Weibel-Palade bodies at a glance.

The vascular environment can rapidly alter, and the speed with which responses to both physiological and pathological changes are required necessitates the existence of a highly responsive system. The endothelium can quickly deliver bioactive molecules by regulated exocytosis of its secretory granules, the Weibel-Palade bodies (WPBs). WPBs include proteins that initiate both haemostasis and inflammation, as well those that modulate blood pressure and angiogenesis. WPB formation is driven by von Willebrand factor, their most abundant protein, which controls both shape and size of WPBs. WPB are generated in a range of sizes, with the largest granules over ten times the size of the smallest. In this Cell Science at a Glance and the accompanying poster, we discuss the emerging mechanisms by which WPB size is controlled and how this affects the ability of this organelle to modulate haemostasis. We will also outline the different modes of exocytosis and their polarity that are currently being explored, and illustrate that these large secretory organelles provide a model for how elements of secretory granule biogenesis and exocytosis cooperate to support a complex and diverse set of functions.