Ultrastructure of retinal cells transplanted to the rabbit choroid.
OBJECTIVE
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 ultrastructure of such transplants, looking for features that might explain the phenomenon.


METHODS
Rabbit retinal tissue fragment transplants were produced with previously described methods. The donor age was 15 days and the transplants were examined by standard electron microscopy when the transplants were 1-2 months (3 transplants) or 3-4 months old, of postconception age (3 transplants).


RESULTS
The transplants survived and developed as expected from previous observations. Rosettes were seen, but they were not as common as in transplants produced with the same technique in the subretinal space of rabbits. Photoreceptor outer segments were not seen in the transplants. At 1 month, there was an incomplete sheath of Müller cells around the transplants, and a complete one at 3-4 months. There was also a well-developed basement membrane around the transplant at 3-4 months, but less so at 1 month. Blood vessels did not enter the transplant. The fenestrations in the choriocapillaris were not affected as long as the pigment epithelium was normal.


CONCLUSIONS
The enclosure of the transplants by Müller cells might help to insulate them from the immune system of the host, but it is a late phenomenon and it is not likely to have much effect for the first few weeks after the transplantation. We suspect that either the rabbit choroid is an immune-privileged 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.
Lütjen-Drecoll
E
E
Department of Anatomy II, University of Erlangen-Nürnberg, Germany.
Bergström
A
A
Ehinger
B
B
D000818:Animals; D001485:Basement Membrane / Q000648:ultrastructure; D002455:Cell Division; D017690:Cell Transplantation; D002829:Choroid / Q000601:surgery* / Q000648:ultrastructure; D006085:Graft Survival; D020419:Photoreceptor Cells, Vertebrate / Q000648:ultrastructure; D011817:Rabbits; D012160:Retina / Q000166:cytology* / Q000648:ultrastructure; D014184:Transplantation, Homologous