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).