Researchers led by Holger Gerhardt at the Max-Delbrück-Center for Molecular Medicine in the Helmholtz Association (MDC) found that primary cilia endow endothelial cells, the innermost cells lining our blood vessels, with a particular sensitivity to a blood borne signaling protein BMP9 that stabilizes vessel connections. How endothelial cells sense and react to blood flow during vascular remodeling is poorly understood. The research team studied endothelial cells in culture under simulated blood flow conditions and investigated progressive blood vessel network changes in a genetically engineered mouse model that lacks primary cilia just in endothelial cells. They found that flow levels which normally stabilize vessel structures fail to do so when cells lack primary cilia. Their data suggest that primary cilia, flexible structures that extend and bend in the blood flow, function as mechanosensors and signaling structure by combining chemical and mechanical input into a strong molecular signal that changes endothelial gene expression.