Abstract
Purpose :
Our research is aimed at understanding how the pure-rod retina of the elasmobranch Little skate (L. erinacea) can perform rod and cone functions with a monotypic population of photoreceptors. The skate retina allows us to describe the properties of rod circuitry within the context of a functional, evolutionarily optimized visual system, where all downstream components co-evolved to process signals from a single cell type. Here, we reveal the ultrastructural characteristics of a hybrid rod, its postsynaptic partners, and the wiring in the OPL of a pure-rod retina.
Methods :
SB-3DEM was performed on individual retinal samples of the little skate. The datasets analyzed are from a region of interest (ROI) in the OPL and from a full cross-section of the skate retina. The ROI dataset XYZ dimensions are 27.6μm x 27.6μm x 21.5μm with a voxel size of 4.5nm x 4.5nm x 70nm. The full cross-section dataset dimensions are 88μm x 304μm x 21.8 μm with a voxel size of 10nm x 10nm x 70nm. 3D reconstructions and measurements were done with Reconstruct and Amira3D software.
Results :
~12-14 anatomically distinct invaginating processes diverge from rod terminals. These are presumed to belong to ON bipolar and horizontal cells. Additionally, ~3-4 processes made basal contacts outside of the invagination and are presumed to belong to OFF bipolar cells. However, it is also possible that there is a switched basal and invaginating identity for ON and OFF bipolar cells, respectively (as seen in some amphibians). The skate rod does not have an axonated terminal and exhibits ~4-6 telodendria (~2-18 μm), which can be traced to neighboring rods, or unidentified postsynaptic processes. Putative synaptic contacts along telodendria are presumed based on local neurotransmitter vesicle clusters along the length of telodendria.
Conclusions :
Skate rod terminal morphology suggests a hybrid rod-cone anatomy. The lack of cones in skates may explain why rods are able to light-adapt and process visual information typically in the domain of cones. To fulfill the function of cones, while also retaining rod capabilities, the rod may need to make extra contacts with neighboring rods and other retinal neurons. We hypothesize that the anatomical hallmarks we see in our data suggest that skate rods have a hybrid rod-cone architecture, which also extends to the downstream circuitry.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.