At the ultrastructural level at E85, the morphology of Wt and TgP23H photoreceptors were similar. Although outer segments had not developed, their photoreceptors had inner segments with connecting cilia (
Fig. 2A). Photoreceptor synaptic terminals also had not developed in the outer plexiform layer (
Figs. 4A, 4B). E105, Wt and TgP23H rod photoreceptors differed in several ways. Wild-type rod photoreceptors had both inner and outer segments (
Fig. 2B), whereas TgP23H rods either lacked outer segments or those present appeared truncated with no evidence of ordered stacked discs. At E105, Wt rod photoreceptor nuclei (RN) had axons and spherules that contained a few synaptic ribbons (
Fig. 4B, black arrowheads), although clear triadic profiles were absent. Rod photoreceptors in TgP23H littermates at that age lacked axons and spherules. From P0 onward, Wt rod spherules (
Figs. 4C–E, white arrows) contained ribbons, as well as synaptic and triadic profiles. By P0 some TgP23H rod photoreceptor nuclei appeared pyknotic due to densification of chromatin (
Fig. 3, black arrowhead). At P0, TgP23H rods failed to develop triadic profiles and their synaptic terminals could not easily be identified (
Figs. 4C–E, black arrows). At P3, ribbons were occasionally found in rod photoreceptor terminals; however, no triads were found (
Fig. 4D, black arrows).