Ultrastructure of human retinal cell transplants with long survival times in rats

Human fetal retinas (6-12 weeks post-conception) were obtained from elective abortions, transplanted to rat retinas and examined by electron microscopy. The oldest transplants that form the basis of this report were obtained 40 and 41 total weeks post-conception. The host rats were immunosuppressed with cyclosporin A. The transplants developed according to their intrinsic, genetically determined timetable. The development was heterogeneous with some parts showing almost normal differentiation and others, little. Both rods and cones developed with inner and outer segments and synaptic terminals. In regions corresponding to the inner plexiform layer, bipolar cell processes were seen in the typical dyad arrangement. Likewise, amacrine cell processes formed typical conventional synapses. Serial synapses were seen, engaging amacrine cell synapses as well as a few reciprocal synapses at the bipolar cell dyads. Monad-type synaptic complexes, a sign of immaturity, were common in bipolar cell processes. Similarly, incompletely differentiated synapses of both the amacrine and bipolar cell types were often observed. Ganglion cell processes could not be identified with certainty. A structure with morphological characteristics similar to the inner limiting membrane was noted to form inside the transplant. Both epi-retinal and sub-retinal transplants were obtained. Transplant cells touched host photoreceptor cells or pigment epithelium without any obvious specializations. The host pigment epithelium microvilli were absent adjacent to the graft. However, graft cells did appear in the host retina, and nerve cell processes were observed to cross the membrane separating the transplant and host.