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points along their shafts (Fig. 3 . 1 C). The shafts of filopodia can also give
rise to new lamellipodia or filopodia (Fig. 3 . 1 D, also see Fig. 3.2 A,D). Most
filopodia are transient and are retracted back into the cell following their
initial protrusion. An invariant and defining feature of filopodia is that they
are dependent on the actin filament cytoskeleton. Specifically, the filopo-
dium is supported by a bundled array of actin filaments (Fig. 3 . 1 E). In neu-
rons, filopodia are found along axons and dendrites, and also at the growth
cone, both in vitro and in vivo (Fig. 3 . 1 F). Actin filaments have fast growing
“barbed ends” and less dynamic “pointed ends.” In filopodia, as a general
rule, actin filaments have their barbed ends directed toward the tip of the
filopodium, while the pointed ends of the filaments are located at the base
of the filopodium, or deeper into the cell. Actin filaments underlying cel-
lular protrusions are highly dynamic with half-lives ranging from seconds to
minutes. However, actin filaments in neuronal growth cone filopodia have
markedly longer half-lives (20-25 min; Mallavarapu and Mitchison, 1999 ).
Figure 3.2 Examples of in vitro sensory axons (A-D) and nonneuronal cells (E-F) stained
with phalloidin to reveal actin filaments. The arrowheads in (A) denote a filopodium that
gave rise to additional filopodia from its shaft. The inset in panel (D) shows a filopodium
that has given rise to a small lamellipodium along most of its length. While most of the
filopodia in panels (A), (C) and (D) arise directly from the leading edge of the growth
cones, panel (B) shows an example of filopodial “ribs,” the tips of which extend only a
short distance from the leading edge and the shaft/rib of the filopodium is embedded
in a lamellipodium (see inset for higher magnification). Panel (E) shows a fibroblast and
the inset denotes, as in panel (B), a filopodium with its base (b) and a shaft/rib (s) that is
largely located within the cytoplasm of the cell, and its tip (t) projecting outward. Panel
(F) shows the leading lamellipodium of a peripheral nervous system glial cell termed a
Schwann cell. These cells have the same developmental origin as sensory neurons (neu-
ral crest) but clearly different morphology and functions.
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