The vertebrate circulatory system, to sum it up glibly and way too simply, is a heart pumping blood. But how and when in an organism’s development does the flow begin?

A series of movies that capture the process in developing zebrafish show that it requires a ‘go’ signal. Hundreds of blood cells wait in the equivalent of a vascular starting gate, tethered in place to the blood vessel walls. Then, all at once, they are set free by an enzyme called ADAM8, which acts as a ‘molecular scissors,’ Atsuko Sehara-Fujisawa of Kyoto University and her colleagues describes in a paper published online in Current Biology yesterday. Without the enzyme, blood cells stagnate in the starting gate. (Press release here.)

The researchers don’t yet know whether a similar mechanism works in other organisms, but they suggest three possibilities for why blood cells wouldn’t be released to circulate in the body one by one, as they come off the production line. First, an active signal would provide control for which cells go and which cells wait. Or, it might help the developing organism to make sure vessels are fully formed before they fill with blood. Finally, there are some hints, the researchers say, that blood cells need plasma to flow first in order to fully mature.

In timelapse photos from the researchers’ videos, below, you can see red blood cells accumulating a blood vessel, then beginning to freely flow. (The action takes place in the PCV, or posterior cardinal vein. In the first image, taken 24 hours postfertilization (hpf) the lumen of the dorsal aorta (DA) contains few erythrocytes. In the second and third image, the vessel is more fully formed and begins to fill with blood cells.The fourth image shows the onset of circulation, with the process in full force in the final image.)