Photoreceptor plasticity in reaggregates of embryonic chick retina: rods depend on proximal cones and on tissue organization

Plasticity of photoreceptors and their integration into epithelial structur es homologous to an outer nuclear layer (ONL), was investigated in embryoni c chick retinal cell reaggregates by immunohistochemistry using an antibody specific for red plus green cones (RG-cones) and an antibody for rods. If reaggregates are raised in the presence of pigmented epithelium (RPE), comp letely reconstructed, stratified retinal spheres are produced, where all ro ds and cones are integrated into an outer laminar ONL, similar to a normal retina. In the absence of RPE, 'rosetted' spheres form which contain intern al rosettes homologous to an ONL. Only a minor fraction of cones and rods o f 'rosetted' spheres are located within rosettes, while a larger fraction i s diffusely displaced in nonorganized areas, thus, not contributing to an O NL-like epithelium. In both types of spheres, the total percentage of RG-co nes was similar to the in vivo retina, indicating that expression of cones is autonomous. Following cones, after about one day, rods developed only wi thin already existing RG-cone clusters. Thereby, the ratio of rods to RG-co nes increases as the tissue organization decreases: for stratified spheres this ratio is, 0.50 (1 rod/2 cones; similar to mature retina); for rosettes , 0.74 (3 rods/4 cones) and for nonorganized areas, 1.09 (1 rod/1 cone) - a higher ratio under our conditions has never been detected. Thus, rod expre ssion depends strictly on the presence of nearby cones; their relative numb ers are distinctively adjusted according to the cytoarchitecture of the tis sue environment. The biomedical implications of these findings are briefly discussed.