Biology Reference
In-Depth Information
CHAPTER
5
Animal Cell Shape: The Importance
o f the Cytoskeleto n
Unlike cells found in other Kingdoms of life, cells of animals have no cell walls. This
makes them unusually flexible, both in the literal sense of being easily deformed by an
external force, and in the sense that they can use internal mechanisms of force generation
to acquire, rapidly, a range of specialized shapes. This shape-changing lies at the foundation
of much of animal morphogenesis.
The shape of any flexible body is governed by the second law of thermodynamics, which
favours an arrangement that minimizes the free energy of the system over any other possible
arrangements (Chapter 3). If the plasma membrane were a simple phospholipid bilayer, an
isolated cell would minimize its energy by minimizing its surface area. An isolated cell
would therefore become spherical (neglecting a small distortion due to gravity) and groups
of adhering cells would have the shapes of adhering soap bubbles. Indeed, early mathemat-
ical treatments of cell shape modelled cells precisely according to these simple laws of
surface tension. 1 The shapes of non-adherent cells, or of cultured cells that have just been
trypsinized for passage and have therefore lost their junctions with the matrix and so on,
are indeed bubble-like ( Figure 5.1 ). Not all cells are like this, however, even in suspension;
the biconcave disc of a mammalian erythrocyte is an exception that is familiar to most biol-
ogists, and swimming protozoa come in a vast variety of shapes that seem to defy the prin-
ciple of minimizing surface area ( Figure 5.1 ). It is clear, therefore, that other forces must be at
work even for some cells in suspension, and certainly for those that associate with each
other.
While surface tension at the plasma membrane is no doubt important, the dominant influ-
ence on the shapes of animal cells is the cytoskeleton. This modifies membrane shape both
by forming a contractile network in the cortical region, just inside the plasma membrane,
and by forming systems of long filaments that run across the cell and can push or pull on
the membrane. Of the three basic types of cytoskeleton (microfilaments, intermediate fila-
ments and microtubules), microfilaments and microtubules play the largest role in control-
ling the shape of most cells. This point is illustrated by the relatively mild phenotype of the
vimentin knockout mouse, 2,3 in which cells that ought to express vimentin and that have
no alternative intermediate filament system are still able to play their normal roles in
development.
 
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