Organization of the intercellular junctions in the endothelium of cardiac valves

The intramembranous organization of valvular endothelial cells and the structure of their intercellular junctions were studied using both thin section and freeze fracture electron microscopy. Using the double replica method, large areas of fractured endothelial cell plasma membrane are exposed. On both faces, the intramembranous particles are randomly distributed, but they are 2-3 times more frequent on the P face than on the E face: on the former, their number varies from 340 to 1700 particles/micron 2. The density of vesicular openings seems to be slightly higher on the tissue front (29-43 openings/micron 2) than on the blood front (24-34 openings/micron 2). The vesicular stomata are absent in parajunctional areas. The intercellular junction structure appears as a variation to that described for arteries. The occluding junctions appear as a network of 1-6 (most frequently 3-4) interconnected ridges on P faces or grooves on E faces. Occasionally, the presence of strands formed by association of short bars can be observed on the P face ridge. In addition, there are a small number of occluding junctions with low profile ridges, free or marked by few particles, similar to those described for the venules. These junctions are probably involved in the inflammatory reaction occurring during clinically manifested valvular disease. The communicating (gap) junctions, small or large, are free, partially or completely associated. In all valves examined we observed a special kind of communicating junction the particles of which are disposed in 1-4 rows, forming branched or circular patterns. Intracellular injection of 6-carboxyfluorescein shows transfer of the dye to the neighboring cell, suggesting that the cells are coupled. In both atrioventricular and sigmoid valves, the endothelial junctions have a similar pattern with some differences in the degree of complexity. The ventricular aspect of the valves contains junctions with a larger number of junctional strands than the atrial or arterial aspect. This suggests a possible relationship between the number of strands and the stress factors (i.e. blood hydrostatic pressure). The presence of functionally communicating junctions of various dimensions and shapes suggests that the endothelial cells of valvular endocardium are metabolically coupled. At short exposure times (5-10 min), filipin-incubated valves exhibit characteristic filipin-sterol complexes (FSC) around the vesicular stomata of the endothelium. After a longer exposure (30-90 min) FSC labeled randomly the rest of plasma membrane except for coated pits, gap junction regions and area boundering tight junctional strands.