Weibel-Palade body

In physiology, Weibel-Palade bodies are organelles in the endothelial cells, the cells which make up the endothelium, the exceedingly thin cell sheet which lines all blood vessels, and the heart. They are named after the two scientists who first described them in 1964. They play a dual role in blood coagulation haemostasis and inflammation.

There are two major constituents of Weibel-Palade bodies. One is von Willebrand factor (vWF), a multimeric protein involved in blood coagulation. The second is P-selectin, which binds to passing immune cells (leukocytes). This allows the fast moving leukocytes in the blood to bind to the cells lining the blood vessels and slow down in a series of steps called the leukocyte adhesion cascade. Subsequently, leukocytes transmigrate across the endothelium and enter the surrounding tissue where they can migrate to the site of infection. 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.

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 characterised by prolonged and variable mucocutaneous bleeding. Type III von Willebrand Disease is a severe bleeding disorder, not unlike severe haemophilia type A or B. VWF acts in primary haemostasis 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 &alpha;-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. Their observation was published in: Weibel ER, Palade GE. New cytoplasmic components in arterial endothelia. Journal of Cell Biology 1964;23:101-112 (fulltext).