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common molecular and cellular signaling mechanism in general, including
the classic PCP events in the fly. Needless to say, the incredible versatility of
the PCP signaling module will need to be achieved by many different up-
stream input and downstream output in different morphogenesis events,
which will be interesting topics to study.
4. IS THE GROWTH CONE POLARIZED?
Axonal growth cones have been studied for many years. They are
known to be very sensitive to concentration differences and are able to turn
to areas with higher or lower concentration of molecular guidance cues.
However, whether growth cones are polarized or asymmetric has never
been established or at least there has not been a widely accepted consensus.
The highly motile membrane and cytoskeletal structures tend to lead one to
think that growth cones are highly “fluid” and “dynamic” and not polarized.
However, when looking inside the growth cone, there is ample evidence of
polarity ( Fig. 6.2 ).
First, microtubules are polarized with plus ends pointing toward the dis-
tal end of axons and represent the “forward” direction of the growth cone.
This polarized organization of microtubule structure is established as early as
the axon is formed. This “proximal-distal” growth cone axis controls not
only the direction of microtubule polymerization (and depolymerization)
but also the direction of vesicular trafficking.
Second, actin filaments also show polarity. The plus ends of actin fila-
ments (barbed ends) point to the tips of filopodia and the minus ends point
to the inside of the growth cone. Actin filaments undergo retrograde flow
with monomers moving inside the growth cone due to treadmilling ( Yang,
Zhang, Pollard, & Forscher, 2012 ).
Third, endo- and exocytosis are found polarized in growth cones and
involved in navigation ( Itofusa & Kamiguchi, 2011 ). Whether endo- and
exocytosis can provide sufficient membrane translocation to turn growth
cones still needs experimental evidence. However, the existence of pola-
rized membrane trafficking suggests that this could at least be a mechanism
of setting up an asymmetric signaling gradient within the growth cone.
Based on the aforementioned polarized microtubule and actin organiza-
tion and membrane trafficking ( Etienne-Manneville, 2011; Itofusa &
Kamiguchi, 2011 ),
the growth cone is a highly polarized structure.
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