The endothelium is a thin layer of endothelial cells and forms the inner lining of our blood vessel, which separates blood from the underlying tissue. The vessel wall is equipped with a number of self-healing properties that prevent infection and excessive blood loss upon vascular injury. Following trauma the endothelium rapidly releases a vascular first aid kit, the Weibel-Palade body (WPB), which helps to recruit leukocytes to sites of inflammation. From the WPB it also releases long strings of Von Willebrand factor (VWF), which are adhesive for platelets and which are crucial for the formation of a platelet plug. Failure to release (sufficient amounts of) VWF leads to bleeding, such as in Von Willebrand disease (VWD). In this project I will use endothelial cells from patients to study the formation and release of WPBs in order to better understand the mechanisms that prevent excessive bleeding in healthy individuals and how abnormalities in these mechanisms result in disease.