A grid was superimposed on endothelial cells and endothelial cell thickness measured each place the grid crossed the vessel. Average thickness was calculated from each individual measurement. Magnification x 5,000. Definition of ultrastructural features: B. Pinocytotic vesicles are 0.07–0.08 μm in diameter (arrow), magnification x20,000; C. Space between cells (arrow), magnification x20,000; D. Large vacuoles are > 0.1 μm (arrows); x20,000.