Biomedical Engineering Reference
In-Depth Information
characteristic global properties of whole living cells that contain millions of
such elements. On the other hand, information-processing networks, i.e., signal
transduction and gene regulatory networks, are full abstractions in that every
individual interacting element that occurs only once in the model actually repre-
sents hundreds to billions of copies of that particular type of molecular species.
Another difference between the structural and information networks is that the
former takes into account position and physicality, whereas interactions in the
latter can be represented mathematically as a graph because, in a first approxi-
mation, space does not play a role (see also this volume, Part II, chapter 4, by
Wuchty, Ravasz, and Barabási).
At first glance, one might think that the structural cytoskeletal network
maintains the cell's shape, whereas the biochemical information-processing net-
work determines the behavioral state of the cell. However, as will be discussed
below, evolution has led to assembly of cells in which structure and informa-
tion-processing functions are tightly coupled—a fundamental property of living
systems at all size scales. Another central property of biological systems, such as
a cell, is that they need to be stable, yet flexible. Cells are continuously chal-
lenged by chemical and physical stimuli: not only do cells have to resist random
perturbations and maintain their structure and behavioral program, they also
have to be flexible enough to respond appropriately to specific external signals
that require distinct changes in both cell mechanical and biochemical behaviors,
such as during cell migration and differentiation. Interestingly, death of both
cells and whole organisms is characterized by a rapid increase in rigidity (
rigor
mortis)
, with a complete loss of the flexibility that dominates the living state.
Thus, this unification of
robustness
with
flexibility
, both in terms of cell struc-
ture and behavior, is a hallmark of complex living systems.
4.
RESULTS
4.1. Structural Networks in Cells
4.1.1. From Molecular Biochemistry to Cellular Mechanochemistry:
The Cytoskeleton
Cells are comprised of thousands of molecules that are arranged and con-
nected in specific ways so as to produce distinct structures and biochemical
functions; they are not membranes filled with viscous colloidal solution. In par-
ticular, mammalian cells contain an internal molecular framework or "cytoskele-
ton" that provides shape stability to the cell, and orients much of the cell's
metabolic and signal-transducing machinery (35,36). The cytoskeleton is an
interconnected 3D network or lattice comprised of three major classes of fila-
mentous protein polymers—microfilaments, microtubules, and intermediate
filaments. A subset of the microfilaments that contain myosin as well as actin
