International Business Department           Liu Bojia           October 02, 2023

  The beating of the heart has always been a fascinating subject, and over 2000 years ago, Aristotle, who was later called the "Father of Biology", observed the beating of the heart in a chicken embryo.

  In general, our hearts beat regularly and rhythmically. From birth to old age and death, a heart beats about 3 billion times in a human lifetime, never stopping. However, the first of billions of times - the initiation of the heartbeat - is still a hidden process.

  In a paper published this week in the leading academic journal Nature, a team of researchers co-led by Dr. Adam Cohen, a professor of chemistry, chemical biology, and physics at Harvard University, and Dr. Sean G. Megason, a professor of systems biology at Harvard Medical School, captured for the first time the moment of initiation of the heart cells from resting to beating during early heart development in zebrafish.

  How does the heart turn on its first beat? At the beginning of the experimental exploration, the researchers confessed that they didn't know what would happen; perhaps a few cells would start beating first, and then the beating area would slowly expand; perhaps, it would be different parts of the heart that would start beating separately, and then eventually converge and harmonize; or, perhaps, it would start out weakly, and then beat more and more forcefully as time went on.

  However, the experiments found that none of the above.

  The researchers cultured zebrafish in the lab. This small, transparent, fast-growing fish can provide an excellent model for observing the heart, as it takes just 24 hours after fertilization to see a heartbeat emerge in the developing embryo, while the process can be captured on camera.

  Using fluorescent proteins and high-speed microscope imaging, the researchers captured changes in calcium ion levels and electrical activity in heart cells in real time in zebrafish embryos, indicating heart cell beats by massive bursts of calcium ion levels and electrical activity signals.

  The researchers were amazed to find that as if someone flipped a switch at a certain moment, the developing zebrafish heart cells would suddenly start beating in unison.

  After further recording the first few beats of the zebrafish embryo's heart, the researchers found that the first few beats were infrequent and irregularly spaced, and that the heart rate gradually accelerated and became more uniform as it developed.

  Interestingly, with each heartbeat, one region was the first to discharge, triggering an electric current that quickly passed through other cells and prompted them to discharge. In different individual zebrafish, the region of cells that first discharged was not consistent, meaning that there was no one region of cells that was specifically responsible for initiating each heartbeat.

  This was also surprising to the researchers, because it's quite different from what we usually see in the developing heart: in the adult heart, there are specialized pacemaker cells in specific regions that act as commanders to coordinate the heartbeats, whereas in the developing heart, each cardiac cell has the ability to beat on its own.

  These new findings not only satisfy the scientists' curiosity, but more importantly, the researchers believe that the zebrafish heart development process is likely to be consistent with that of other species, including humans, and therefore studying the basic biology of the heartbeat will also help scientists to understand more about how arrhythmias occur in humans and how to intervene.