Weibel-Palade bodies are endothelial-specific organelles that store von Willebrand factor. (Left) Oblique section of a Weibel-Palade body from pulmonary artery endothelium of a rat, showing parallel arrangement of internal tubules (H). Reproduced with permission from Weibel and Palade.¹¹  (Right) Electron micrograph (Wagner et al¹² ) of human umbilical vein endothelial cells stained for von Willebrand factor with peroxidase showing a cluster of positively stained Weibel-Palade bodies. Left bar is 0.1 μM, and right bar is 0.5 μM. Reproduced from The Journal of Cell Biology.^11,12  Copyright 1964 and 1982 The Rockefeller University Press.