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Journalism of Courage
The vertebrate heart is a complex organ, with chambers, valves and other features. But in the early stages of embryonic development, things are a lot simpler. The heart is basically a tube. Still, it has to pump blood, and in human embryos, it begins to do that at about three weeks. Scientists have thought that it moves blood the way the esophagus moves food, through peristalsis, a wave of contractions of the tube wall.
Now Arian S. Forouhar and Morteza Gharib of the California Institute of Technology and colleagues have shown that the tube doesn8217;t function through peristalsis. Rather, they suggest in a report in the journal Science, it is a suction pump.
The researchers used advanced imaging techniques to study zebrafish embryos. Zebrafish are often used in embryology because they develop quickly, are transparent and can be genetically modified to express fluorescent green proteins that make elements of the embryo easier to see. In their studies, the researchers were able to observe and time the movement of the tube walls, and they found evidence that peristalsis could not be the mechanism. For one thing, in peristalsis, the speed of blood flow should be equal to the speed at which the contraction wave travels through the walls. But they found that the blood flowed at a faster rate.
In peristalsis the wave should also travel in only one direction. But the imaging showed that the contractions begin at a midpoint and propagate in both directions. The wave reaches the ends and reflects back, causing rapid expansion in the middle. This expansion, the researchers say, creates the suction that accelerates the blood through the tube. The whole cycle takes slightly more than half a second.
