Observations on the synaptic ribbon morphology in retinas of two human subjects at autopsy

Retinal photoreceptor and bipolar cell axon terminals possess synaptic ribbons (SR) that aid in the trafficking of synaptic vesicles at active zones. In rodents, besides SR, a number of other synaptic elements [e.g., synaptic spheres (SS)] are known to appear when exposed to altered ambient illumination. Here, we report changes of ribbon shape in photoreceptor and bipolar cell axon terminals in retinas of two persons at autopsy who suffered from brain hemorrhage. In both subjects, retinal hemorrhage was present in the outer and inner nuclear layers. SR were bent or swollen and transformed into SS. A count revealed that about 54-60% of the photoreceptor axon terminals over the nasal to temporal retina possessed SS. They were associated with synaptic triads or remained floating in cytoplasm. The bipolar cell axon terminals possessed either SR or sphere-like bodies. As these features were not seen in control retinas of donors who died of other causes, we assume that in hemorrhagic subjects, SR underwent transformation into SS, in which perhaps ischemia (caused due to vascular obstructions by hemorrhage) played a leading role.