Is EmbryonicLimulusHeart Really Myogenic?

Abstract

Although the neurogenic nature of the heartbeat in adult Limulus has been well studied and is undisputed, we contest the reports that the embryonic heartbeat is myogenic. This notion, based on histological, calorimetric, and drug studies, is challenged by evidence from transmission electron microscopy and intracellular recording. The first, infrequent heartbeats occur at the time of the third embryonic molt when only the anterior portion of the heart tube is formed and functional. Contractions extend further caudad concomitant with lumen formation in the rear heart segments. All lumen-containing heart sections that we have examined, from the earliest on, have revealed neural elements in a bundle at the dorsal midline of the heart. Axons 1/m or less in diameter are prevalent: vesicle-filled terminal-like areas adjacent to muscle cells are often present as well, even in the youngest beating hearts. Myocardial cells show excitatory postsynaptic potentials as soon as heartbeat has begun, but they often fail to summate in the earlier stages so that contractions are few. Resting potentials remain at −65 to −70 mV from the onset of heartbeat until well after the larva has hatched, but heartbeat frequency, regularity, depolarization height (never overshooting) and duration all increase as embryos get older, probably as innervation of muscle fibers increases and coordination between pacemaker and follower neurons improves. We have found no evidence that embryonic Limulus heart passes through a myogenic phase and believe that it is neurally driven from the beginning.