Biology Reference
In-Depth Information
Dissipative Oscillations
The previous sections have considered instability of trivial equilibria without regard
to time, which gives rise to the distinctively complex and purposeful biological
structures. We have also considered the irreversibility that emerges on the cellular
level and imposes directionality on the development of the cellular structures with
time. In this section, temporal instability will be treated. In general, it may be argued,
instability of the latter kind gives rise to the perpetual nonrandom movement that is
perhaps the best known attribute of living matter. Within the scope of this topic we
will be concerned here with the temporal instability that arises from the system-level
mechanics of the interaction of the cell body and boundary. The cyclic motion of the
cell body and boundary that is the cycle of cell divisions is the central mechanical
phenomenon on which rests the existence of all advanced life forms. Considerable
progress has been made in the systems-biomechanical approach to simpler cyclic
motions of the cell body alone, which will be considered in this section.
Before embarking on this special subject, as permitted by today's state of knowl-
edge, additional general reasons for studying the emergent periodic cell motion may
be mentioned. Experimental observations of periodic movement have the potential
to furnish dynamic data of higher quality for comparison with quantitative theory,
because measurements in this case are effectively repeated on the same cell under
the same conditions, eliminating the main sources of variability in quantitative cell
research. Further, it is conceivable that the modes of oscillatory movement are the
same as the modes of movement that appears non-oscillatory. Decomposition of
non-periodic movement into periodic modes may lead to deterministic mechanical
explanation for the complex movement that is the hallmark of cellular activity as
observed under the microscope.
The cell body is known to oscillate within the cell boundary in various physio-
logical contexts. An example of cyclic but complex, apparently multiperiodic and
three-dimensional movement of the cell body within the confines of the cell bound-
ary is presented by cytotoxic T lymphocytes of the immune system. These cells
were the subject of an irreversibility model discussed in the last section and will be
further treated here in regard to their periodic movements. In experiments, T lym-
phocytes display oscillations of their microtubule aster and of the mechanically
coupled nucleus and membranous organelles. The cell body comprised of these
linked structures oscillates within the lymphocyte's largely unchanging boundary
next to and between the interfaces that the boundary has formed with target cells
(Kuhn and Poenie 2002 ). The oscillations next to one target may facilitate the extru-
sion of the cytotoxic granules at that target, and the oscillations between interfaces
appear to be the nonintuitive way in which the T cell achieves reorientation of its
secretory apparatus between the simultaneously engaged targets.
A different example is provided by neuroblasts in the developing brain. In the
pseudostratified layer of neural progenitors, nuclei undergo a cyclical motion which
is coupled with the cell division cycle (Sidman et al. 1959 ). The orientation of these
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