The Questions of Developmental Biology

Such nerve outgrowth is led by the tip of the axon, called the growth cone (see Figure
12.23). The growth cone does not proceed in a straight line, but rather "feels" its way along the
substrate. The growth cone moves by the elongation and contraction of pointed filopodia called
microspikes. These microspikes contain microfilaments,
which are oriented parallel to the long axis of the axon. (This
mechanism is similar to the one seen in the filopodial
microfilaments of secondary mesenchyme cells in
echinoderms; see Chapter 8) Treating neurons with
cytochalasin B destroys the actin microspikes, inhibiting
their further advance (Yamada et al. 1971; Forscher and
Smith 1988). Within the axon itself, structural support is
provided by microtubules, and the axon will retract if the
neuron is placed in a solution of colchicine. Thus, the
developing neuron retains the same mechanisms that we
have already noted in the dorsolateral hinge points of the
neural tube; namely, elongation by microtubules and apical
shape changes by microfilaments. As in most migrating cells,
the exploratory filopodia of the growth cone attach to the
substrate and exert a force that pulls the rest of the cell
forward. Axons will not grow if the growth cone fails to
advance (Lamoureux et al. 1989). In addition to their
structural role in axonal migration, the microspikes also have
a sensory function. Fanning out in front of the growth cone,
each microspike samples the microenvironment and sends
signals back to the cell body (Davenport et al. 1993). As we
will see in Chapter 13, the microspikes are the fundamental
organelles involved in neuronal pathfinding.