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A
B
C
D
FIGURE 5.10 Views of an Aplysia growth cone. A. Namarski image showing the growth cone. The
bulging central domain and the thin peripheral domain containing actin cables are visible. B. Labeling the
actin filaments with a fluorescent probe reveals they are concentrated in the peripheral domain and the filopo-
dia. C. Labeling the microtubules reveals that these structures are in the central domain. D. Pseudo-colored
merged image of actin filaments (red) and microtubules (green). (From Paul Forscher)
B
Filopodia
Receptors
A
Ligands
P-Zone
Actin
cable
C
Lamellipodia
Actin
monomers
Actin fibers
D
Actin
cable
Actin
Microtubule
Filopodia
swing rearward
Filopodia
retract
Myosin
Tubulin
E
C-Zone
Cytoskeleton
associated
proteins
Actin meshwork
Microtubules with cargo
F
cargo
microtubule
motor
protein
FIGURE 5.11 The structure of the growth cone. A. Actin bundles fill filopodia, that are bounded by mem-
branes with cell adhesion molecules and various receptors, poke out at the advancing edge and are retracted
at the trailing edge of the growth cone. Between the filopodia are sheets of lamellipodia that extend forward.
They are filled with an actin meshwork that is continuous with that in the main body of the growth cone.
Here also microtubules push forward and carry cargo to and from the cell body along the axon shaft as
they enter the growth cone and fan out toward the filopodia. B-F. Close-ups of various regions show some
of the molecular components of the cytoskeletal network that are localized in the growth cone.
detached from the surface of a culture dish using a fine
glass needle, the growth cone snaps into a new direc-
tion that is consistent with the tension exerted by the
remaining filopodia (Wessells and Nuttall, 1978).
Microtubules are the other main cytoskeletal ele-
ments of the growth cone. Pools of unassembled
tubulin are concentrated in the growth cone, which
is the most sensitive part of the axon to the effects
of microtubule depolymerizing agents such as
nocodozole (Brown et al., 1992). Natural microtubule
stabilizing proteins, such as Tau, are also highly con-
centrated near the growth cone, suggesting that this is
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