Background: Proper binocular vision depends on the routing at the optic chiasm of the correct proportion of retinal ganglion cell (RGC) axons that project to the same (ipsilateral) and opposite (contralateral) side of the brain. The ipsilateral RGC projection is reduced in mammals with albinism, a congenital disorder characterized by deficient pigmentation in the skin, hair, and eyes. Compared to the pigmented embryonic mouse retina, the albino embryonic mouse retina has fewer RGCs that express the zinc-finger transcription factor, Zic2, which is transiently expressed by RGCs fated to project ipsilaterally. Here, using Zic2 as a marker of ipsilateral RGCs, Islet2 as a marker of contralateral RGCs, and birthdating, we investigate spatiotemporal dynamics of RGC production as they relate to the phenotype of diminished ipsilateral RGC number in the albino retina.
Results: At embryonic day (E)15.5, fewer Zic2-positive (Zic2+) RGCs are found in the albino ventrotemporal (VT) retina compared with the pigmented VT retina, as we previously reported. However, the reduction in Zic2+ RGCs in the albino is not accompanied by a compensatory increase in Zic2-negative (Zic2-) RGCs, resulting in fewer RGCs in the VT retina at this time point. At E17.5, however, the number of RGCs in the VT region is similar in pigmented and albino retinae, implicating a shift in the timing of RGC production in the albino. Short-term birthdating assays reveal a delay in RGC production in the albino VT retina between E13 and E15. Specifically, fewer Zic2+ RGCs are born at E13 and more Zic2- RGCs are born at E15. Consistent with an increase in the production of Zic2- RGCs born at later ages, more RGCs at E17.5 express the contralateral marker, Islet2, in the albino VT retina compared with the pigmented retina.
Conclusions: A delay in neurogenesis in the albino retina is linked to the alteration of RGC subtype specification and consequently leads to the reduced ipsilateral projection that characterizes albinism.