Before the Season: How a Heart Takes Shape

The Screening

1Before the soccer season, every player on the Westfield Middle School team gets a sports physical. When the doctor listened to eighth grader Camila Hartigan's chest, she heard a soft extra sound and sent Camila for a heart ultrasound to be safe. The ultrasound showed a healthy heart with four chambers: two upper chambers called atria, which receive blood, and two lower chambers called ventricles, which pump it out. Camila was cleared to play the same day.

2The heart doctor, Dr. Noor Halvorsen, keeps a poster in her office that shows how hearts form before birth or hatching. She explained that a human heart does not start with four chambers. Its first form is a single tube that begins to beat about three weeks after fertilization. The tube then bends into a loop, and for a while blood flows through one atrium and then one ventricle in a row. Walls grow inside to divide them, and by about week 8 the heart has its four chambers. Table 2 shows this timeline.

3At the same time, several pairs of arteries form in the neck region of the embryo. These arch arteries pass through the folds behind the head. In fish, they become the arteries that carry blood through the gills. In a human embryo, most of them later disappear or are reshaped into the arteries to the head, part of the aorta and the arteries to the lungs, so an adult has no trace of a row of paired arteries.

4The poster also compares six kinds of embryos (Table 1). Adult hearts differ a lot. An adult zebrafish heart has one atrium and one ventricle. A frog's has two atria and one ventricle, and a turtle's has two atria and one ventricle that is partly divided. Chickens and humans have four chambers. A fruit fly, an insect with no backbone, has a heart that is a tube along its back. Biologists find this kind of comparison useful because organisms that share a pattern of early development often share a common ancestor.

5Camila asked how anyone could study embryo hearts. Dr. Halvorsen said that zebrafish embryos are clear, so a beating heart can be seen under a microscope about one day after the egg is fertilized, and a chick embryo's heart can be seen beating on about day 2 of its 21 days in the egg.