Developmental changes in the pattern of retinal projections in pigmented and albino rabbits.

The distribution of retinal axons and/or terminals in the retino-recipient nuclei of pigmented and albino rabbits varying in age from the 24th postconceptional day (24PCD) to adulthood was examined following unilateral intraocular injections of the enzyme horseradish peroxidase. Both in pigmented and albino rabbits contralateral retinal axons and/or terminals in the dorsal and ventral lateral geniculate nuclei (DLG and VLG), superior colliculi (SC), pretecta (PT) and accessory optic tract nuclei (AON) were already present on 24PCD. In the period 26-30PCD the contralateral projection occupied the entire volume of the DLG, VLG and SC. Although 32PCD (the day of birth) the proportions of the volumes of DLG and VLG occupied by the contralateral projections were slightly reduced, their volume continued to increase in absolute terms up to adulthood. In pigmented rabbits the ipsilateral projections to all retino-recipient nuclei were most dense and extensive on 26PCD. From 26PCD, the relative extent of the ipsilateral projections was gradually reduced, but a reduction in their absolute extent did not become evident until 32PCD. By 32PCD the ipsilateral projection to the AON had disappeared completely. The distribution of ipsilateral axons and/or terminals and the relative proportion of the nuclei occupied by the ipsilateral projection in all other retino-recipient nuclei had become adult-like by 34PCD. In albino rabbits only a sparse ipsilateral projection to the presumptive superficial collicular layers was present on 24PCD. In the remaining retino-recipient nuclei an ipsilateral projection was present on 26PCD. From 26PCD the relative extent and from 30PCD the absolute extent of ipsilateral retinal axons and/or terminals was gradually reduced. The relative extent of the ipsilateral projection had become almost adult-like by 34PCD. Throughout development ipsilateral projections in albinos were consistently less dense and less extensive than those in pigmented rabbits, and unlike in pigmented rabbits, the ipsilateral projections to the VLG and PT were only transient. The differences between the two strains in the pattern of retinofugal projections were further enhanced during the period of segregation of the ipsilateral and contralateral projections. Considering the fact that in both strains there is a partial correspondence between the period in which the spatial extent of the ipsilateral projections is reduced and the period of retinal ganglion cell (RGC) death, it is likely that RGC death plays a role in the process of segregation of the retinal afferents into ocular domains. However, our data suggest that other mechanism(s) also play an important role in the process.