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 V on Willebrand factor (vWF) is a large multimeric plasma protein that plays a central role in hemostasis. 12 It is synthesized in megakaryocytes 34 and endothelial cells 56 and stored in the a-granules of platelets 7 and in the endothelial cell storage organelles, the Weibel-Palade (WP) bodies. 8 P-selectin is a component of both the platelet a-granule and WP body membrane, 9 but other components of the a-granules have not been identified in WP bodies. 10 vWF is released from endothelial cells both constitutively and via the regulated pathway from WP bodies after endothelial cell stimulation." Release from WP bodies makes available the highly platelet-interactive, high-to abnormally high-molecular-weight multimeric forms of vWF. 2 vWF is released from endothelial cells in culture in response to stimulation by a variety of secretagogues, including thrombin.1112 It is generally considered that the mechanism of release involves the fusion of the limiting membrane of the WP bodies with either the luminal or abluminal membranes of the endothelial cell. This process has been inferred from observation in two settings. In vitro studies, using immunofluorescence 8 " 13 or electron mi- 10 demonstrated that the release of vWF was associated with a reduction in the numbers of WP bodies and the appearance of vWF in the supernatant. In studies in vivo, WP bodies appeared to release their contents into the circulation after fusing with the luminal membrane of endothelial cells.14 Although this observation was reported after the recognition that WP bodies stored vWF, 8 this was not appreciated by the authors, who considered the observation to indicate that WP bodies were secretory in nature but of unknown function.14 To our knowledge, there has been no in vivo study in which plasma vWF levels have been quantified in parallel with alterations in the WP bodies and vWF distribution within endothelial cells.We have developed a model for the induction of thrombin generation in vivo in various animal species. This is achieved through the infusion of two defined components of the prothrombinase complex 15 : factor Xa (activated factor X [FXa]) and a coagulant active phospholipid surface provided by phosphatidylchol...

Two cases of benign tumor-like mesenteric lesions are presented. The limited literature on comparable and similar lesions is reviewed, and the histologic findings are correlated. The lesions are composed of chronically inflamed adipose and fibrous tissue in various proportions. They probably represent different stages of a reparative process initiated by damage of the mesenteric adipose tissue of various etiologies. Whereas lesions in the younger age groups (mean 39.9 years) are predominatly characterized the fibrosis, those in the older age groups (mean 55.8 years) usually show a chronic inflammatory cell infiltrate rather than fibrosis. More than a dozen terms have been used for these lesions. The summarizing term "sclerosing mesenteritis" is proposed.

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