because the cells are too small and frail to be impaled with microelectrodes; it is also not possible to apply conventional electrophysiological techniques to the early embryonic heart. For these reasons, complete understanding of the ontogeny of electrical activity and related physiological functions of the heart during early development has been hampered. Optical signals from voltage-sensitive dyes have provided a new powerful tool for monitoring changes in transmembrane voltage in a wide variety of living preparations. With this technique it is possible to make optical recordings from the cells that are inaccessible to microelectrodes. An additional advantage of the optical method for recording membrane potential activity is that electrical activity can be monitored simultaneously from many sites in a preparation. Thus, applying a multiple-site optical recording method with a 100- or 144-element photodiode array and voltage-sensitive dyes, we have been able to monitor, for the first time, spontaneous electrical activity in prefused cardiac primordia in the early chick embryos at the six- and the early seven-somite stages of development. We were able to determine that the time of initiation of the contraction is the middle period of the nine-somite stage. In the rat embryonic heart, the onset of spontaneous electrical activity and contraction occurs at the three-somite stage. In this review, a new view of the ontogenetic sequence of spontaneous electrical activity and related physiological functions such as ionic properties, pacemaker function, conduction, and characteristics of excitation-contraction coupling in the early embryonic heart are discussed.
