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morphology of single LGN axon terminals have been
examined in the visual cortex of cats and primates,
and individual LGN terminals display a significant
refinement during postnatal development, consistent
with the autoradiographic studies (Florence and
Casagrande, 1990; Antonini and Stryker, 1993).
The development of retinal arbors in the LGN illus-
trates how targeting errors are prevalent, yet subtle in
appearance. In mammals, retinal axons grow to the
correct area of the LGN from the outset, but they also
make two kinds of inappropriate targeting projections.
First, nearly all of the axons produce a few small col-
laterals, about 10 to 20 mm in length, in a part of the
LGN that will eventually be innervated solely by
axons from the other eye. Interestingly, the formation
of an eye-specific innervation pattern in the LGN
begins prior to visual experience. In the cat, these side
branches are selectively eliminated in utero (Figure
9.6A and B).
A second phase of refinement occurs postnatally
when retinal terminals become more focused within a
portion of the correct eye-specific layer (Figure 9.6C).
Retinal ganglion cells that have small visual receptive
fields (cf. X-cells) decrease the width of LGN tissue
that they innervate by a small (60 mm) but significant
amount (Sur et al., 1984; Sretevan and Shatz, 1986).
This would be the equivalent of pulling into your
neighbor's driveway after completing a trip from
hundreds of miles away. Thus, immature projections
are numerous but modest in size.
Are synapses actually being formed by these tran-
sient projections? The answer can be found by first
filling entire axons with a tracer, such as horseradish
peroxidase, and then examining the terminals with an
electron microscope. In fact, labeled presynaptic ter-
minals have been found in parts of the target where
they never remain in the adult, indicating that these
structurally mature synapses will eventually have to
be broken (Reh and Constantine-Paton, 1984; Camp-
bell and Shatz, 1992).
The natural elimination of neuromuscular synapses
can be observed over several days in living animals
(Figure 9.7). Two strains of mice are genetically
engineered to express a unique fluorescent protein in
their motor neurons. When the two strains are mated,
muscle cell fibers can be identified that are co-inner-
vated by motor terminals that contain one or both flu-
orescent proteins (Walsh and Lichtman, 2003). By
imaging the muscle fiber in vivo over successive days,
one can observe the withdrawal of one synapse and
the enlargement of the other to occupy the entire end-
plate (termed t akeove r).
Just how commonplace is axonal refinement and the
loss of synaptic connections? It is found in a great
A
LGN
HRP
chiasm
Eyes
Eye-specific layers: prenatal loss of
branches in the wrong layer
B
Topography: postnatal loss of
exuberant branches
C
FIGURE 9.6 Development of retinal ganglion cell terminals in
the cat lateral geniculate nucleus (LGN). A. When individual retinal
fibers are labeled with horseradish peroxidase (HRP) at embryonic
days 43-55, they have many side branches in the inappropriate layer
of LGN (arrows). B. By birth, most of the side branches have been
eliminated, and terminals arborizations have been restricted to the
correct layer. However, the terminal zone remained wider in the eye-
specific lamina at 3-4 weeks postnatal (arrows). C. When fibers of
retinal X-cells were filled in adult cats, they were found to have
retracted (black arrows). (Adapted from Sur et al., 1984; Sretavan
and Shatz, 1986)
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