Biomedical Engineering Reference
In-Depth Information
sults were obtained with heterogeneous models in which different filaments ex-
hibited different levels of stiffness.
Thus, a key feature of the cellular tensegrity network—the level of cy-
toskeletal prestress—is critical for control of both static and dynamic mechani-
cal behavior in whole cells. As predicted by the model, the global system
architecture and inhomogeneity of time constants between individual elements
also significantly contributes to the emergent properties of the system: the whole
network behaves differently than an individual Kelvin-Voigt cable. This finding
that both elastic and frictional behaviors of living cells naturally fall out from
the tensegrity model indicates that the viscous properties of mammalian cells are
not due to fluid behavior of the cytosol. Rather, these complex mechanical prop-
erties of cells emerge from collective mechanical interactions among the distinct
molecular filaments that comprise the cytoskeletal network. These results em-
phasize the importance of the tensionally prestressed cytoskeleton for cell me-
chanical behavior and add further support for the universality of the cellular
tensegrity model (37,39).
4.1.4. Biological Implications of Tensegrity Beyond the Cytoskeleton
In a more encompassing biological interpretation, a mechanical design prin-
ciple that uses networks composed of discrete elements rather than a single me-
chanical continuum allows molecules (e.g., the proteins that form the filaments)
to bridge the gap between microscopic structureless biochemistry and macro-
scopic mechanics and pattern in just one step of self-assembly. However, as
mentioned in the introduction, living systems harbor a hierarchy of many levels
of emergence over many size scales. Of interest thus is that the principle of ten-
segrity is scalable, and in fact operates at various size scales, from molecule to
organism (9,37,39). For example, tensegrity may govern how individual mole-
cules, such as proteins, and multimolecular structures (e.g., lipid micelles) gain
their mechanical stability and 3D form (19,37,39,75). Geodesic forms also are
dominant in molecular systems including viruses, the simplest example of a "liv-
ing" system; interestingly, tensegrity was used to explain the geodesic structure
of viral capsids (7).
At a larger size scale in living tissues, cells are attached to anchoring scaf-
folds that are also 3D structural networks composed of fibrillar extracellular
matrix molecules. Because cells apply cytoskeletally generated tractional forces
on their adhesions, these extracellular matrix networks are also prestressed and
hence stabilized through tensegrity. Local increases in tissue tension are sensed
by individual adjacent cells that respond by switching into a proliferative state,
thereby increasing cell mass to match increases in applied macroscopic forces.
In this manner, tension-dependent changes in cell growth allow higher-order
tissue and organ structures, such as glandular buds and brain gyri, to be sculpted
