Fig. 5. Normal retinal axon projections in Brn3b^3a/3a knock-in mice. Lipophilic dye tracings of the proximal visual pathway in E17.5 mice. (A) Schematic view of the visual system at the ventral surface of the brain. Anterior is towards the top, posterior towards the bottom. Contralateral pathway is shown in red and ipsilateral pathway in blue. The area within the broken lines is imaged with a Nikon fluorescent dissecting microscope. (B) Wild-type mouse. The majority of axons cross the optic chiasm towards the contralateral pathway. Certain axons from ventrotemporal quadrant of the retina do not cross the optic chiasm and form the ipsilateral projection pathway. (C) Brn3b-null mouse. Abnormally high percentage of axon fibers travel towards the ipsilateral optic tract and alongside the left optic nerve to the left eye. (D) Brn3b^3a/3a knock-in mouse. Normal axon projection pattern is restored by Brn3a^ki expression in the absence of Brn3b. ON, optic nerve; OT, optic tract; Ipsi, ipsilateral projection; Contra, contralateral projection. Asterisk indicates the optic chiasm. Scale bar: 200 µm.