Take a Look at The Very Moment a Heart Starts Beating in an Animal Embryo

Credits: Jia et al., Nature, 2023

Researchers have documented the incredible instant a heart initiates its beat, an accomplishment made even more astounding given the short lifespan of a zebrafish. Around 20 hours into the development of zebrafish, their embryonic hearts began pulsing, transitioning from a group of individual cells into a unified entity.

Leveraging advanced microscopic imaging, the team from Harvard University expanded on previous research on the inaugural heartbeats in chickens, rats, and mice. They meticulously observed the infant zebrafish embryos to determine precisely when the heart cells commence their rhythm.

Historical research has revealed that the initial heartbeat sets in even before the formation of the elementary heart structure, a rudimentary tube. The earliest indications of activity are found within clusters of heart muscle cells, known as cardiomyocytes. These cells are found to be drenched with calcium ions, albeit in a rather chaotic manner.

In a fully developed heart, calcium ions dart in and out of cardiomyocytes, producing action potentials (electricity spikes) that catalyze heart contractions. Bill Jia, the head investigator, along with Cohen and their team, built on this foundational knowledge.

They analyzed how calcium ion waves organized and spread in young zebrafish embryos, leading to the heart’s inaugural coordinated pulse. As cardiomyocytes began aligning into a circular formation at the zebrafish embryos’ center, the once sparse calcium waves intensified in size and frequency.

Subsequently, a sudden surge in calcium levels was witnessed, followed by the heart cells emitting waves of electrical signals across the tissue. The first couple of heartbeats appeared slightly uneven but quickly synchronized.

The team noted, “A consistent, orderly heartbeat is established well before it’s connected to the bloodstream or begins pumping,”. Additionally, Jia’s team detected that around 90 minutes before the heart’s debut beat, zebrafish heart cells transitioned into a responsive state, seemingly preparing for their pivotal role.

Intriguingly, the calcium ion waves that paved the way for the first heartbeat didn’t always start at the same position among different zebrafish embryos. This indicates no specific cells are preordained to initiate the process.

Typically, this starting point, or the “locus of initiation”, was situated centrally within the cardiac circle, rather than at its peripheries where adult heart pacemaker cells are found.

The team postulates that the preliminary activity in heart cells, prior to their first contraction, might stimulate heart development.

Considering the parallels between zebrafish and embryos of chicks, rats, and mice, the scientists propose that the foundational principles of heart development might be universal among vertebrates, a category encompassing humans. Such insights might pave the way for future investigations into cardiac anomalies like arrhythmias in humans.