Graft-host connections in long-term full-thickness embryonic rabbit retinal transplants

PURPOSE. To establish neuronal connections in the rod and cone pathway betw een laminated rabbit retinal transplants and the host retina. METHODS. Fourteen adult rabbits received a complete full-thickness embryoni c transplant. After survival times of 3 to 10 months, the retinas were stud ied under light microscope and with immunohistochemistry. Antibodies agains t protein kinase C (PKC), parvalbumin, and calbindin were used to label rod bipolar cells, AII amacrine cells, and cone bipolar cells, respectively. T he AB5 antibody was used to label ganglion cells. RESULTS. The transplants displayed laminated morphology with layers paralle l to the host retinal pigment epithelium. In the oldest specimens (10 month s after surgery), laminated layers of graft and host approached each other and almost reconstructed the normal retinal appearance. The ganglion and co ne bipolar cells of the host survived well, as was seen with AB5 and calbin din double-labeling. Connections between cone bipolar cells in the graft an d ganglion cells in the host were not common. PKC-labeled rod bipolar cells and parvalbumin-labeled AII amacrine cells of host and graft showed sprout ing activity directed toward an intermediate plexiform layer located betwee n the graft and host. In specimens double-labeled with PKC and parvalbumin, this intermediate plexiform layer was seen to contain numerous PKC- and pa rvalbumin-labeled processes. Direct connections between rod bipolar and AII amacrine cells in host and graft were seen in the 10-month specimens. CONCLUSIONS. Full-thickness embryonic transplants survive for at least 10 m onths, and normal laminated morphology develops. Host and graft fuse and to gether contribute nerve cell processes to an intermediate plexiform layer. Direct graft-host contacts are also present between neuronal types that in the normal retina participate in the rod pathway.