Essential Cell Biology 5th edition

596 CHAPTER 17 Cytoskeleton
of actin-binding proteins are involved in their assembly. We begin with
the cell cortex and its role in cell shape and movement, and we conclude
with the contractile apparatus of muscle cells.
A Cortex Rich in Actin Filaments Underlies the Plasma
Membrane of Most Eukaryotic Cells
Although actin is found throughout the cytoplasm of a eukaryotic cell,
in many cells it is highly concentrated in a layer just beneath the plasma
membrane. In this region, called the cell cortex, actin filaments are linked
by actin-binding proteins into a meshwork that supports the plasma
membrane and gives it mechanical strength. In human red blood cells,
for example, a simple and regular network of fibrous proteins—including
actin and spectrin filaments—attaches to the plasma membrane,
providing the support necessary for the cells to maintain their simple discoid
shape (see Figure 11–29). In other animal cells, the cortex includes
a much denser network of actin filaments that are cross-linked into a
three-dimensional meshwork. The rearrangements of actin filaments
within the cortex provide much of the molecular basis for changes in
both cell shape and cell locomotion.
Cell Crawling Depends on Cortical Actin
Many eukaryotic cells move by pulling themselves across surfaces, rather
than by swimming by means of beating cilia or flagella. Carnivorous
amoebae crawl along in search of food. The advancing tip of a developing
axon migrates in response to growth factors, following a path of
chemical signals to its eventual synaptic target cells. White blood cells
known as neutrophils migrate out of the blood into infected tissues when
they “smell” small molecules released by bacteria, which the neutrophils
seek out and destroy. For these cells, chemotactic molecules binding to
receptors on the cell surface trigger changes in actin filament assembly
that help steer the cells toward their targets (see Movie 17.7).
The molecular mechanisms of these and other forms of cell crawling
entail coordinated changes among many molecules in different regions of
the cell; there is no single, easily identifiable locomotory organelle, such
as a flagellum, responsible. In broad terms, however, three interrelated
processes are known to be essential: (1) the cell sends out protrusions
at its “front,” or leading edge; (2) these protrusions adhere to the surface
over which the cell is crawling; and (3) the rest of the cell drags itself forward
by traction on these points of anchorage (Figure 17–33).
All three processes involve actin, but in different ways. The first step, the
protrusion of the cell surface, is driven by actin polymerization. A crawling
fibroblast in culture regularly extends thin, flattened lamellipodia (from
the Latin for “sheet feet”) at its leading edge. These extensions contain a
dense meshwork of actin filaments, oriented so that most of the filaments
have their plus ends close to the plasma membrane. Many cells also
extend thin, stiff protrusions called filopodia (from the Latin for “thread
feet”), both at the leading edge and elsewhere on their surface (Figure
17–34). These are about 0.1 μm wide and 5–10 μm long, and each contains
a loose bundle of 10–20 actin filaments (see Figure 17–29C), again
oriented with their plus ends pointing outward. The advancing tip (growth
cone) of a developing nerve cell axon extends even longer filopodia, up to
50 μm long, which help it to probe its environment and find the correct
path to its target cell. Lamellipodia and filopodia are both exploratory,
motile structures that form and retract with great speed, moving at
around 1 μm per second. These protrusions are thought to be generated
by the rapid, local growth of actin filaments, which assemble close to the