High-voltage electron microscopy of capillary endothelial vesicles.

Conventional electron microscopy of thin sections through capillary walls is inadequate to discern the relationships between endothelial vesicles and their association with the cell surface. High-voltage electron microscopy of thick sections through diaphragm muscle capillaries has been employed to visualize the three-dimensional structure of the vesicular system. Stereopairs of thick sections provide for direct three-dimensional observations of samples several vesicle diameters deep. A variety of simple and compound vesicular forms are present but not all are conjoined or maintain connections with the endothelial cell surface. This contributes to the concept of a dynamic interaction between free and attached vesicular structures where fission and fusion events compartmentalize and reconnect repeatedly. Such interactions would provide for a discontinuous pathway across the capillary wall but with a higher degree of complexity than a simple shuttling mechanism.