Mesostructures of the aortic valve.

Background And Aim Of The Study: The aortic valve cusp is commonly described as a three-layered structure containing circumferentially aligned fiber bundles. Little is known, however, regarding fiber bundle sizes, branching patterns, or how they are connected. This is because previous morphological studies relied primarily on histological sectioning and staining techniques, which tend to affect all of the collagen, regardless of structure or orientation.

Methods: To address this problem, a novel system was developed for the visualization and analysis of the intermediate-scale 'mesostructures' of aortic valve cusps. Mesostructures are defined as the branching fiber bundle and membrane structures that make up the valve. This system uses elliptically polarized light to provide contrast between collagen mesostructures without the need for embedding, staining, or other contrast-enhancing techniques. Using this system, high-resolution images of 42 whole porcine aortic valve cusps were acquired in an unloaded (i.e. resting) condition and during application of controlled manipulation. Image-processing algorithms were developed to quantify fiber bundle morphological features and produce detailed maps of the fiber bundle patterns.

Results: Fiber bundle sizes and patterns were found to be significantly different for each of the three cusps. The non-coronary cusp had a significantly smaller bundle diameter (0.9 +/- 0.07 mm) than the left and right coronary cusps (1.1 +/- 0.08 mm). The left and non-coronary cusps appeared to be mirror images of each other, whereas the right coronary cusp was self-symmetric. When applying controlled loads to the cusp specimens, thin, overlapping, collagenous membranes were often found which connected the fiber bundles. Interesting pinnate fiber branching patterns were also found.

Conclusion: These morphological results were strikingly different than the currently accepted three-layer description, and may provide valuable insight into aortic valve structure-function relationships.