Platelets take up and release fibrinogen. Platelets engulf fibrinogen from the blood and load it into endosomes through the process of endocytosis. Endosomes containing fibrinogen fuse with granules containing VWF. The protein VAMP-3 facilitates this membrane fusion. During platelet activation, platelets release fibrinogen and VWF into the blood to promote platelet aggregation and thrombosis. Professional illustration by Somersault18:24.