Abstract
Purpose :
The elasmobranch little skate (L. erinacea) possesses a retina with only one type of photoreceptor, i.e. rods. A simplex retina like this allows for unique opportunities to study the vertebrate visual system. Unlike genetically modified rod-only models, this pure-rod retina has evolved naturally to the present state. Thus, we can study and describe the properties of rod circuitry within the context of a functional, evolutionarily optimized visual system, where all downstream components co-evolved to process signal from a single type of cell. Little is known about the wiring and synaptic architecture in the OPL of a pure-rod retina, where the first steps in visual information processing take place.
Methods :
Eyes from little skate were hemisected and choroid-attached pieces of retina from the tapetal area were obtained. Retinal pieces were embedded in resin blocks and SB-3DEM was performed. The dataset analyzed here was from a region of interest in the OPL of the skate retina; width/height = 27.6μm, section thickness = 0.070μm, depth = 21.5μm. Voxel size was 4.5nmX4.5nmX70nm. 3D reconstructions and measurements of rod terminals with all invaginating and basal processes were done with Reconstruct and Amira software.
Results :
There was great diversity in the types and # of invaginating and basal processes making contact with rods. ~12 anatomically distinct invaginating processes can be traced to one, or several different synaptic ribbons. These are presumed to belong to ON bipolar and horizontal cells. The full identity of each unique invaginating process could not be determined from this limited dataset. An additional ~3-4 processes made basal contacts outside of the invagination and are presumed to belong to OFF bipolar cells. The skate rod does not have an axonated terminal and ~4-6 filopodia of variable length can be traced to either neighboring rods, or yet unidentified cellular processes.
Conclusions :
Skate rod terminal morphology suggests a hybrid rod-cone anatomy. The number of invaginating processes exceeds > 3-fold the usual number of four invaginating processes into a typical vertebrate mammalian rod. The multiple invaginating processes contacting skate rods are somewhat similar to the anatomical features typically observed in cone terminals.
This is a 2021 ARVO Annual Meeting abstract.