Photoreceptor and glial markers in human embryonic retina and in human embryonic retinal transplants to rat retina.

The purpose of this study was to compare the development of photoreceptor and glial cells in human embryonic retinal transplants with the development of normal human embryonic retina (13-20 weeks postconception). Human embryonic retinal cells (donor age 6-11 weeks postconception) were transplanted to the retinas of adult immunosuppressed rat hosts. Host animals were killed when the transplants were of 13-37 weeks total age (donor age+survival time after surgery). Immunohistochemistry was performed with antibodies specific for neuron-specific enolase (NSE), synaptophysin (SYN), cone-specific opsins, rhodopsin, rod alpha-transducin, S-antigen, vimentin, cellular retinaldehyde-binding protein (CRALBP) and glial fibrillary acidic protein (GFAP). With regards to photoreceptors, NSE and SYN immunoreactive cones were seen in transplants from 14-16 weeks of age, but cone opsin immunoreactivity was not seen until 25 weeks. Developing graft rods became S-antigen immunoreactive at 17-18 weeks. At 20 weeks, inner segments and some cell somas of graft rods stained faintly for alpha-transducin and rhodopsin. At 31 and 37 weeks, inner and outer rod segments were intensely labelled for the rod-specific antigens. The grafts exhibited areas of varying maturation with different staining intensities. Concerning the glial cells, vimentin immunoreactivity was seen in the earliest transplants studied (total age 14-16 weeks), but only in stages older than 19 weeks was the immunoreactivity of graft Müller cells comparable in intensity to those of the host retina. Host Müller cells were immunoreactive for GFAP near the lesion site at all times. At 20 weeks, some GFAP immunoreactive processes were seen inside the graft, apparently coming from the host retina. At 25 weeks, faintly stained Müller cells intrinsic to the graft were observed, indicating a gliosis within the graft. Graft Müller cells were first seen to express CRALBP immunoreactivity at 19-20 weeks and, at 25 weeks, intense immunoreactivity was seen in the transplant, mostly in regions near the host. In the transplants only the Müller cells were stained, whereas both Müller and retinal pigment epithelium cells were CRALBP immunoreactive in the host retina. The development of human embryonic retinal transplants appears to parallel approximately normal in utero development. Transplant cones, rods and Müller cells all express their cell-specific proteins. The photoreceptors develop both inner and outer segments and contain several essential proteins for processing light. The transplants can reach a degree of maturity comparable to newborn retina.