Animal Cell Shape: The Importance of the Cytoskeleton - Mechanisms of Morphogenesis - page 39

BUILDI NG AND PLACEMENT OF TENSILE MICROFILA MENTS
The actin microfilaments of animal cells are arranged in two main ways: as a network of
fine filaments in the cortical region of the cytoplasm and as a network of tense filament
bundles, often called 'stress fibres', that connect the sites at which cells adhere to each other
and to the basement membrane.
Branched networks of actin are characteristic of the cell cortex and of the motile leading
edge (Chapter 8). In most cells, the cortex immediately below the plasma membrane is occu-
pied by a highly cross-linked network of microfilaments that forms a viscous gel. This gel is
capable of resisting tangential tension forces and all compression forces although, because
this resistance is based on viscosity, short-term forces are resisted better than long-term
ones. In morphologically simple cells such as adult mammalian erythrocytes ( Figure 5.4 ),
the cortex is less than 1 m m thick but it plays a major role in controlling cell shape. Indeed
it has to, for there is no other cytoskeletal system in these cells. 9 Erythrocytes deficient in
components of the cortical cytoskeleton lose their biconcave shape, round up and become
fragile. Erythrocytes from which membrane lipids have been extracted, however, retain their
basic shape, which confirms that the protein network is more important than the membrane
FIGURE 5.4 The cortical cytoskeleton of a mammalian erythrocyte (for clarity, the diagram shows only the most
significant of the many components present).