Page 389 - Handbook of Properties of Textile and Technical Fibres

362 Handbook of Properties of Textile and Technical Fibres

11.10 Mechanotransduction

Collagen fibrils are a major factor in the conversion of mechanical forces and work into
stored energy; in many cases this energy is stored in the form of high-molecular-weight
polymers such as collagen fibers (Silver, 2006). When muscles and tendons are loaded,
the muscle does work on the collagen fibers and energy is stored as work in the tendon.
During tendon stretching the applied force stores work elastically, by stretching the
triple helix; the energy is released after the load is removed.
Collagen fibrils are attached to cell membranes through attachment molecules such
as integrins and other cell surface macromolecules. During this stretching of the
collagen fibers, tension is transmitted through the cell membrane setting into motion
the activation of the phosphorelay pathways inside the cell (Silver, 2006). Tension
typically activates the MAP kinase pathways leading to synthesis of new collagen
to “bolster” the ability of the tendons to support loads (Silver, 2006). Thus, some of
the mechanical energy generated from the muscle tension is stored in the form of
highemolecular-weight polymers; energy is stored in the form of covalent bonds
that link amino acids together in the newly synthesized collagen fibers. When external
tensile loads decrease, the collagen fibers atrophy and release the energy from the
broken covalent bonds. This can occur during nonloading events, such as prolonged
bed rest or when an astronaut is in a low gravitational field.
In this manner, collagen fibers are dynamic structures that are constantly growing or
resorbing depending upon the level of tension they experience. The interactions
between the collagen fibers and cells in tissues are an exciting part of the dynamics
that occur in ECM biology that ultimately affects health and the pathogenesis of
disease processes such as cancer.

11.11 Conclusions

Collagen fibers are the structural elements found in vertebrate tissues that transmit
forces, store, and dissipate energy. Collagen fibers limit the deformation of tendon
and other load bearing tissues and have a hierarchical structure that includes collagen
molecules, microfibrils, fibrils, fibers, and fascicles. Collagen molecules are packed
into a quarter-stagger arrangement with neighboring molecules staggered by multiples
of D, which is about 22% of the molecular length. During mechanical deformation
collagen molecules as well as the gap region of the D period are stretched. At larger
strains, molecules and fibrils slide by each other, which leads to energy losses. Finally,
collagen fiber failure occurs by disintegration of some of the hierarchical structure
yielding collagen subfibrils that lose much of their mechanical strengths.
The ability of collagen-cell interactions to provide dynamic structural alterations in
the mechanical properties of ECMs provides clinicians with the ability to monitor
changes in tissue structure. However, this will require new techniques to measure
changes in properties that occur during the disease process. For this to occur a detailed