Ultrastructure research of the endocardial endothelium of monkeys.

The surface structure of the endocardial endothelium of normal monkeys (Macaca fuscata and M. irus) was studied using scanning and transmission electron microscopy. The endocardium was covered by a layer of endothelial cells, each of which was recognized by the presence of nuclear bulge and marginal folds. The free cell surface was covered by a number of microvilli. The size of the endothelial cell and its surface morphology varied considerably in the different portions of the heart. The endothelial cells were packed more densely along the free margin of the valves especially at the noduli valvularum semilunarum. The microvilli over the cell surface were denser and longer on the ventricular side of the mitral valve and on the aortic valve, where the marginal folds were not "folds" but were formed by an array of numerous microvillous projections. These cytoplasmic projections had a topographical correlation with micro- and macro-pinocytotic vesicles, thus suggesting their role in the interaction with circulating biologically active substances.