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A
Live imagining
Fix tissue
Bulk labeling
eye
ablated
lobe
optic
tectum
ipsi
contra
Ipsilateral axon branching
B
Dominated by
contralateral axons
Lost branches
Transient branches
Stabilized branches
Dominated by
ipsilateral axons
C
Behavior of added ipsilateral branches
Territory
Stabilized
ipsi
contra
Lost
0
10
20
30
40
50
60
70
Fraction of branches (%)
FIGURE 9.12 Selective addition and retraction of axon branches during competition for postsynaptic
space. A. The retinotectal projection in Xenopus tadpoles is normally to the contralateral tectum, but ablation
of one tectal lobe causes RGC axons from the ipsilateral eye to innervate the spared tectum. A single RGC
axon is labeled with a fluorescent dye and imaged over an eight-hour period. Following this, the brain is
fixed, and the projection from each eye is completely labeled with two different dyes (purple and yellow)
such that the innervation pattern in the tectum is evident. B. Time-lapse images (top) and drawings (bottom)
of a single ipsilateral eye axon in a living tadpole are shown. Branches are color coded for their behavior
during the recording period: lost branches (green), transient branches (red), and stabilized branches (blue).
In this case, the largest number of stabilized branches was formed in the region dominated by ipsilateral
axons. C. New ipsilateral axon branches are more likely to remain for the entire recording period (i.e., stabi-
lized) if they are formed in territory dominated by ipsilateral axons. In contrast, they are preferentially elim-
inated from the contralateral territory. (From Ruthazer et al., 2003)
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