Ultrastructure of retinal cells transplanted to the rabbit choroid

Purpose: Allogenic rabbit-to-rabbit retinal cell transplants survive in the choroid, which is not as expected because it has not been shown that this is an immune-privileged site. We have therefore examined the ultra structur e of such transplants, looking for features that might explain the phenomen on. Methods: Rabbit retinal tissue fragment transplants were produced with previously described methods. The donor age was 15 days a nd the transplant s were examined by standard electron microscopy when the transplants were 1 -2 months (3 transplants) or 3-4 months old, of postconception age (3 trans plants). Results: The transplants survived and developed as expected from p revious observations. Rosettes were seen, but they were not as common as in transplants produced with the same technique in the subretinal space of ra bbits. Photoreceptor outer segments were not seen in the transplants. At 1 month, there was an incomplete sheath of Muller cells around the transplant s, and a complete one at 3-4 months. There was also a well-developed baseme nt membrane around the transplant at 3-4 months, but less so at 1 month. Bl ood vessels did not enter the transplant. The fenestrations in the chorioca pillaris were not affected as long as the pigment epithelium was normal. Co nclusions: The enclosure of the transplants by Muller cells might help to i nsulate them from the immune system of the host, but it is a late phenomeno n and it is not likely to have much effect for the first few weeks after th e transplantation. We suspect that either the rabbit choroid is an immune-p rivileged site, even though there is no previous direct evidence for this, or that the retinal tissue itself is responsible for the prolonged survival at this site. Copyright (C) 2000 S. Karger AG, Basel.