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Cross section of growth cone on adhesive substrate
Cross section of growth cone on less adhesive substrate
FIGURE 5.17 Growth cones and adhesion. A. On a very adhe-
sive substrate growth cones are flattened, have lots of filopodia, and
do not move rapidly (top). On a less adhesive substrate, growth
cones are more compact, rounded, have fewer processes, and often
move more quickly. B. Neurites in culture given a choice between
an adhesive and a nonadhesive substrate will tend to follow the
adhesive trails.
Artificial substrate
in cut commissure
FIGURE 5.16 Axons may follow mechanical pathways. A. The
axons of neurons on a dried collagen matrix growing through the
cracks. B. Axons of the corpus callosum can use an artificial sling to
grow from one side of the brain to the other.
have been moved proximally, suggesting that axons
readily grow onto more adhesive membranes but will
not onto less adhesive ones (Nardi, 1983).
To measure growth cone attachment to various cell
adhesion molecules, a pipette can be positioned in
a culture dish containing growing axons, and culture
media can be squirted at the growth cones in an attempt
to “blast” them off the substrate (Figure 5.18). The
longer the growth cone stays attached to the surface, the
stronger its adhesion must be. Neurite growth rate can
then be measured on these same substrates for compar-
ison (Lemmon et al., 1992). Moreover, when axons are
given a choice between pairs of naturally occurring cell
adhesion molecules, they do not necessarily grow pref-
erentially on the more adhesive substrate. Interestingly,
it turns out that the most adhesive substrates, such as
the lectin concanavalin A, are not good supporters of
axon outgrowth. In fact, growth cones tend to get stuck
on such a surface. They seem unable to retract their
filopodia efficiently and thus grow very slowly. Thus,
the ability to detach is just as important as the ability to
attach, and the molecules that have just the right
amount of adhesion, not too little and not too much, are
best able to support axon growth.
when they are plated on polycationic substrates, such
as polylysine, that stick well to negatively charged bio-
logical membranes, the neurons are much more likely
to initiate axonal outgrowth. The growth cones of such
neurons are flattened against the substrate as though
they adhere very strongly to it (Figure 5.17). When
axons are grown on a patterned dish that offers them
the choice between a nonadhesive substrate, versus an
adhesive substrate, the growth cones follow the adhe-
sive trail (Letourneau, 1975; Hammarback et al., 1985)
(Figure 5.17). These findings led to the idea that
growth cones might simply follow gradients of adhe-
sion in the developing organism. In fact, this may be
the case for some neurons.
In the wing of the moth, sensory axons of the wing
grow in the distal to proximal direction along a basal
lamina of epithelial cells (Nardi and Vernon, 1990).
Examination of this epithelium microscopically shows
that it becomes increasingly complex toward the base.
Transplantation of the epithelium suggests that this
change corresponds to a gradient. Axons readily cross
onto a transplant that has been moved in the proximal
to distal direction, but avoid distal transplants that
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