Survival and integration of neural retinal transplants in rd mice

PURPOSE: To examine the ultrastructure of the rd retina after transplantation of small micro-aggregates of neural retina in order to determine their survival and integration with the host retina and for sites of communication between transplant and host neurons. METHODS: Neonatal micro-aggregates from transgenic mice expressing a LacZ gene reporter gene in their rods were transplanted into the subretinal space of transgenic rd mice expressing a LacZ reporter gene in their rod bipolar cells. The mice were killed at various times after transplantation surgery and studied by light and electron microscopy. RESULTS: Retinal transplants survived well, as long as 8 months, without signs of rejection and were well integrated into the host retina. Cell bodies of transplanted rods made membrane-to-membrane contacts with rod bipolar cells of the host at areas where there were gaps in the host external plexiform layer. One synaptic process of a transplanted rod was found on the vitreal side of the host's external limiting membrane. In two cases, a postsynaptic process in a transplanted rod spherule contained an Xgal label, implying that it belonged to a host rod bipolar. There was evidence of extension of processes between host and transplant retinas involving astrocytic rather than neural structures. CONCLUSIONS: Retinal allografts to the subretinal space of rd mice survive indefinitely. Close but non-synaptic contacts occur between transplant and host neurons that could allow ephaptic communication between these two retinas. Evidence of synaptic contacts between transplant and host was difficult to find.