Biomedical Engineering Reference
In-Depth Information
by organ-level mechanical forces during morphogenesis. Finally, provision of
stability and flexibility within a biological network through a tensegrity force-
balance is most obvious at the highest level of organization in the hierarchy of
life. The musculoskeletal system that allows humans to walk and hold our bod-
ies in various positions gains its stability through a balance between continuous
tension (muscles, tendons, ligaments) and local compression (bones) that gener-
ates a tensile prestress (tone).
Thus, tensegrity appears to represent a fundamental design principle that is
used to stabilize biological networks at all size scales in the hierarchy of life, as
well as throughout evolution (37). The flexibility and stability provided by use
of tensegrity also may have contributed significantly to the process of hierarchi-
cal self-assembly and environmental selection that first led to the origin of cellu-
lar life (38), as well as to the development of multicellular organisms comprised
of interconnected networks of cells, tissues and organs (39).
Importantly, the complex mechanical behaviors of a tensegrity system rep-
resent emergent properties of the whole network, and not properties of the indi-
vidual structural members. For these reasons, tensegrity may provide a means to
incorporate "physicality" and spatial constraints into models of complex network
systems that commonly are only thought of in terms of information flow. Inter-
estingly, most biochemical reactions proceed in a "solid-state" in living cells,
i.e., many of the enzymes, substrates, and reactants are physically immobilized
on insoluble cytoskeletal scaffolds (6,36,50). Thus, mechanical properties of
structural networks, and hence tensegrity principles, may also directly impact
information flow in biological systems, as will be discussed below.
4.2. Information Networks
On the hardware side, the mechanical properties of the cells are the obvious
properties that emerge from interactions between structural proteins. In contrast,
on the software side, the emergence of some simple, fundamental, higher-level
features from interactions among regulatory genes and proteins is not immedi-
ately apparent. Here we show that, despite the complexity of molecular path-
ways within a cell, global cell behaviors associated with a change of phenotype
exhibit simple rule-governed properties that emerge from interactions in the
regulatory network of the cell.
4.2.1. Cell Fates as Emergent Properties
The global behavior of a cell within a tissue in a multicellular organism can
be reduced to a few behavioral modes or phenotypes, the so-called "cell fates":
the proliferative state, the migratory state, differentiation, senescence or the state
