Atherosclerosis suppression in the left anterior descending coronary artery by the presence of a myocardial bridge: an ultrastructural study.

Ultrastructural changes of the left anterior descending coronary artery (LAD) by the presence of myocardial bridge (MB) were studied. In contrast with various atherosclerotic lesions in the intima both proximal to MB and in the whole length of the LAD having no MB, intimal thickness beneath MB was remarkably thin. Neither lipid deposition nor foam cells were present there even in the aged. The intima beneath MB consisted of only normal smooth muscle cells (SMCs) in some layers. Collagen fibrils loosely stuffed among SMCs showed a spiraled appearance. The intima distal to MB was thicker than that beneath MB. Modified SMCs were present along with normal SMCs, and foam cells were intermingled with them. By scanning electron microscopy, endothelial cells proximal to MB were arranged in a pavement-like appearance, and they were polygonal and flat. Endothelial cells beneath MB became spindle-shaped and regularly engorged along the direction of blood flow. Such regularity was lost in endothelial cells distal to MB. These endothelial changes indicate that the intima beneath MB is stressed by high shear and that intima proximal or distal to MB is stressed by low shear. It is, thus, suggested that alteration of hemodynamic factors that arise from contraction of MB greatly affects the evolution of atherosclerosis through the regulation of intimal lipid infiltration within the LAD.