Ultrastructural comparative study on lymphatic capillaries of the subendocardium, myocardium, and subepicardium of the heart left ventricle.

A comparative study, with quantitative analysis, was done on the fine structure of the lymphatic capillaries in the subepicardium, myocardium, and subendocardium of the left ventricle from dog hearts. In the three layers, the vesicular system of lymphatic capillaries was quite similar. More than 63% of the plasmalemmal vesicles are in contact with the abluminal or luminal endothelial membrane of cardiac lymphatic capillaries. Volume density measurements showed that plasmalemmal vesicles occupied between 8.0 and 9.3% of the endothelial cytoplasm. Overlapping and interdigitation contacts were frequent, whereas end to end contacts were scarce. The frequency of open junctions wider than 50 nm was inferior to 1%. The intercellular cleft was often obstructed by a focal tight junction (macula occludens). Intercellular spaces without specialized junctions were more frequent with subepicardial and myocardial lymphatic capillaries as compared with subendocardial vessels. Important quantitative variations of the fine structure were encountered on the abluminal side of the endothelial wall. In the subendocardium, the basement membrane covered 33% of the lymphatic endothelium whereas for the subepicardium and myocardium lymphatic capillaries, the values were reduced significantly to 18% (P less than 0.05) and 10% (P less than 0.001), respectively. An inverse relation was observed between the proportion of endothelium covered by basement membrane and the frequency of abluminal endothelial projections associated with the endothelial wall. Endothelial projections (EP) decreased in the following order: myocardium (19 EP/100 micron), subepicardium (15 EP/100 micron), and subendocardium (10 EP/100 micron). Quantitative morphological variations in the fine structure of lymphatic capillaries are believed to result from physiological (muscular movements, metabolic activity) and morphological (thickness of the connective tissue support surrounding lymphatic capillaries) differences existing between the three cardiac layers.