Postnatal development of the ipsilaterally projecting retinal ganglion cells in normal rats and rats with neonatal lesions.

The postnatal development of normal and anomalous uncrossed retinofugal projections in albino rats was studied using horseradish peroxidase and lectin-conjugated horseradish peroxidase as retrograde neuronal tracers. In normal rats, the number of ipsilaterally projecting retinal ganglion cells (IPRGCs) decreased continuously from more than 3000 cells on the day of birth (day 0) to slightly more than 1000 on postnatal day 5. In contrast, the number of IPRGCs in rats which received either unilateral eye enucleation or thalamectomy at birth increased abruptly to more than 4000 after 24 h on postnatal day 1, thereafter the number decreased rapidly reaching an adult level of slightly more than 2000 on postnatal day 5. The overall pattern of changes of the number, density and distribution of the IPRGCs was similar in rats which received eye enucleation or thalamectomy, although minor differences regarding the number of cells labelled in the temporal and nasal parts of retinas in rats with different lesions were detected. These findings imply that a neonatal lesion such as monocular enucleation or unilateral thalamectomy may not only result in retention of normally transient IPRGCs but also cause an increased number of IPRGCs by misrouting, rerouting or collateral sprouting of optic axons at the optic chiasma that could significantly affect the developing optic pathway. Furthermore, the differential effects of the two types of neonatal lesions upon the IPRGCs provide further evidence to previous findings that different mechanisms might be involved in generating the anomalously enlarged uncrossed retinofugal projections in rodents.