Fiber and synaptic connections between embryonic retinal transplants and host retina

The aim of this study was to investigate (a) whether embryonic retinal transplants can sprout fibers into a lesioned adult host retina and (b) if these fibers established synaptic connections with the host. Embryonic rat (E16-22) or human (9-13 weeks) retinal cells were transplanted to adult rats. Normal Long-Evans rats received rat transplants. The hosts for human transplants were athymic nude rats. After varying survival times (3 to 11 months), animals were perfused with 4% paraformaldehyde (sometimes with added 0.1% glutaraldehyde). Glass microneedles, coated with DiI (a carbocyanine dye) were placed into the transplants which were then stored at room temperature in 2% paraformaldehyde for 3-15 months. This filled the cells that had processes in the area where the needle had been placed. Gelatin-embedded eyecups were cut on a vibratome. DiI-labeled transplant cells exhibited fiber outgrowth into the host retina. After photoconversion of the dye to an electron-dense precipitate, these neuronal processes could be followed with better resolution than with fluorescence. Occasionally, host cells could also be labeled by DiI placed into the graft, indicating fiber ingrowth of host fibers into the transplants. Selected photoconverted sections were embedded for electron microscopy. Synapses could be found along transplant processes that had grown into the host inner plexiform layer. These results indicate that neuronal fibers originating from embryonic retinal transplants form synapses in the host retina.