Heart Valves as a Functional System
Understand the molecular and cellular components of the heart valves will allow us to understand the biomechanical features of the valves.
The aortic valve is the most frequently diseased and studied valve, and represents the perfect paradigm to represent the complex and highly specialized extracellular matrix configuration of heart valves. During diastole, the aortic valve leaflets stretch to avoid blood backflow. The change in its configuration is predominantly dependent on collagen, with directional realignment and crimping of the fibers. The collagen orientation determines tissue compliance to tensile stress. This complex fiber architecture is very sensitive to pressure, and very low pressures at the beginning of the cardiac cycle are capable of inducing collagen fibers arrangements, with loss of the fibrosa corrugations and collagen crimps (15–18). Mechanical functions of collagen also include limitation of cuspal stretching to avoid proplapse (achieved mainly by the strained and aligned collagen in the fibrosa) (23).
During systolic valve opening, the tissue becomes relaxed, with the elastin from the ventricularis layer recoiling to make the cusp retracted again (20, 26). In this phase of the cardiac cycle, fibers have a random directional distribution and crimps of collagen fibrils are restored (23).
All these continuous and coordinated alterations in valve configuration depend on the quality and quantity of its components, such us collagen, elastin and GAG. Indeed, they are the major determinants of the functional mechanisms and long-term durability of the valve. It has already been showed that the cell source used in bioprosthesis is important for long-term durability since the lack of cells in bioprosthesis has been shown to be the main source of failure for implants (26). That is the reason why all the previously described components are essential to maintain valve homeostasis. Thus, cell source and function are an important component of heart valve tissues, as they depend on the viability and function of active valve cells (23).