Clusters of VVOs are deployed at intervals in the cytoplasm of endothelial cells. They are often concentrated near lateral borders of endothelial cells, that is, parajunctionally, and provide a direct link between the vascular lumen and ablumen. Approximately 12 percent of VVOs extend from luminal to the abluminal plasma membranes when viewed in single, random 80-nm electron microscopic sections, and in serial sections, this value approaches 100 percent . The individual vesicles and vacuoles comprising VVOs are bounded by membranes and interconnect with each other and with the endothelial cell plasma membranes by means of stomata that are closed by thin diaphragms. The stomatal diaphragms measure less in diameter than the diameter of individual VVO vesicles and vacuoles. The presence of tracers within adjoined vesicles-vacuoles, as well as within the stomata connecting them, provides prima facie evidence that such vesicles and vacuoles are in open communication with each other. However, in some cases, the stomata joining individual vesicles and vacuoles were closed by a diaphragm, and passage of macromolecular tracers was restricted such that one vesicle or vacuole contained tracer whereas the other did not. It would seem, therefore, that the diaphragms that separate individual stomata serve to restrict the passage of cargo and may be opened and closed individually.
VVO structure cannot be fully appreciated by electron microscopy of standard 70- to 100-nm sections. In serial 12-to 14-nm ultrathin sections for electron microscopy and computer-assisted three-dimensional reconstructions, a network of interconnecting vesicles and vacuoles was revealed and established that VVOs provide a continuous, often serpentine pathway across venular endothelium, extending to both the lumen and ablumen at multiple sites . Subsequent studies have revealed that the VVOs of adjacent endothelial cells also open to the interendothelial cleft, and in some instances, VVOs from adjacent endothelial cells connect across the intercellular cleft between adjacent endothelial cells, raising the possibility that plasma may extravasate by a VVO pathway that extends across overlapping endothelial cells.
The large size and complexity of VVOs suggest that they are sessile structures and that their component vesicles and vacuoles do not shuttle back and forth across endothelial cell cytoplasm. In evaluation of ultrathin (12- to 14-nm-thick) serial sections, only 9 of 1,395 (0.65%) uncoated vesicles were found to be single entities, free and unattached in the cytoplasm, that is, > 99 percent were attached to other vesicles and vacuoles as parts of VVOs .
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