Morphology of porcine aortic valve cusp elastin

While the structure and function of heart valve cusp collagen have been relatively well defined, the role and morphology of elastin remains poorly understood, despite the fact that it comprises up to 13% of the cusp dry weight.

The elastin structure of 24 hot-alkali-digested porcine aortic valve cusps was investigated with scanning electron microscopy. Elastin structures were categorized according to their morphology and a model of the distribution of these structures within the cusp was developed.

The two main types of elastin observed, amorphous and fibrillar, were further categorized based on their morphology. Amorphous structures included continuous sheet, sheet with integrated fibers on the surface and sheet with fenestrations. Fibrillar structures identified were loose fibers, loose mesh/woven fibers and compact mesh. By imaging samples of digested fibrosa and ventricularis that had been microdissected apart, we were able to produce maps of the elastin structure in the two layers. The ventricularis contains a large continuous sheet of amorphous or compact mesh elastin that covers the entire layer. Elastin in the fibrosa is much more complex, consisting of large tubes that emerge from the aortic attachment and extend circumferentially across the cusp. The tubes, constructed of amorphous fenestrated sheet and loose mesh elastin, likely surround the large circumferential collagen bundles observed in the fibrosa.

The elastin structures that we have identified help explain the measured mechanics of this tissue and suggest that collagen and elastin are highly integrated. As a result, we believe that elastin plays an important functional role in the cusp and that a full explanation of heart valve cusp mechanics must incorporate the contributions of both collagen and elastin.