Here is the developing zebrafish heart at 3 days post-fertilization. Unlike mammals, which have four chambers, the zebrafish heart consists of only two: a single ventricle (left) and a single atrium (right). Despite the difference in the number of chambers, the heart is the first organ to form in both mammals and zebrafish. Can you guess why? The developing embryo needs nutrients and as its cells multiply and grow, the nutrients in the blood need help to get to all parts of the embryo.  This is where the heart comes in: it pumps blood around the body, sending nutrients essential for development. The image above was created to help us visualize the muscles in the heart, muscles that allow the heart to pump blood. You will notice that not only is the heart composed predominantly of muscle, but so are the surrounding regions that form to support the developing heart.

Contributed by Michka Sharpe, a fifth year graduate student in the Biological and Biomedical Sciences program at Harvard Medical School, and our Featured Artist for August 2017.