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A
Embryonic
frog brain
A
B
Optic
tectum
Neuron
Retina
Optic
tract
Lens
Optic
nerve
head
Optic
nerve
Optic
chasm
Ta r get
C
B
Purkinje
Park
Kinase
Court
Cut
Synapse
School
Axon of retinal ganglion
cell still navigating to tectum
Cerebellum
Center
FIGURE 5.5 Axons grow from the retina to the tectum using
their growth cones to guide them. A. A dorso-lateral view of the
embryonic frog brain. B. Images of single retinal ganglion cell axons
through the plane of section indicated in (A) shows that they as they
grow to the tectum, they are always tipped by active growth cones.
C. When the axon is separated from the cell body by cutting the optic
stalk, time-lapse imaging shows that isolated growth cones still
grow along the correct pathway. (After Harris et al., 1987)
FIGURE 5.4 An axon growing to its target (A) is like a driver
navigating through city streets (B). See text for details.
cell bodies continue to grow (Bray et al., 1978) and
even navigate correctly (Harris et al., 1987) (Figure 5.5).
the 1930s, Speidel observed growth cones live in vivo
at the ends of growing sensory axons in the trans-
parent growing tail fin of a frog (Speidel, 1941). He
followed single-growth cones over days, even weeks,
and he watched how they responded to obstacles
and injuries. He was impressed with the ability of
these growth cones to change directions, branch, and
respond to stimulation. Speidel described the growth
rates as high as 40 mm/hr (Figure 5.6C). Such growth
cones in vivo advanced in an amoeboid fashion and
showed a number of delicate transient processes, con-
sistent with Harrison's earlier in vitro observations.
Growth cones assume several morphologies and
may travel at different speeds as they navigate through
different parts of their pathways (Tosney and
Landmesser, 1985; Bovolenta and Mason, 1987).
Pioneer axons that are growing straight ahead have
more traditional growth cones with several active
filopodia and a few lamellipodia. The growth cones of
follower axons that grow along earlier pioneer axons
THE GROWTH CONE
Growth cones were recognized more than a
hundred years ago by the famous Spanish neuro-
anatomist, Ramon y Cajal, as expansions at the tips of
axons in fixed embryonic material. He imagined the
growth cone as a sort of soft battering ram that extend-
ing axons used to force their way through the packed
cells of the embryonic brain (Ramon y Cajal, 1890)
(Figure 5.6A). In 1910, Ross Harrison took pieces
of embryonic neural tube and put them into tissue
culture where he saw axons tipped with growth cones
growing live across a microscope slide (Harrison,
1910). He was astounded to see the growth cones move
and wiggle in real time. They seemed to feel their way
along the surface, sending out long, thin filopodia and
forming veils between the filopodia (Figure 5.6B). In
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