Systems Biomechanics of the Cell - Systems Biomechanics of the Cell - page 2

organelles. The degree of mechanical association between these components and
therefore the structural composition of the cell body proper may vary between cell
types and stages of development, but the presence of a single dominant, mechani-
cally linked complex inside the cell boundary seems to be typical, giving rise to the
cell body concept. The similarity of prokaryotic cells to the eukaryotic body-
boundary organization appears increasing in the light of the recent research into the
prokaryotic cytoskeleton; here we will be concerned, however, with the eukaryotic
structure as the one which is currently better studied and which gives rise to the
more complex forms and movements that exemplify advanced life.
It seems natural to structure the present approach to the system biomechanics of
the cell by focusing on the system-level effects in the mechanics of the body, the
boundary, and their interaction. Furthermore, in the first approximation it seems
worthwhile exploring the effects limited to those arising from the mechanics of the
body reduced to an idealized aster (or asters) or microtubules flexing elastically, and
of the boundary reduced to an idealized shell, either rigid or under line tension. The
interaction between the body and the boundary can be limited to simple contact;
cortically anchored molecular motors exerting dissipative forces on the contacting
microtubules can be considered in addition. Although this representation of the cell
mechanics is extremely idealized and simplified, the presented analysis shows that
the limited set of simple physical forces, when placed under the structural con-
straints of the generic heritable cell structure (body and boundary), can produce
remarkably complex and life-like behavior. The particularly fundamental and gen-
eral emergent effects that will be considered in this topic are instability of symmetry
(which makes the specific cases of stability nontrivial), irreversibility, and dissipa-
tive oscillations. The thesis of this short monograph is that system-level mechanical
effects on the cellular level deserve further theoretical study, must be taken into
account when designing and interpreting experiments, and may ultimately prove to
be responsible for many of the most intriguing manifestations of life.
Instability of Symmetry
Unipolar Cell Body
In many cell types that have been studied experimentally, the prominent structural
feature is the aster of microtubules, which are converging and anchored on the cen-
trosome (Bray 2001 ). Besides forming the structural basis for the cell body as
defined in the introduction, the aster's microtubules direct transport of organelles
and secretory vesicles. This makes the position of the centrosome a reliable marker
for the position of the cell body within the cell boundary as well as functionally
important for various cellular activities, such as wound closure, migration, and
interactions between cells of the immune system (Gotlieb et al. 1981 ; Kupfer et al.
1982 , 1994 ; Kupfer and Singer 1989 ; Ueda et al. 1997 ). In particular, induction of