Pathways of macromolecular extravasation across microvascular endothelium in response to VPF VEGF and other vasoactive mediators

Objective: The goal of these studies was to define the anatomic pathways by which circulating macromolecules extravasate from the hyperpermeable micro vessels that supply tumors and from normal venules that have been rendered hyperpermeable by vasoactive mediators. Methods: Extravasation pathways of circulating macromolecular tracers were followed by several morphological techniques: light and fluorescence micros copy, transmission electron microscopy of routine as well as ultrathin and serial sections? computer-assisted three-dimensional reconstructions, and m orphometry. Results and Discussion: Macromolecules extravasated across tumor microvesse ls or across normal venules rendered hyperpermeable by VPF/VEGF, histamine, or serotonin by three primary pathways: 1) Vesiculo-vacuolar organelles (V VOs), clusters of cytoplasmic vesicles and vacuoles that. span endothelial cytoplasm from lumen to ablumen; 2) trans-endothelial cell (EC) pores, and 3) fenestrae. We also present data concerning the structure and function of VVOs as well as evidence that VVOs form as the result of Linking together and fusion of caveolae-sized unit vesicles. Under suitable conditions VVOs also afforded a pathway for macromolecular transport in the reverse directi on, i.e., from vascular ablumen to lumen. Finally, in addition to opening V VOs to the passage of macromolecules, mediators such as VPF/VEGF may also i nduce structural rearrangements of VVOs, transforming them into tr-ans-EC p ores or fenestrae.