Weibel–Palade body

Weibel–Palade bodies are the storage granules of endothelial cells, the cells that form the inner lining of the blood vessels and heart. They store and release two principal molecules, von Willebrand factor and P-selectin, and thus play a dual role in hemostasis and inflammation. They are named after the two scientists who first described them in 1964.

Constituents
There are two major components stored within Weibel–Palade bodies. One is von Willebrand factor (vWF), a multimeric protein that plays a major role in blood coagulation. The other is P-selectin, which plays a central role in the ability of inflamed endothelial cells to recruit passing leukocytes (white blood cells), allowing them to exit the blood vessel (extravasate) and enter the surrounding tissue, where they can migrate to the site of infection or injury.

Additional Weibel–Palade body components are the chemokines Interleukin-8 and eotaxin-3, endothelin-1, angiopoietin-2, osteoprotegerin, the tetraspanin CD63/lamp3 and α-1,3-fucosyltransferase VI.

Clinical significance
The importance of Weibel–Palade bodies are highlighted by some human disease mutations. Mutations within vWF are the usual cause of the most common inherited bleeding disorder, von Willebrand disease. VWD has an estimated prevalence in some human populations of up to 1%, and is most often characterized by prolonged and variable mucocutaneous bleeding. Type III von Willebrand Disease is a severe bleeding disorder, not unlike severe hemophilia type A or B. VWF acts in primary hemostasis to recruit platelets at a site of injury, and is also important in secondary hemostasis, acting as a chaperone for coagulation factor VIII (FVIII).

Production
Multimeric vWF is assembled in the Golgi apparatus from vWF dimers. The Golgi then buds off vesicles, covered in a lipid bilayer, which consist almost exclusively of vWF. The only parallel organelle in physiology is the alpha granule of platelets, which also contains vWF. Weibel–Palade bodies are the main source of vWF, though, and α-granules probably play a minor role.

History
Weibel–Palade bodies were initially described by the Swiss anatomist Ewald R. Weibel and the Romanian physiologist George Emil Palade in 1964. Prof. Palade was to win the Nobel Prize in Physiology or Medicine in 1974 for his work on the function of organelles in cells.