Q. I have a question about von Willebrand Factor – where is it stored? All that I can gather is that it’s stored in ‘Weibel-Palade bodies’, but where are they? Are they in the vessel wall, in the platelet, free-floating in the blood? It’s confusing me a little bit.
A. First, a little background. Von Willebrand Factor is a huge multimeric protein that is made by megakaryocytes and endothelial cells. It functions in both the initial, platelet-plug phase of hemostasis (in which it glues the platelets to the endothelium), and in the second, fibrin-forming phase of hemostasis (in which it serves as a carrier molecule for factor VIII that keeps factor VIII from being prematurely degraded).
In Von Willebrand disease, von Willebrand factor is either decreased or abnormal. This means that patients have a hard time gluing platelets to their endothelium during clot formation (so the initial platelet plug doesn’t form as well). They also may have a hard time forming fibrin, since von Willebrand factor is a carrier (degradation-preventing) molecule for factor VIII, and factor VIII is a critical factor in the coagulation cascade.
So. Where is von Willebrand factor stored? In a few different places, it turns out:
1. Endothelial cells. Endothelial cells synthesize von Willebrand factor and store it in little organelles called Weibel-Palade bodies (named after the two scientists who discovered them). The main things found in Weibel-Palade bodies are von Willebrand factor (described above) and P-selectin (a cell-adhesion molecule that endothelial cells use to catch passing leukocytes, allowing them to leave the blood vessel and migrate into the surrounding tissue).
2. Platelets. Megakaryocytes (big cells in the bone marrow that give rise to platelets) synthesize von Willebrand factor, and it gets stored in the alpha granules of platelets.
This is a good idea, because the main job of von Willebrand factor is to act as the glue that sticks the platelets down onto the subendothelium! So it’s stored in places near the place it’s needed most.