Transmission electron micrographs of the outer plexiform layer in Crx-/- retinas. (A) In Crx+/+ retina at P21, newly formed rod spherules were abundant (arrow demonstrates one example). The spherules were club-shaped and contained a single invaginating synapse with one ribbon complex. Cone terminals form large, flat pedicles that contain many invaginating synapses with separate ribbon structures. These terminals were more scarce, but apparent in Crx+/+ retinas at P21 (one example in box). (B) In the outer plexiform layer (OPL) of Crx-/- retinas, photoreceptor terminals appeared highly disorganized at P21 (arrows). Well-formed pedicles and spherules were not evident. Putative terminals containing ribbon-like structures were apparent, suggesting at least limited generation of synapse components. Many terminals appeared to contain multiple ribbon-like structures, instead of a singule ribbon. For example, terminal 1 and 2 contained two ribbons each, whereas terminal 3 appeared to contain only one. opl, outer plexiform layer. Scale bar = 2 μm.