The researchers tracked the movement of blood cells against the background of fluorescently stained blood serum. High-speed imaging (1,000 frames/s) and digital particle-tracking yielded movement vectors for the blood cells. The cells moved at speeds of up to 1.5 mm/s in a primitive, valveless heart and 0.5 cm/s in a more developed heart. The resultant churning forces should be more than enough to activate the many endothelial genes known to respond to shear stress.
The high speeds surprised the group. The small scale of the developing heart was thought to slow down blood cells because of frequent collisions of cells with vessel walls. But apparently the zebrafish has a mighty heart, even at the age of 37 h.
If flow was blocked with a bead, the juvenile hearts continued to beat but failed both to form one heart chamber and to loop correctly. Thus, says Köster, “it's not just a genetic program that directs heart development.” He hopes to determine which developmental genes are responding to flow forces to shape the development of the heart. ▪