Healthcare and Medicine Reference
In-Depth Information
A Growth cone at fascicle
E
E
S
D
D
S
F
T
B
C
S
10:30
10:50
11:10
11:30
11:50
12:30
4:00
A
G
A
B Growth cone at choice point
H
B
C
FIGURE 5.6 Early observations of growth cones. A. In the late
1800s, Ramon y Cajal saw expansions of axons near the ventral
midline of the chick neural tube. B. In the 1930s, Speidel observed
growing nerve fibers tipped with axons in the frog tail. C. In the
early 1900s, Harrison grew neural explants in culture and watched
them extend axons tipped with motile growth cones. (From Ramon
y Cajal, 1890; Harrison, 1910; Speidel, 1941)
C Growth cone in target
are generally simple and more bullet-shaped with few
filopodia (Figure 5.7A). Growth cones get particularly
complex, when they arrive choice points along the
pathway (Figure 5.7B). Upon reaching an appropriate
target of innervation, growth cones once again alter
their shape. They display thin and highly branched
terminals (Harris et al., 1987; Halpern et al., 1991)
(Figure 5.7C). Static observations from fixed tissue
provide neither the rate of growth nor the sampling
strategy that growth cones employ as they make
decisions. While Cajal's remarkable visual memory
and artistry allowed him to produce accurate draw-
ings of growth cones nearly a century ago, simple
video microscopy systems now permit most laborato-
ries to view and measure the growing axon.
Over the past 20 years, the standard compound
microscope has been embellished with a number of
technical wonders. These advances include low light-
sensitive video cameras, scanning laser illumination,
and specialized image processing software. Coupled
with the recent introduction of fluorescent labels that
are rapidly transported along living axons, one is now
able to produce time-lapse movies of process out-
growth and innervation (Glover et al., 1986; Honig and
Hume, 1986; Harris et al., 1987). Vital fluorescent dyes
are commonly used to observe living processes.
The lipophilic dyes, DiI and DiO, intercalate into the
neuron membrane and diffuse rapidly down axonal
processes; they are then visualized with epifluorescent
Target region
Optic tract
Tectum
D
60
40
20
0
FIGURE 5.7 The different shapes of a growth cone. A. Growth
cones that are fasciculating with other axons tend to be simple and
have few filopodia. B. Growth cones at choice points are complex
with filopodia and lamellipodia. C. Growth cones in the target
region become even more complex and sprout backbranches along
the axon shaft. D. Speed versus location plot for retinal axons in
Xenopus embryos. They grow at about 60 mm/hr along the optic
tract. When they arrive at the tectum, they slow down dramatically.
illumination. Fluorescent proteins of different colors
such as Green Fluorescent Protein, (GFP), RFP (Red),
and YFP (Yellow) are engineered variants of natural
proteins found in certain types of jellyfish. When a
growing neuron is transfected with such a GFP gene,
it creates its own fluorescent protein. By genetically
Search Pocayo ::




Custom Search