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Fig. 2. Zic2 is sufficient to change axonal behavior at the midline. Left
panels show axonal trajectories of targeted RGCs at the optic chiasm. Middle
panels show the location of the targeted RGCs (green) in retinal whole-mounts
(gray). Right panels are cartoons summarizing the results for each particular
case. (A) In mouse embryos electroporated only with plasmids bearing
EGFP, many green axons originating from the central retina have reached or
crossed the midline (dashed line). (B) In embryos ectopically
expressing Zic2/EGFP in the central retina, many targeted axons have crossed
the midline but in this case some growth cones are positioned at the midline
and a few axons are also observed turning to the ipsilateral side. (C)
In embryos electroporated only with plasmids bearing EGFP at E13.5 and
sacrificed at E17.5, all green axons coming from the central retina cross the
optic chiasm midline. (D) By contrast, embryos ectopically expressing
Zic2 plus EGFP in the central retina show a significant increase in the number
of axons that project ipsilaterally at the chiasm. (E) In embryos
electroporated at E14.5 with plasmids bearing only EGFP and sacrificed at
E17.5, all green axons coming from the central retina cross the midline at the
optic chiasm. (F) The increase in the proportion of axons projecting
ipsilaterally after Zic2 ectopic expression is modest when electroporation is
performed at E14.5, as compared with electroporation at E13.5. Yellow
arrowheads point to the ectopic ipsilateral projection. ON, optic nerve; cOT,
contralateral optic tract; IOT, ipsilateral optic tract; d, dorsal; n, nasal;
t, temporal; v, ventral. Scale bars: chiasm panels, 200 µm; retina panels,
500 µm.
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