Transmission electron micrographs of the outer plexiform layer in Crx-/- retinas at P21. (A) Crx+/+ rod spherules contained a single invaginating synapse with one ribbon complex. The spherule was a blunt or club-shaped structure. (B) Crx+/+ rod terminals contained a single ribbon structure (r). Two processes, from horizontal cells (h), contacted the rod laterally. An additional process, the postsynaptic bipolar dendrite (b), was situated more centrally. Terminals were filled with synaptic vesicles. One coated vesicle originatinf from the cell membrane was observed (arrow). (C) In the OPL of Crx-/- retinas, photoreceptor terminals appeared highly disorganized. Putative terminals containing synaptic vesicles and ribbon-like structures were apparent (arrows), suggesting at least limited generation of synapse components. However, well formed spherules and pedicles were not observed. Further, many terminals appeared to contain multiple ribbon-like structures (r). The majority of these ribbons were not associated with the synaptic membrane, but instead were found free floating and, in some instances, were perinuclear (D, arrow). H, horizontal cell; B, bipolar cell; N, nucleus; r, ribbon. (A) Scale bar = 500 nm, (B) Scale bar = 250 nm, (C and D) Scale bar = 500 nm.