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By capturing movies of both the blood and vasculature of zebrafish embryos, each less than two millimeters long, researchers have been able for the first time to see the very moment that blood begins to flow [as reported by ScienceDaily]. The observations, reported online on June 3 in Current Biology, show that the earliest blood flow, involving what appear to be hundreds of cells, begins all at once.
Remarkably, that onset of life-giving circulation takes more than a beating heart. In fact, red blood cells remain stuck to the blood vessel wall initially, even after the heart starts to beat, says Atsuko Sehara-Fujisawa of
These findings raise an obvious question: Why would the onset of primitive blood circulation require such an active protease instead of just going with the flow, with blood cells entering the circulation one by one? First, the researchers say, proteolysis would allow for control over which blood cells enter the circulation and which get held back. It might also help to stop blood cells from entering the circulation too early, preventing leaks that might occur if blood vessels aren't fully formed, or avoiding stagnation before an adequate flow of plasma is established with the heartbeat, the researchers add. It may be that blood cells need plasma to flow before they can reach maturity.