THE VESICULO-VACUOLAR ORGANELLE (VVO) - A DISTINCT ENDOTHELIAL-CELL STRUCTURE THAT PROVIDES A TRANSCELLULAR PATHWAY FOR MACROMOLECULAR EXTRAVASATION

The vesiculo-vacuolar organelle (VVO) is a recently described organell e found in the cytoplasm of endothelial cells that line tumor microves sels and normal venules, VVOs are grape-like clusters of interconnecti ng uncoated vesicles and vacuoles, bounded by trilaminar unit membrane s, that span the entire thickness of vascular endothelium, thereby pro viding a potential trans-endothelial connection between the vascular l umen and the extravascular space, Macromolecular tracers preferentiall y cross hyperpermeable tumor microvessels through VVOs, The present in vestigation was undertaken to elucidate further the ultrastructure and function of VVOs in a murine ovarian carcinoma (MOT) and in normal ve nules, Morphometry revealed that VVOs were enormous cytoplasmic struct ures (median area, 0.12-0.14 mu m(2) in single electron micrographs). Moreover, the individual vesicles and vacuoles that comprised VVOs wer e on average substantially larger than capillary caveolae and followed a non-normal distribution that was skewed to the right, Specimen tilt ing provided conclusive evidence that individual VVO vesicles and vacu oles communicated with each other and with the endothelial cells' plas ma membranes by stomata, some of which were closed by diaphragms compo sed of a single membrane, Studies with two tracers, ferritin (FE, diam eter similar to 11 mm) and horseradish peroxidase (HRP, diameter simil ar to 5 nm), revealed that passage of macromolecules through VVOs was regulated at the level of stomatal diaphragms, thereby demonstrating a mechanism for controlling the passage of macromolecules across endoth elial cells, Thus, compared with tumor microvessels, little circulatin g FE and HRP entered the VVOs of normal venular endothelium because st omata joining vesicles and vacuoles to each other and to the lumen and ablumen were closed, VVOs and their component vesicles/vacuoles were readily distinguished from endosomal organelles such as coated vesicle s and multivesicular bodies, which also accumulated FE and MRP, Our fi ndings indicate that VVOs provide a major pathway for the extravasatio n of circulating macromolecules across endothelia taller than capillar y endothelium and suggest that upregulated WO function accounts for th e well-known hyperpermeability of tumor blood vessels.