Transcapillary transport of solute by the endothelial vesicular system: evidence from thin serial section analysis

Upon crossing the walls of continuous capillaries, perfused electron-dense solutes often appear to become lodged in the attenuated space bounded by pericytes and the ablumenal capillary surface. In single thin sections, it is difficult to determine whether such accumulations result from transcellular transport via the endothelial vesicular system or whether tracer has filled these spaces retrogradely after paracellular transport through interendothelial clefts. We have taken continuous thin serial sections through patches of terbium trapped between pericytes and the endothelial cells of continuous capillaries in the rete mirabile of the eel. Sampling these areas in three dimensions has revealed that the terbium deposits are bounded and limited to regions of the capillary wall devoid of interendothelial associations. Dense interstitial terbium deposits were also continuous with ablumenal endothelial caveolae and could also be seen in clusters of fused vesicles. These observations strongly implicate transcellular transport by the endothelial vesicular system as the route by which accumulated terbium had crossed the capillary wall.