Heart Development
The zebrafish heart emerges at 24 hours post fertilization as a linear tube that undergoes a process of remodeling that ends in the formation of its two chambers. In humans the heart forms from the mesoderm 18 days after fertilization, and begins beating around day 21. In both zebrafish and humans the heart is the first definitive organ to develop and become functional. Zebrafish embryos can take in oxygen via passive diffusion, so mutations that lead to poor blood circulation can survive and be studied. In addition, in most organisms there is a period where the heart is functional but not essential, though this period is very short. This time period is several days in fish, while many other organisms can only survive a few hours. This extended time allows for the analysis of mutants with very low or no cardiac function, and gives zebrafish an advantage over a model organism like the mouse, where a similar phenotype would result in early lethality.
Above: Zebrafish heart at 24 hours. At this point the heart is linear, but beating.
Below: Zebrafish heart at 48 hours.
Cellular Structure of heart
In humans and zebrafish there are two main types of cells that make up the heart: cardiomyocytes and cardiac pacemaker cells.
The atria, where blood enters the heart, and ventricles, where blood is pumped out of the heart are
made of cardiomyocytes. Cardiomyocytes are elongated and somewhat tubular. The basic unit of a cardiomyocyte is the sarcomere. Cardiomyocytes differ from other muscle cells in that they often branch, and have intercalated discs. The epicardium, the layer immediately outside of these heart muscles, has been found in zebrafish to be essential for their regenerative properties.
The pacemaker cells are what send and receive the electrical impulses that coordinate heart function. They are mostly found in the sinoatrial (SA) node. They have natural automaticity, meaning they generate impulses themselves. They can be distinguished from other cardiomyocytes based on their location, and their automaticity.
Humans have two atria and two ventricles, the right side receives low oxygen blood and pumps it to the longs and the left side receives that blood and sends it back out to the rest of the body. Zebrafish only have a single atria and ventricle, making their heart simpler than a human heart. However, similarities in cellular structure, as well as conservation of genes and pathways make the zebrafish heart a viable model for the human heart.
