Morphological alterations in endothelial cells associated with the release of von Willebrand factor after thrombin generation in vivo

von Willebrand factor (vWF) is synthesized by endothelial cells and stored in endothelium-specific granules, the Weibel-Palade (WP) bodies. The release of vWF from endothelial cells in vitro in response to secretagogues such as thrombin is considered to result in the loss of WP bodies through the fusion of the WP bodies with the plasma membrane. Biochemical and morphological techniques, including transmission (TEM) and scanning (SEM) electron microscopy, were used to examine the plasma profile of vWF in parallel with morphological alterations in endothelial cells associated with the generation of thrombin in vivo. There was a rapid loss of high-molecular-weight multimers of the circulating vWF, with full recovery within 1 hour. Simultaneously, TEM demonstrated that the endothelial cells lost WP bodies and became severely vacuolated; this was associated with the appearance of craters in the endothelial surface on SEM. Release of stored vWF in WP bodies seemed to follow the fusion of multiple rather than individual WP bodies, with the resulting vacuole fusing and rupturing through the plasmatic membrane. Within 1 hour there was increased morphological evidence of metabolic organelle activity associated with replacement of WP bodies, presumably due to de novo synthesis of the basic protomer and its packaging in high-molecular-weight multimeric form in the storage organelles.