Scanning and light microscopy of human aortic leaflets in stressed and relaxed states

Abstract

A two-year investigation of the anatomy of stressed aortic leaflets compared to that of leaflets in the relaxed state is reported. Intact fresh human aortic valves were fixed at an intra-aortic root pressure of 80 mm. Hg with a pressure-autoregulatory roller pump. The best fixation method found was with 4 per cent glutaraldehyde, osmium tetraoxide, and a critical point drying process. Marked differences between the pressure-fixed and relaxed specimens were found with respect to surface character and thickness. The aortic side of the leaflets has a rough surface, with striations in the circumferential direction, whereas the ventricular surface is much smoother, with very fine striations in the radial direction. These patterns are reflections of the character of the fiber layers lying immediately below the epithelial layers. Five distinct layers of fibers were found in midline cross sections, by both scanning and light microscopy. Further, the elastic components are located toward the ventricular surface. The thickness measurements show a marked difference between the stressed and relaxed states. These data provide anatomic explanations for the uniaxial stress-strain behavior, bending characteristics, and remarkable durability of human aortic leaflets. The concept that fixation of biological tissues in the stressed state is of importance in developing structural, physiological, and pathological hypotheses is advanced.