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. Emb ryonic rat (E16-22) or human (9-13 weeks) retinal cells were transplan ted to adult rats. Normal Long-Evans rats received rat transplants. Th e hosts for human transplants were athymic nude rats. After varying su rvival times (3 to 11 months), animals were perfused with 4% paraforma ldehyde (sometimes with added 0.1% glutaraldehyde). Glass microneedles , coated with DiI (a carbocyanine dye) were placed into the transplant s which were then stored at room temperature in 2% paraformaldehyde fo r 3-15 months. This filled the cells that had processes in the area wh ere the needle had been placed. Gelatin-embedded eyecups were cut on a vibratome. DiI-labeled transplant cells exhibited fiber outgrowth int o the host retina. After photoconversion of the dye to an electron-den se precipitate, these neuronal processes could be followed with better resolution than with fluorescence. Occasionally, host cells could als o be labeled by DiI placed into the graft; indicating fiber ingrowth o f host fibers into the transplants. Selected photoconverted sections w ere embedded for electron microscopy. Synapses could be found along tr ansplant processes that had grown into the host inner plexiform layer. These results indicate that neuronal fibers originating from embryoni c retinal transplants form synapses in the host retina. (C) 1995 Acade mic Press, Inc.