Fig. 5. Zic2-induced turning of ipsilateral retinal axons away from the midline is mediated by both EphB1-dependent and -independent pathways. (A) Quantitative RT-PCR for EphB1 performed on non-electroporated or electroporated mouse retinas at E13.5 with CAG-EGFP or with CAG-Zic2/CAG-EGFP plasmids. EphB1 levels significantly increase after ectopic expression of Zic2 in the center of the retina by in utero electroporation, as compared with control retinas. More than ten retinas for each condition were pooled per experiment. Data from seven different experiments are shown. Error bars indicate s.e.m.; ^*P<0.005. (B) 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 in each particular case. (a) In wild-type embryos electroporated at E13.5 with Zic2/EGFP, many of the green axons originating from the central retina turn at the midline. (b) In EphB1^-/- embryos ectopically expressing only EGFP in the central retina, no ipsilateral axons are seen at the chiasm. (c) When Zic2/EGFP are ectopically expressed in the central retina of EphB1-null embryos at E13.5, the number of axons projecting ipsilaterally does not change as dramatically as when wild-type embryos are electroporated with Zic2/EGFP, but a significant proportion of axons that project ipsilaterally can be still detected. White arrows indicate ipsilateral projections. 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.