Biology Reference
In-Depth Information
FIGURE 5.1 The shapes of some cells in suspension. Mammalian cells that normally grow attached to
a substrate and to each other typically round up and have soap bubble-like morphologies, minimizing their surface
area, when released (the drawings in the top row are of mIMCD3 renal epithelial cells during passage). Erythro-
cytes, on the other hand, normally exist in suspension but nevertheless they maintain a biconcave discoid shape and
do not round up into spheres. Protozoa can maintain very complex shapes.
TENSEGRITY
One of the most comprehensive models of cell shape 4,5 treats the cell as an assembly of
tension and compression elements. This model views the cell as analogous to buildings
and sculptures that gain their structural stability partly from tension, rather than from the
pure compression that stabilizes an arch. It is therefore referred to by the same term,
'tensegrity', that was coined by the architect R. Buckminster Fuller 6 to describe the 'tensional
integrity' of buildings and of the bar-and-wire sculptures by the artist Kenneth Snelson. 7 The
principle of tensegrity can be illustrated by constructing a simple assembly from two pencils
and an elastic band ( Figure 5.2 ); tension in the band holds the pencils firmly while compres-
sion in the pencils prevents the tension in the band from being released.
In most cells, tension is borne and generated mainly by the microfilaments, especially in
conjunction with myosin, and also by intermediate filaments and by the membrane itself
as surface tension. Compression is borne mainly by microtubules and by the extracellular
FIGURE 5.2 A desktop model of tensegrity, in which the balance of forces between compressed pencils and
a tense elastic band stabilizes a simple structure. In a better version, the pencils would be filed down in the middle so
that they do not quite make contact as they cross and there would be no possibility of frictional forces between them.
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