Bilateral branching contributes minimally to the enhanced ipsilateral projection in monocular Syrian golden hamsters

Studies of the plasticity of retinal projections following neonatal eye ablations in rats and hamsters have demonstrated an enhanced ipsilateral projection which affects both the terminal fields and the number of ipsilaterally projecting cells. The enhancement has been attributed to an excess of bilateral branching by single optic nerve axons, but this claim was not supported by the present study. Double retrograde labels were used to quantify the ipsilateral enhancement and measure the extent of bilateral branching in hamsters with neonatal eye removals. In 19 normal Syrian golden hamster retinas the average number of bilaterally projecting retinal ganglion cells (BPRGCs) was 7.4 (SD = 3), and the average number of ipsilaterally projecting retinal ganglion cells (IPRGCs) was 1349 (SD = 478) (Hsiao, K., G. M. Sachs, and G. E. Schneider (1984) J. Neurosci. 4: 359-367). In nine retinas of hamsters with removal of one eye at birth, the average number of BPRGCs was 25.3 (SD = 9.2), and the average number of IPRGCs per retina was 2036 (SD = 414). Although the increase in BPRGCs is statistically significant, it accounted for only 2 to 3% of the 51% increase in the number of IPRGCs. Various possible mechanisms underlying the formation of the enhanced projection are enumerated, and it is concluded that the predominant mechanism is a reduction of cell death in the developing population of IPRGCs. The responses of IPRGCs and BPRGCs to early eye enucleations were also examined with respect to age at enucleation and cell size. No difference in size was found between IPRGCs and BPRGCs in monocular hamsters compared to normal hamsters.(ABSTRACT TRUNCATED AT 250 WORDS)