Reversed ratio of color-specific cones in rabbit retinal cell transplants

Recently, we have reported on the emergence of various retinal cell types in embryonic rabbit retina transplanted to adult rabbits. When comparing the relative numbers of the spectrally different cone types in the transplants to those in the host or age-matched control retinas, a surprising shift was observed. While in the normal rabbit retina the middle-wavelength-sensitive (M) cones are considerably more abundant than the short-wave-sensitive (S) cones, the S/M cone ratio was found to be the opposite in the graft. The number of rosettes containing only S-cones in high density was found to be considerably higher than that of M-cone rich rosettes. The number of S-cones also exceeded that of the M-cones in each rosette that contained both cell types. Our results were obtained from the systematic immunocytochemical analysis of 15 different transplants derived from transplantations of embryonic rabbit retinas into adult hosts of the same species. The emergence and proportion of the two cone types were followed between 14 and 63 days after transplantation (between 29 and 78 postconceptional days of the donor tissue). Sections from various parts of the transplants were reacted with the monoclonal antibodies COS-1 and OS-2, specific for the middle- and short-wavelength-sensitive cones, respectively. The explanation for the reverse cone ratio in these transplants is not known yet, however, the observed phenomenon may indicate differences between the specification of the two basic cone types.