Optical indications of pace‐maker potential and rhythm generation in early embryonic chick heart.

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Abstract
Pacemaker type action potentials and rhythm generation in very early embryonic chick hearts were monitored using a voltage-sensitive merocyanine-rhodanine dye. Rhythmicity in recurrence of the spontaneous action potentials was evident at the 7 somite developmental stage, and the rhythm was completely organized by the early period of the 9 somite stage i.e., just before the 1st contraction. The rhythmic recurrence of action potentials increased in frequency as development proceeded from the 7 to the 9 somite stage, and presumably gave rise to the rhythm of the initial contractions. Optical signals resembling the pacemaker type action potential with a diastolic depolarization phase were first detected in embryonic hearts at the 8 somite stage. At this stage pacemaker type action potentials were detected from various regions, such as the ventricle and the unfused primordia at the atrium level. Regionalization of pacemaker type action signals was exhibited at the early period of the 9 somite stage. At this stage the pacemaker type signals were often evident at the atrium level, while the cardiac type signals were detected in the ventricular region. Rhythmicity is already generated at the 7 somite developmental stage. Pacemaking cells are widely distributed in the embryonic hearts at the 7-8 somite stages. The appearance of the pacemaker potential is initially localized to the atrium level at about the 9-10 somite stages.